8.1 Cell Division plays many important roles in the lives of organisms
When cell undergoes reproduction, cell division, the two daughter cells that result are genetically identical to each other/the parent cell
Before parent cell splits, duplicates chromosomes. One set distributed to each daughter cell
Daughter cells receive identical sets of chromosomes from original parent cell
Offspring cell genetically identical to other/to original parent cell
Cell division can result in reproduction of whole organism (single-celled organisms). Asexual reproduction, parent/offspring are identical genetically (clone)
Sexual reproduction requires sperm, egg, gametes
Production of gametes involves special type of cell division in reproductive organs
Gamete has half as many chromosomes as parent cell that makes it, chromosomes have unique combos of genes
Offspring in sexual reproduction not identical to parents, generally resemble parents
Each offspring has unique combo of genes from parents, making unique combo of traits
Cell division allows sexually reproducing organisms to develop from a single cell (egg/zygote) into adult organism
After an organism is grown, cell division continues in renewal/repair, replacing dead cells
Type of cell division for growth/maintenance of multicellular organisms or asexual reproduction called mitosis
Production of egg/sperm involves meiosis
8.3 The large, complex chromosomes of eukaryotes duplicate with each cell division
Euk cells have more genes, which are grouped into chromosomes
Each euk species has a characteristic number of chromosomes in each cell nucleus
Each euk chromosome has one long DNA molecule and protein molecules, attached to DNA. Proteins help maintain chromosome’s structure/control activity of genes
Entire ^ complex known as chromatin
Most of the time, chromatin is is made of lots of thin fibers
As cells prepare to divide, chromatin coils, forming tight, distinct chromosomes. Does to compact DNA into manageable packages
Chromosomes of euk cell are duplicated before they condense/cell divides
DNA of each chromosome replicated, new protein molecules attach
Each chromosome consists of two copies, sister chromatids (joined copies of the original chromosome)
Sister chromatids attached at centromere
When cell divides, sister chromatids of duplicated chromosome separated from each other. Each chromatid considered an individual chromosome, identical to cell’s original chromosome
During cell division, one of newly separated chromosomes goes to daughter cell, other goes to other daughter cell. Each daughter cell receives complete/identical set of chromosomes
8.4 The cell cycle includes growth and division phases
Cell division is the basis of reproduction for every organism, allows multicellular organisms to grow, replaced worn out/damaged cells
Cells divide a certain amount depending on type
Some mature cells never divide, explaining why certain kinds of damage can never be reversed
Process of cell division important to cell cycle
Cell cycle has two main stages: growing stage (interphase, where cell doubles everything in cytoplasm/replicates DNA) and miotic stage (actual cell division)
Cells spend most time in interphase. Cell performs normal functions/has high metabolic activity. Cell duplicates chromosomes
Interphase divided into 3 subphases
G1 phase
First gap
S phase
Synthesis of DNA (DNA replication)
Chromosomes duplicated
G2 phase
Second gap
Cell completes preparations for cell division
At beginning of S phase, chromosome single. At end of DNA replication, chromosomes doubled, each having two sister chromatids joined along lengths
Miotic phase (M phase) is when cell physically divides
Miotic phase divided into 2 overlapping stages
Mitosis
nucleus/contents (incl chromosomes) divide, distributed to two daughter nuclei
Cytokinesis
Cytoplasm divided in two
Combination of mitosis/cytokinesis produces two genetically identical daughter cells, which repeat the cycle
Mitosis unique to euk.s, is solution to problem of allocating identical copy of whole set of chromosomes to 2 daughter cells
Accuracy of mitosis essential to development, ensures all body cells receive all chromosomes from original one cell
8.5 Cell division is a continuum of dynamic changes
Dramatic changes during cell division, mitotic phase
Mitosis is continuous process, but has 5 main stages
Prophase
Prometaphase
Metaphase
Anaphase
Telophase
Chromosomes movements depend on mitotic spindle, a structure of microfibers/associated proteins that guide separation of two sets of daughter chromosomes
Spindle microtubules emerge from two centrosomes, microtubule-organizing regions in cytoplasm of euk
8.6 Cytokinesis differs for plant and animal cells
Cytokinesis overlaps with telophase
Cytokinesis proceeds differently for plant/animal cells
Animal cells
Occurs by cleavage
First sign of ^ is cleavage furrow (groove in cell’s surface)
At furrow, cytoplasm has microfilaments made of actin. When they interact with myosin, the ring contracts
Contraction of myosin ring pinches parent cell in two, making two separate daughter cells
Plant cells
Stiff cell walls prevent contraction
During telophase, membranous vesicles w/ cell wall material collect at middle of parent cell
Vesicles fuse, forming cell plate, which grows outward, getting more cell wall materials as more vesicles fuse with it
Eventually, membrane fuses w/ plasma membrane, cell plate’s contents join parental cell wall, resulting in 2 daughter cells, each bounded by own plasma membrane/cell wall
8.7 The rate of cell division is affected by environmental factors
Timing of cell division needs to be carefully controlled to grow, dev normally, maintain tissues
Researchers have found many factors (chemical/physical) that influence cell division
Physical factors
Animal cells exhibit anchorage dependence, meaning they need to be in contact with a solid surface to divide (like culture dish/extracellular matrix of a tissue)
Density-dependent inhibition, where crowded cells stop dividing
Animal cells growing on surface of a dish multiply to form a single layer, stop dividing when touch each other
If some cells removed, those bordering open space begin dividing again, continue until vacancy filled
Believe physical contact of cell-surface proteins between adjacent cells responsible for inhibiting cell division
Chemical factors
Cells can’t divide if an essential nutrient is left out
Most mammalian cells only divide if certain proteins, growth factors, are present
Dozens of different growth factors
Different cell types respond to certain growth factors/to a certain combo of them
Importance of proper cell division to health clear
Cancerous cells are different from normal body cells, no longer exhibit types of regulation, and grow/divide (even without an anchorage surface or in high densities, etc)
8.8 Growth factors signal the cell cycle control system
Reproductive behavior of cells (division) results from interactions among dif molecules
In a living animal, most cells anchored in fixed position, bathed in solution of nutrients supplied by blood. Usually don’t divide unless signaled by other cells to
Growth factors are main signals
Cell cycle control system is a set of molecules that triggers/coordinates key events of cell cycle
Ex. during mitosis, metaphase does not automatically lead to anaphase. Proteins of the cell cycle control system trigger anaphase to begin
Checkpoint in cell cycle critical control point where stop/go signals can regulate the cycle
Default of animal cells is to stop cell cycle at checkpoints unless overridden by go signals from growth factor proteins
Major checkpoints include G1 and G2 subphases of interphase.
Intracellular signals detected by control system tell system if key cellular processes ave been completed/whether cell cycle should proceed
Control system receives messages from outside the cell (ex. Env conditions/presence of growth factors)
For most cells G1 checkpoint is most important during cell division.
If cell gets go signal at G1 checkpoint, usually enters S phase, going on to complete cycle/divide
if signal never arrives, cell switches to permanently non dividing stage called G0 phase
8.9 Growing out of control, cancer cells produce malignant tumors
Cancer cells don’t heed normal signals that regulate cell cycle. Divide excessively, invade other body tissues. May continue to grow/spread until kill the organism
Cancer begins when a single cell undergoes changes that convert a normal cell to a cancer cell.
Cancer cell often has altered proteins on surface, body’s immune system recognizes cell as foreign and destroys it
If cancer cell not destroyed, may multiply to form a tumor, a mass of abnormally growing cells in normal tissue
If abnormal cells remain at original site, called benign tumor. Cause problems if grow in/disrupt certain organs. Often can be removed or left alone
A malignant tumor is a mass of abnormally reproducing cells that can spread to neighboring tissues/invade other parts of the body. Can displace normal tissue, interrupt organ function
Person w/ malignant tumor said to have cancer
Cancer cells may separate from original tumor/secrete signal molecules that cause blood vessels to grow toward tumor
Few tumor cells may enter blood/lymph vessels, and move to other parts of body where can proliferate/form new tumors
Spread of cancer cells beyond original site called metastasis
Cancers named according to organ/tissue in which they originate
Cancer cells do not heed normal signs that regulate cell cycle
Many have defective cell cycle control systems that proceed past checkpoints w/o growth factors
Some make their own growth factors, causing cells to divide continuously
Stop dividing randomly, if they stop at all
Can divide indefinitely, as long as have the supply of nutrients
Many tumors can be successfully treated
Localized tumors can be surgically removed or treated w/ concentrated w/ beams of high-energy radiation (which usually damages DNA in cancer cells)
Radiation damages normal body cells, producing harmful side effects
Chemotherapy used to treat metastatic/widespread tumors. IV drugs used to disrupt steps in cell cycle.
Side effects of chemotherapy are due to drugs’ effects on cells that rapidly divide in parts of body other than where cancer cells are (ex. Hair loss from effects on hair follicle cells)
9.2 The science of genetics began in an abbey garden
Heredity is the transmission of traits from one generation to the next
The field of genetics studies heredity, started when Gregor Mendel deduced fundamental principles of genetics by breeding garden peas
Mendel’s research was influenced by study of physics, math, chemistry from university
Education helped Mendel design studies that were experimentally/mathematically rigorous (responsible for success)
Mendel argued parents pass discrete “heritable factors” to offspring.
Stressed that heritable factors (genes) retain individuality (identity) generation after generation (never mixed, even with time)
Chose to study garden peas because have short generation times, produced lots of offspring, had many varieties (ex some have white or purple flowers), matings can be controlled
Heritable feature that varies among individuals (like flower color) called a character
Each variant of character (ex purple or white flowers) called traits
Pea plants able to self-fertilize. Sperm-carrying pollen released from stamens land on egg-containing carpel of same flower.
Mendel covered flower with a bag to ensure no pollen from another plant could reach the carpel
When he wanted cross-fertilization…
he prevented self-fertilization by cutting off immature stamens before produced pollen.
To cross fertilize, dusted carpel with pollen from another plant.
After pollination, carpel dev.ed into pod w/ seeds (peas). Seeds grew into offspring plants
^ methods, Mendel could make sure of parentage of new plants
Mendel’s success due to experimental approach/choice of organism/selection of characters to study
Chose to observe 7 characters, occurred as 2 distinct traits
Chose true-breeding varieties, which produced only same variety as parent plant.
ex, plant w/ purple flowers it true-breeding if all next generations produced through self-pollination always consists of plants w/ purple flowers
Mendel wanted to know what would happen when crossed different true-breeding varieties with each other
What offspring would result if plants w/ purple flowers and plants w/ white flowers were cross-fertilized
Offspring of two different varieties are called hybrids. Cross fertilization itself known as hybridization, or genetic cross
True-breeding parents called P generation (p for parental) and their hybrid offspring called F1 generation (F for filial, son)
When F1 plants self-fertilize or fertilize each other, offspring are F2 generation
Mendel’s quantitative analysis of F2 plants from thousands of genetic crosses allowed him to deduce fundamental principles of heredity
9.3 Mendel’s law of segregation describes the inheritance of a single character
Mendel’s tracked inheritance characters that occur in two forms (like flower color). Results led him to formulate several hypotheses about inheritance
Mendel observed (by cross breeding true-breeding pea plant w/ purple flowers/true-breeding pea plant w/ white flowers) that all resulting F1 plants had purple flowers
By mating F1 plants, Mendel found that ¾ had purple flowers, ¼ had white flowers
Reasoned that heritable factors for white flowers didn’t appear bc masked when purple flower factor present
Deduced that F1 plants must have carried two factors for flower-color character (one purple, one white)
Developed four hypotheses
Alternative versions of genes that account for variations in inherited characters
Gene for flower color in pea plants has two versions (purple or white)
Alternative versions of a gene are called alleles
For each character, organism inherit two alleles of a gene, one from each parent
Alleles may be identical or not
Organism that has two identical alleles for a gene is hom*ozygous
Organism that has two different alleles for gene is heterozygous
If two alleles of an inherited pair differ, one that determines organism’s appearance called dominant allele. Other that has no noticeable effect on appearance called recessive allele
Sperm or egg only carries one allele for each inherited character bc allele pairs separate from each during production of gametes
When sperm/egg unite during fertilization, each contributes its allele, restoring paired condition in offspring
Mendel’s law of segregation explains inheritance pattern
Hypotheses predict that when alleles segregate during gamete formation in F1 plants, half gametes receive purple flower allele, other half white flower allele
During pollination among F1 plants, gametes unite randomly
An egg w/ purple flower allele has = chance of being fertilized by sperm w/ purple-flower allele or w/ white-flower allele
Same true for egg w/ white-flower allele
Total four equally likely combos of sperm/egg in F2 generation
Punnet square visually highlights four possible combinations of gametes/resulting four possible offspring in F2 generation
each square represents equally probable product of fertilization
¼ plants have two alleles for purple flowers
½ have one for purple, one for white
¼ have two alleles for white flowers (express recessive trait)
Because organism’s appearance doesn’t always reveal genetic composition, geneticists distinguish between organism's observable traits phenotype and genetic makeup, genotype
Mendel found that each of 7 characters studied exhibited same inheritance pattern: one parental trait disappeared in F1 generation, reappeared in F2 offspring
Mechanism underlying ^ inheritance pattern stated by Mendel’s law of segregation
Pairs of alleles segregate during gamete formation; the fusion of gametes at fertilization creates allele pairs once again
Research since est that law of segregation applies to all sexually-reproducing organisms
9.9 Many inherited traits in humans are controlled by a single gene
Trait being dominant doesn’t mean it’s normal or more common
Recessive traits may be more common (ex. Freckles are dominant, but more common for them to be absent)
Mutant traits refers to trait that is less common in nature
Genetic disorders like albinism vs pigment or freckles vs none are inherited as dominant or recessive traits controlled by a single gene
Human disorders show simple inheritance patterns
Recessive Disorders
Albinism and Tay-Sachs are inherited as recessive traits
Most people w/ recessive disorders are born to normal parents who are both heterozygotes (both are carriers of the recessive allele for the disorder, but are phenotypically normal)
Albinism
If two people who are heterozygous carriers for albinism, each child of them has a ¼ chance of getting two recessive alleles/getting that disease.
Child w/ normal pigmentation has ⅔ chance of being Aa carrier. ⅔ offspring with pigmented phenotype will be albinism carriers
Cystic fibrosis
Recessive CF allele found in 10 mil Am.s. Are silent carriers
Person with two copies of allele has cystic fibrosis
Secretion of thick mucus from lungs/other organs. Can interfere w/ breathing, digestion, liver function. Makes vulnerable to bacterial infections
No cure, certain ways to treat
Most common in Caucasians
Most genetic disorders not evenly disturbed across all ethnic groups
May be due to prolonged geographic isolation of certain populations.
Isolation can lead to matings between close blood relatives, who are more likely to carry same recessive alleles than unrelated people.
Matings between close relatives may cause frequency of rare allele to increase in that community
Geneticists observed increased incidence of harmful recessive traits among inbred animals
With increased mobility in human populations today, unlikely that two people who carry a rare, harmful allele will meet and mate
Dominant Disorders
Most harmful alleles are recessive, a number are caused by dominant alleles
Some are harmless, like extra fingers or toes (polydactyly) or webbed fingers/toes
Achondroplasia
a form of dwarfism where head/torso develop normally but arms/legs short
hom*ozygous dominant (AA) causes death of embryo, so only heterozygotes only have this disorder (Aa)
Person w/ this has 50% chance of passing to children
Everyone who doesn't have it are hom*ozygous recessive (aa)
Dominant alleles that cause lethal diseases less common than recessive alleles that do
1 reason bc dominant lethal allele can’t be carried by heterozygotes w/o affecting them
Many lethal dominant alleles come from mutations in sperm/eggs that kill embryo
If afflicted individual born but doesn’t survive long enough to reproduce, can’t pass allele to future generations (unlike lethal recessive, pass through healthy heterozygous carriers)
lethal dominant allele can escape elimination if doesn’t cause death til advanced age.
Huntington’s disease doesn’t appear to mid-life. Once starts deterioration of nervous system, irreversible and fatal.
Because allee for it dominant, any children born to parent w/ allele has 50% chance of inherited the allele/disorder
9.20 Chromosomes determine sex in many species
Term sex means classification into group with a shared set of anatomical/physiological traits (biological gender)
Gender has come to encompass what people want to be, not what they are biologically
Sex determined by chromosomes in humans/lots of animals
humans/other mammals have two types of sex chromosomes, X and Y
Person with 2 X chromosomes usually is a female. Males have 1 X and 1 Y
Y chromosome smaller than X.
Short segments at ends of Y chromosomes that are hom*ologous w/ regions on X. Regions allow X/Y in males to pair and behave like hom*ologous chromosomes during meiosis in testes
After meiosis, each gamete has one sex chromosome and haploid set of autosomes (nonsex chromosomes, humans have 22)
All eggs contain a single X chromosome
Half of sperm cells have X and half have Y
Offspring's sex depends on whether sperm cell that fertilizes egg has X or Y chromosome
Sex determination is matter of chance, 50/50
Human Y chromosomes have 78 genes, half expressed only in testis.
One gene is known for crucial role in sex determination. SRY codes for proteins that regulate other genes on Y chromosome
Anatomical signs of sex emerge when embryo is 2 months old. Before then, rudiments of gonads are genetic. Early gonads develop testes or ovaries, depending if SRY gene active
If SRY not turned on, develop ovaries, even in XY embryo
If SRY produces its protein, turns on other genes, leading to testis development
In humans, sex determined by interactions of several proteins produced by dif genes
Development of female gonads requires gene called WNT4, which encodes a protein that promotes ovary development
Embryo that’s XY but has extra copies of WNT4 gene may develop rudimentary female gonads
Biochemical, physiological, anatomical features associated w/ females/males are more complex than previously realized. Many genes involved in development
Sex is not a binary state, w/ 2 outcomes. Bc of complexity of genes/proteins involved in sex determination, many variations exist
Some ppl born w/ intermediate sexual (intersex) characteristics
Some transgender people
In some animals, environment can determine sex
For some reptiles, temperature of incubation of eggs during specific period of embryonic development determines sex
If green turtle hatchlings develop above 30°C, usually male.
Leads some to worry climate change will affect makeup of turtle populations
9.11 Incomplete dominance results in intermediate phenotypes
Mendel’s two laws explain inheritance in terms of discrete factors (genes) that are passed from generation to generation according to probability
Laws valid for sexually reproducing organisms (garden peas, humans, etc)
Mendel’s laws don’t fully explain all patterns of genetic inheritance for most sexually reproducing organisms. More complex
The F1 offspring of Mendel’s pea crosses always looked just like one of parental varieties. Called complete dominance (dominant allele has same phenotypic effect if in 1 or 2 copies)
For some, appearance of F1 hybrids falls between phenotypes of two parental varieties, called incomplete dominance. (ex. Mixing flowers colors)
In incomplete dominance, instead of having 3:1 chances of purple to white say, you would have 1:2:1 (2 being mixed)
Examples of incomplete dominance in humans
Hypercholesterolemia (High levels of cholesterol in blood)
Involves recessive allele (h)
Normal ppl have HH.
People w/ Hh have blood cholesterol levels 2x normal. Prone to atherosclerosis (blockage of arteries by cholesterol buildup)
May have heart attacks around age 30
Can be controlled through diet changes or medications
People with hh have it more serious
5x normal amount of blood cholesterol
May have heart attacks as early as 2 years old
Harder to treat
Doses of cholesterol-lowering drugs, organ surgeries, transplants, filtering lipids from blood
H specifies cell-surface receptor protein called LDL receptor. Low density LDL is transported in blood
In certain cells, receptors take in excess LDL particles from blood, promote breakdown. Helps prevent accumulation
Hh have half normal number of LDL receptors
hh have no receptors
Lack of receptors results in buildup (no way to rid of excess) and can be deadly
9.12 Many genes have more than two alleles that may be codominant
Most genes can be found in populations in more than two versions (multiple alleles)
Each individual carries (at most) two different alleles for a particular gene, but more can exist in the population
Ex. ABO blood group phenotype involves three allees of a single gene
Various combinations produce four phenotypes
A, B, AB, O
Matching compatible blood types critical for safe blood transfusions so that the immune system doesn’t reject blood that has antibodies for (since don’t recognize proteins of it)
Certain antibodies bind specifically to foreign carbs of dif blood cells, cause them to clump together, potentially killing recipient
Four blood groups result from various combo.s of three different alleles
IA (Adds carbohydrate A to red blood cells)
IB (Adds carbohydrate B to red blood cells)
i (Adds neither carbohydrate A or B to red blood cells)
Each person inherits one of these alleles from each parent
Bc are three alleles, 6 possible genotypes
Ia and IB alleles dominant to i allele
IAIA and IAi people have type A. IBIB and IBi have type B
Recessive hom*ozygotes, ii, have type O (no carb)
IA and IB alleles are codominant (where heterozygote expresses distinct trait of both alleles): both are expressed in heterozygous ppl (IAIB) who have type AB)
Codominance different from incomplete dominance
9.13 A single gene may affect many phenotypic characters
In many cases, one gene influences multiple characters. Called pleiotropy
Ex. called sickle cell disease (makes red blood cells produce abnormal hemoglobin proteins)
Molecules link, hemoglobin crystalizes. Normal shope red blood cells deform to sickle shape with jagged edges
Sickled cells destroyed rapidly by body, destruction lowers red cell count, causing anemia/weakness
Because of angular shape, sickle cells don’t flow smoothly in blood, accumulate/clog tiny blood vessels.
Blood flow in some parts of body reduced, causing fever, pain, damage to organs
Blood transfusions and drug treatment may help symptoms, but no cure
Usually, only people who are hom*ozygous for sickle-cell allele have it
Heterozygotes are usually healthy
Disease considered recessive
In rare cases, heterozygotes may have some effects when oxygen in blood is reduced
Heterozygote displays incomplete dominance for sickle-cell trait, w/ phenotype between hom*ozygous dominant/hom*ozygous recessive phenotypes
Two alleles are codominant. Blood cells contain both normal and abnormal hemoglobins (sickle cell)
Most common in Africans, rare among other Americans
Frequency of sickle-cell allele might be lower because many hom*ozygotes die before passing genes to next generation
High frequency in tropical Africa, where malaria prevalent. Parasite that causes malaria triggers sickling
Body destroys sickled cells, killing parasite too
Sickle-cell carriers have increased resistance to Malaria, allows to live longer nad have more kids, pass on sickle-cell
9.14 A single character may be influenced by many genes
Mendel saif genetic characters could be classified on either-or basis (purple or white). Many characters, like human skin color/height, vary in population among a continuum
Many features result from polygenic inheritance, additive effects of two or more genes on single phenotypic character (several genes affect one character)
Hundreds of genes/alleles that affect height in humans
Many diseases (ex. Diabetes, heart disease, cancer) display polygenic inheritance
Example
Three hypothetical genes inherited separately, w/ tall allele for each (A,B, C) contributing one unit of tallness to phenotype. Not fully dominant to other alleles (a, b, c)
AABBBCC would be very tall
AaBbCc medium height
aabbcc very short
Because alleles have additive effect, genotype AaBbCc would result in same height as genotype with three tall alleles, like AABbcc
Inheritance of 3 genes could lead to wide variety of height phenotypes
Four inheritance patterns that are extensions (not exceptions) of Mendel’s laws of inheritance
Incomplete dominance
Codominance
Pleiotropy
Polygenic inheritance
Mendel’s idea of genes as discrete units of inheritance is true for all inheritance patterns, even patterns more complex than he considered
9.15 The environment affects many characters
Genetic description always incomplete, bc are other influences
In last module, 3 sets of hypothetical genes could produce 7 different phenotypes for height. But more height phenotypes than 7. Can be influenced by env factors (nutrition/exercise/ etc)
Many characters/traits result from combo of heredity/environment
Clear that many human characters (like risk of heart disease/cancer/susceptibility to alcoholism/schizophrenia) are influenced by genes/environment
Whether human traits are more influenced by genes/environment (nature vs nurture) debated, not agreed on
In certain traits/genotypes, the environment plays no role (ex. ABO blood group)
How red blood cells are circulating in body influenced by env factors like overall health/altitude
Individual features of any organism come from combo of genetic/env factors
9.22 Human sex-linked disorders affect mostly males
Many human conditions are recessive X-linked traits
If man inherits one X-linked recessive allele (from mother) allele expressed. Woman has to inherit 2 of those alleles (one from each parent) to have the trait
Recessive X-linked traits expressed more frequently in men than women
Hemophilia is an X-linked recessive trait (excessive bleeding/pain). Disease traced back from one royal family, which then spread to many others through birth/relations
Another human X-linked recessive disorder is duch*enne muscular dystrophy, where muscles weaken, lose coordination.
People don’t like this.
Traced to recessive mutation in gene of X chromosome that codes for muscle protein
Cell division: how cells reproduce
Cells reproduce for…
Mitosis body cells (every other cell)
Growth
Healing/regeneration
Defense (blood cells)
Miosis (sex cells)
Reproduction
Chromosomes are made of genes, which are made of DNA
Chromosomes are paired in all body cells
Mitosis
One cell w/ 2 chromosomes (pair) (not duplicated yet)
DNA replication to copy chromosomes and get more to have the same amount in each cell (duplicated)
Then 2 chromosomes in each cell (unduplicated)
Paired
One from mom, one from dad
Duplicated
Underwent replication
Did not undergo replication
Paired-diploid
Unpaired-haploid
Cells replaced by cell division called mitosis
Division of cell’s nucleus into 2 nuclei
Usually happens in somatic cells (cells w/ 2 copies of each chromosome)
One copy from each parent
Mitosis results in 2 nuclei w/ identical set of chromosomes to original nucleus
Cytokinesis next, cell divides to form 2 cells
Before enters mitosis, in interphase, where prepares for division
Most time in interphase
In interphases cell grows/duplicates organelles (so that enough when cell divides)
During interphase, chromosomes unwound in nucleus
Through DNA replication, cell copies DNA. After, chromosome has 2 identical DNA molecules (Chromatids)
Chromatids attached at condensed region of chromosome (Centromere). Attached until separate during mitosis
After copying DNA/duplicating organelles, cell enters M phase
M phase (2 stages)
Mitosis (5 stages)
Prophase
Chromosomes condense into shorter/individual structures
Each of these chromosomes has 2 identical chromatids held together at centromere
Previously duplicated centrosomes form microtubules. As form, centrosomes move away from each other bc microtubules lengthen, pushes them apart
2 centrosomes/microtubules form mitotic spindle
Promestaphase
Microtubules lengthen, pushes centrosomes to opposite ends of cell
When centrosomes arrive at opposite ends, called spindle poles
Nuclear envelope breaks down, makes replicated/condensed chromosome accessible to microtubules
Microtubules that extend from spindle poles attach to either side of each chromosome at kinetochores (specialized protein structures that form as centromere of each chromatid)
Microtubules attach to kinetochores called kinetochore microtubules
Each mitotic spindle has 2 other types of microtubules
Polar microtubules (reach across cell, interact w/ each other. Keeps centrosomes separate/defines spindle length)
Astral microtubules (anchor spindle poles to cell membrane)
Metaphase
Kinetochore microtubules arrange chromosomes so lined up along metaphase plate (imaginary line halfway between spindle poles)
Anaphase
Separase separates chromatids of each chromosome
Kinetochore microtubules shorten, pulls chromatids toward opposite spindle poles
Chromatids separated, referred to as individual chromosomes (bc each has own centromere)
Telophase
Mitotic spindle breaks down
Nuclear envelope forms around each set of chromosomes
Chromosomes decondense
Then have 2 nuclei w/ identical genetic material
Cytokinesis
When cytoplasm is divided
Contractile ring squeezes cell into 2 as it shrinks
As contractile ring shrinks, cleavage furrow appears. Then, two separate/identical cells produced
Each cell has own nucleus/set of cellular components
Cells undergo division allowing you to grow/so tissues can renew themselves
Overview
Each chromosome first replicated during interphase
Chromosomes separated/distributed between 2 nuclei during mitosis
Cell divides into 2, new cells genetically identical
Each new cell centers interphase, enters M phase when ready
Anchorage dependence
Need to be attached to something
Density-Dependent Inhibition
When there’s a gap, cells signal for a cell to fill the gap
How much space determines if they divide
Growth factors
Cells are told to divide, don’t just randomly do it
Cancer cells spontaneously divide, don’t listen to signals/rules of division
Cell cycle checkpoints regulate how/if cells divide (control cell cycle)
G1 Checkpoint
Check for nutrients, growth factors, DNA damage, cell size
S checkpoint
DNA synthesis/replication
Look for mutations
G2 checkpoint
Check for cell size, DNA replication, DNA damage
Metaphase checkpoint
Check for chromosome spindle attachment
Cells grow first
Before move on, hit checkpoint. DNA and size have to be okay
Move into S phase, duplicate chromosomes (Making copies of DNA)
Before move into next phase, need to grow
Checkpoint checks that everything is duplicated, cell is right size, and if all genetic material is intact
Fix any damage, move on
Start to divide chromosomes
Separate chromosomes
Cell divides (cytokinesis)
Checkpoints
Specific order
Cannot continue if doesn’t meet requirements (especially DNA)
Strict/specific criteria
Can try to repair, might have to self destruct
Checkpoint regulation
CDKs and Cyclins control this
Proteins
Partners, help to get us through cell cycle
CDK uses phosphate group to activate the molecules, move past checkpoint. Needs to be attached to cyclin to do this
Cyclins cycle, detach after every checkpoint, new one attaches before every checkpoint
Cells can grow, but organisms grow by making more cells through division
Mitosis and cytokinesis allows you to make new body cells
Cell division needs to be regulated
Cancer is in part due to cells that aren’t regulated, and continue to grow/divide a lot
Mutagens can cause cancerous cells
Cancerous cells that divide can create a tumor
Can destroy cancer cells with chemotherapy or radiation
Cell cycle
Interphase, where cells grow, replicate DNA, do cell functions
Most time spent here
M phase, cell division (mitosis/cytokinesis)
May do more or less often, depending on the cell
If a cell has an error, you don’t want it to divide because then another cell will have that issue
Checkpoints that check DNA replication, if DNA is damaged, if it’s growing, resources, functions before allowing to move onto phases
If pass a checkpoint, move onto next phase, goes to another checkpoint til gets to M phase
If a cell fails a checkpoint, it tries to fix it or does apoptosis (self destructs) so can’t replicate, cause more issues
Many proteins regulate cell cycle
Some allow moving forward in cycle
Cyclin
Each checkpoint usually has a different type of cyclin that binds to CDK
CDK
Enzyme (kinase)
Can have different types of cyclin bound to it
Some make cycle stop
Proteins that initiate apoptosis
Sensitive to cues in/outside of cell
Some cells don’t go through phases because are in G0 phase. A resting phase
Still perform cell functions, not preparing to divide
Some stay here permanently, so don’t divide. Can be a reason why major injuries in brain/spinal chord bc don’t replicate, can’t heal
Some stay here for some time, if don’t have enough resources, for ex
Genes are portions of DNA, can code for characteristics/traits
Some traits coded for by a combo of genes
Humans have 46 chromosomes (made of DNA/proteins)
Entire genetic code represented by chromosomes
Specific areas on genes code for traits
Allele is a variant/form of a gene. Could be the same or different in chromosome pairs
Alleles you inherit determines your traits
Alleles usually represented by letters (capital/lowercase matters. Capital means is dominant, lowercase means recessive)
If dominant, will be expressed. Recessive usually not expressed, unless no dominant one present
You have two alleles copies
Genotypes (genetic makeup) can help determine phenotype (physical characteristics)
Have to get an allele from each parent, if you don’t have a combo of letters that they had, not possible
Dominant traits are not always more common than recessive in a population. Dominant allele may be more rare
Alleles are forms of a gene, represented by letters
Recessive allele represented by lowercase letter
Recessive means usually won’t show up, unless no dominant allele present
Dominant allele represented by a uppercase letter, will show up
Genotype (genetic makeup of an organism)
Only takes one dominant allele for a trait to show up
Genotype with same letters considered hom*ozygous (same case. Ex. HH and hh). Also labeled by dominant/recessive after hom*ozygous
Genotype of different letters (Dif cases. ex. Hh) called heterozygous
Punnnett squares can help with monohybrid crossing (seeing what will result in a child from two animals of same species). (But punnet squares only probabilities)
Phenotypes are physical traits of an organism
