Bio 120 Unit 4 Flashcards
Intro to the Cell Cycle
Cells arise through cell division of preexisting cells
Observations of newly developing organisms, or embryos, confirmed that plants and animals
-Start life as a single-celled embryo
-Grow through a series of cells divisions
Meiosis produces reproductive
cells called gametes
Mitosis produces all other cell types= somatic cells
Mitosis
Mitosis and meiosis are usually accompanied by cytokinesis- division of cytoplasm into daughter cells» Mitosis: Genetic material is copied and divided equally between 2 cells» Daughter cells are identical to the parent cell and to each other
Meiosis
Mitosis and meiosis are usually accompanied by cytokinesis- division of cytoplasm into daughter cells»Meiosis: Produces cells with half the amount of hereditary material as the parent cell» Daughter cells are genetically different
How do cells replicate?
Cells mush replicate for life to exist
1. Copying the DNA
2. Separating the copies
3. Dividing the cytoplasm to create 2 complete cells
The Cell Cycle has 4 Phases
The cell cycle is the
orderly sequence of events
that occurs from the
formation of a eukaryotic
cell through the
duplication of its
chromosomes to the time
it undergoes cell division
Cells alternate between M Phase & Interphase
M( mitotic) phase: Dividing
-Chromosomes are condensed into compact structures
Interphase: Nondividing
-G1+S+G2
-Chromosomes are uncoiled
-Cells are growing, preparing and fulfilling their specialized functions
-Cells spend most of their time in interphase
-The gap phase allow cells to grow and replicate organelles
Discovery of S Phase
Chromosomes replication occurs during the S(synthesis) phase of interphase
DNA Replication: S Phase of Interphase Chromosomes Condensation: M Phase
This is chromatin but by convention we still refer to this as the replication of chromosomes
What is a chromosome? A single long double helix of DNA
-Wrapped around proteins called histones
-DNA-Protein materiel is called chromatin
-DNA encodes the cells genetic info
A gene is a section of DNA
-Codes for s specific RNA
-Therefore codes for a specific protein
What happens during M Phase?
M Phase consist of 2 distinct events:
1. Mitosis- the division of the replicated chromosomes
2. Cytokinesis- the division of the cytoplasm
Every species has a characteristic number of chromosomes Humans have 46 chromosomes
Mitosis (M phase) is a continuous process with 5 subphases
- Prophase
- Prometaphase
- Metaphase
4.Anaphase
5.Telophase
Prophase: Chromosomes condense and first become visible in the light microscope
The spindle apparatus forms
-Move replicated chromosomes during early mitosis
- Pull chromatids apart in late mitosis
Prometaphase
-The nuclear envelope breaks down
-Microtubules attach to chromosomes at kinetochores
-Structure that form at the centromere
-2 form of the opposite side of each chromosome
-Microtubules that attach to chromosomes are called kinetochore microtubules
Chromosome are pushed and pulled by microtubules until they reach the middle of the spindle
Metaphase
-Mitotic spindle is complete
-Chromosomes are lined up on the metaphase plate
-Each chromosomes is held by kinetochore microtubules from opposite poles
-Astral microtubules hold spindle poles in place
Anaphase: Cohesins holding sister chromatids together split
-Sister chromatids are pulled by the spindle fiber toward opposite pole of the cell
-Creates 2 identical sets of daughter chromosomes
2 forces pull chromosomes apart:
-Kinetochore microtubules shrink
-Motor proteins of the polar microtubules push the 2 poles of the cell away from each other
Telophase
- A new nuclear envelop begins to from around each set of chromosomes
-The chromosomes begin to decondense
-Mitosis is complete when 2 independent nuclei have formed
Cytokinesis: Typically occurs immediately after mitosis
The cytoplasm divides to from 2 daughter cells
Cytokinesis in Plants
-Vesicles from the Golgi apparatus bring membrane and cell wall components to the middle of the cell
-These vesicles fuse to form a cell plate
Cytokinesis in Animals
-A ring of actin & myosin filaments contract inside the cell
-Pinches in to form a cleavage furrow and the ring shrinks and tighten until division is complete
Bacterial Cell Replication: Bacteria divide via binary fission
-This is a process similar to eukaryotes M Phase
-Bacterial chromosomes are replicated
-Proteins attach to chromosomes and separate them
-Other proteins divide the cytoplasm
Control of the Cell Cycle: Cycle length varies around cell types
-Mostly due to variation in G1
-Rapidly dividing cells essentially eliminate G1 Phase
-Non dividing cells are permanently in G1 phase( G0 state, highway exist)
Division rate can also vary in response to changing conditions
Kinase
Adds a phosphate group
Turns on protein
Phosphatase
Removes a phosphate group
Turns off a protein
Key regulator of cell cycle? MPF (M Phase Promoting Factor)
Kinases
-Cyclin Dependent Kinases(Cdk)
-They depend on cyclin to get activated
Cyclin
-Protein whose concentration cycles up and down during the cell cycle
When cyclin concentrations are high: Cyclin peaks in M phase before decreasing: Cyclin increases during interphase
-MPF is active
-Target proteins are phosphorylated
-Initiating mitosis
How is MPF turned off? 2 key concepts about regulatory systems in cells:
-Negative feedback occurs when a process is slowed of shut down by its products
-Destroying specific proteins is a common way to control cell processes
How is MPF turned off? An enzyme complex(APC/C) is activated during anaphase attaches proteins to the cyclin subunit
Adds a small protein tag called ubiquitin(Ub) that marks cyclin for destruction
3 Cell Cycle Checkpoint Can stop the Cell Cycle( police officers pulls you over)
-All critical points are regulated
-Regulatory molecules at each checkpoint allow a cell to decide whether to proceed with division
-If defective the checkpoint may fail
-Cells that divide without control may from a tumor
G1 Checkpoint: Most important checkpoint, decides if a cell will continue in the cycle
4 factors affect passage through the G1 check
1. Size
2. Availability of nutrients
3. Social signal from other cells
4. Damage to DNA
G1 Checkpoint: If DNA is physically damaged, the p53 protein
-Either activates proteins that pause the cell cycle until damage can be repaired
-Or initiates apoptosis= programmed cell death
p53 is an example of a tumor suppressor
-Damage to the p53 gene can lead to uncontrolled cell division
G2 Checkpoint
-The second checkpoint is between the G2 and M phase
-If chromosome replication has not proceeded properly of if DNA is damaged
-MPF is not activated
-Cells remain in G2 phase
M phase checkpoint: The 3rd checkpoint is really 2 during M phase
-Between metaphase and anaphase: ensures that sister chromatids do not split until all kinetochore are attached to the spindle apparatus
-Between anaphase and telophase: ensures that chromosomes have fully separated
Sexual Reproduction: During sexual reproduction
-Reproductive cells called gametes unite to form a new individual
-This process is called fertilization
-Gamete are called sperm and eggs in animals
Meiosis” lessening act”
Meiosis is a nuclear division that leads to halving of chromosome number
-Gametes must contain half the chromosome number
-At fertilization full chromosome number is restored
Sex chromosomes determine the sex of individual
-In many animals females have 2 X chromosomes and males have X and Y
-Autosomes are non-sex chromosomes
Karyotype
Identifies the number and type of chromosomes
Chromosomes of the same type are called homologous chromosomes or homologs
Homologous pairs
-Contain the same genes in the same position
-Share length, centromere position and staining pattern
-The 2 homologous are not identical may carry different alleles
The Concept of Ploidy: Many organisms including humans are diploid
-They have 2 homologous of each chromosome
-They have 2 alleles of each gene
The Concept of Ploidy: Other organisms including fungi are haploid
-They have only 1 each type of chromosome
-They have just one allele of each gene
Overview of Meiosis
-Just before meiosis begins each chromosome in the diploid parent cell is replicated
-When complete each chromosome has 2 identical sister chromatids
-They remain attached along most of their lenght
-The 2 attached sis chromatids are still considered a single replicated chromosome
Meiosis consists of 2 cell division
Meiosis 1: parent cell is diploid and contains a homologous pair of replicated chromosomes
Meiosis 2: Daughter cells are haploid and contain just 1 homolog
For each division: As in mitosis chromosome movement is coordinated by microtubules of the spindle apparatus
-Microtubules attach to kinetochores at the centromere of each chromosome
-Ends of microtubules at each kinetochore fray, driving chromosome movement
Meiosis 1 is a Reduction Division: Reduces the chromosome number
In plants and animals a diploid cell produces 4 haploid daughter cells
-In animals the daughter cell become eggs or sperm by gametogenesis
Early Prophase 1
-Nuclear envelope begins to break down
-Chromosomes condense
- Spindle apparatus begins to form
-The homolog pairs come together in a pairing process call synapsis
-Result is called a bivalent or tetrad, consisting of 2 homologs
Late Prophase 1: The 2 homologs become attached to microtubules from opposite poles
Homologous begin to separate
-Remain attached at many points called chiasmata
-Exchange or crossing over between homologous non sister chromatids occurs
-Produces chromosomes with a combination of maternal and paternal alleles
Metaphase 1, Anaphase 1, and Telophase 1
Metaphase 1
-The paired homologs line up at the metaphase plate
-Random alignment
Anaphase 1
-Paired homologs separate and migrate to opposite ends
Telophase 1
-Homologs finish migrating to the poles of the cell
-The the cell decides in the process of cytokinesis
The Phase of Meiosis 2: No chromosomes
The Phase of Meiosis 2
Anaphase 2
-Sister chromatids separate
-Resulting daughter chromosomes begin moving to opposite side of the cell
Telophase 2
-Chromosomes arrive at opposite sides of the cell
Meiosis Promotes Genetic Variation: Meiosis results in 4 gametes with a chromosomes composition different from each other and the parent cells
- Crossing over
- Independent Assortment
- Random Feralization
Crossing over
Produces
-New combinations of alleles
-On the same chromosome
-Different in each gamete
Promotes genetic recombination: the creatin of new combination alleles
Independent Assortment
Random separation of homologous chromosomes during meiosis
-Results in variety of combination of maternal and parental chromosomes
-Also promotes genetic recombination
Random Fertilization
-Crossing over and independent assortment assure each gamete is unique
-Random union of gametes ensures that offspring will be genetically different from the parent
-Even if self fertilization occurs
-VS outcrossing gametes from different individuals
Meiosis Promotes Genetic Variation: The essence of evolution via natural selection
-The changes in chromosomes produced by meiosis and fertilization are significant
-Natural selection can only act on variation that exist in a population
-Better genes - better fitness( fit to the environment)
What happens with things go wrong in Meiosis? Errors are common
EX. More than 1/3 of conceptions are spontaneously terminated because of problems
EX. One in every 691 live births in the USA have down syndrome
- Extra copy of chromosome 21
-Called trisomy 21
How do mistakes in Meiosis occur?
If both homologs pair (anaphase 1) or both sister chromatids (anaphase 2) move to the same pole if the parent cell the products will be abnormal»> nondisjunction event , can lead to polyploidy
Errors are random… but there are more in females WHY? : Egg development or oogenesis, in humans
-Primary oocytes ( diploid precursors eggs) enter meiosis 1 during female embryonic development
-Arrest in prophase 1 until sexual maturity is reached
-Don’t complete meiosis until ovulation, years later
-This mean for some oocytes there is up to 50 years wait for meiosis to reach completion
Why does Meiosis Exist? The Paradox of Sex
-Sexual reproduction is relatively uncommon among organisms
-Most organisms undergo asexual reproduction
-Can produce twice as many offspring in the same amount of time» The 2 fold cost of males
Asexual Reproduction: Advantages
-No need for a mate
-Takes less time and energy
-Reliable= fewer steps
-Produces large number of offspring very quickly
-In stable environments with very little change, well adapted organisms can spread and colonize quickly
-Tends to require less parental care
Asexual Reproduction: Disadvantages
-Very little genetic variation in population
-Harmful mutation in parent will be passed on to all offspring
-Entire population of genetically-identical organisms can go extinct if there is a change in the environment
The Purifying Selection Hypothesis 1
In asexual reproduction a damage gene( deleterious) will be inherited by all that individuals offspring
-Sexually reproducing individuals are likely to have offspring that lack deleterious alleles present in the parent
-Natural selection against deleterious alleles is called purifying selection
Test: Compare the same genes in 2 species of daphnia, one who reproduces sexually and one how reproduces asexually
Individual’s in the asexually reproducing species contained many more deleterious alleles
The Changing-Enviornment Hypothesis 2
Offspring produce from sexual reproduction are genetically different from their parents
-They are more likely to survive and produce offspring in the environment changes
-All of the asexually produced offspring are likely to be susceptible to that new strain
Test: Compare rate of sexual vs. asexual reproduction in changing vs non changing environment
Roundworm can reproduce via selfing or outcrossing» absence of pathogen»low rate of outcrossing
Roundworm can reproduce via selfing or outcrossing»evolving pathogens»high rate of outcrossing
Why wasn’t sexual reproduction eliminated? Evolutionary Benefits
- Offspring will not always inherit deleterious alleles
- With variation some may resist rapidly evolving pathogens( or other environmental challenges)
Intro to Mendel and the Gene
Gregor Mendel: Rules of inheritance through a series of experiment of peas
-Chromosomal Theory of Inheritance: Sutton and Boveri
-How genetic info is transmitted from 1 generation to the next and linked inheritance to meiosis
-Asserted that genes are located on chromosomes
In Mendels Time: Interest in selective breeding: Question: What are the oatterne of transmission of traits from parents to offspring?
2 Hypotheses had been formulated:
1. Blending inheritance: Parental traits blend so that their offspring have intermediate traits
2. Inheritance of acquired characteristics: Parental traits are modified through use and then passed on (Lamarck)
First Model Organism in Genetics: Garden Pea
-Practical and had polymorphic traits: A trait that appears commonly in 2 or more different forms
-An individuals observable features comprise its phenotype
-Conclusions drawn from them can be applied to other species
Mendel could control mating: Peas normally self-fertilize but Mendel prevented this and used pollen from one flower to fertilize another, called a cross or cross pollination
- Self Fertilization
Female organ receives pollen and male organs produce pollen grains which produce sperm cells - Cross Fertilization
Collect pollen from another flower and then transfer it to the female part organ of another flower whose male organs have to be removed
Mendel’s Experiments with one trait: The Monohybrid Cross
Mating parents with 2 different phenotypes for a single trait
-The adults: (P) Parental Generation
F1 Generation(first filial): Next generation and so on…F2, F3
Dominant & Recessive Traits
Mendel called
-The genetic determinate for wrinkled seed recessive
-The determinant for round seed dominant
-Repeated these experiments with each of the other traits
-In each case the dominant trait was present in a 3:1 ratio over the recessive trait in the F2 generation
-Reciprocal crosses showed gender has no influences
Mendel proposed the hypothesis of: Particulate Inheritance
-Hereditary determinates do not blend or change through use
-They act as discrete, unchanging particles: we refer to gene as the discrete” particles” or units of inheritance
Mendel’s Principle of Segregation
-The 2 members of each gene pair much segregate( anaphase 1)
-They separate into different gamete cells during the formation of eggs and sperm in the parents
Genes, Alleles and Genotypes
Hereditary determinants for a trait called genes
-Each individual has 2 versions of each gene= alleles
-The combination of alleles found in an individual is called its genotype
-An individuals genotype has a profound effect on its phenotype ( observable features)
Mendel’s Experiment with 2 traits: The Diybrid Cross
- Independent assortment: Alleles of different gene are transmitted independently
2.Dependent assortment: The transmission of one allele depends on the transmission of another
The Dihybrid Cross
Mendel’s results supported independent assortment
-The Punnett square from a dihybrid cross predicts
-9 different offspring genotypes( not 3)
-4 possible phenotypes (not 2) should be present in a ratio 0f 9:3:3:1 (not 3:1)
Mendel used testcrosses to further confirm the principle of independent assortment
-Using the results of a cross to successfully determine the unknown genotype of one parent
Mendel’s Principle of Independent Assortment
Different genes assort independently
-On different chromosomes
-Line up in a random orientation in metaphase 1
Testing the Chromosome Theory
Thomas Hunt Morgan adopted fruit flies as a model organism for genetics
-Wild type is the most common phenotype for each trait
-Other phenotypes arise by mutation
-Mutants are individuals with traits caused by mutations
The white eyed mutant: Morgan did the reciprocal cross
-Suggest a relationship between gender and inheritance of eye color
-Genes being on either sex chromosome is sex linked( X or Y)
-Morgan proposed that the gene for white eye color x-linked
What happens when genes are linked? Mostly, inherited together
Linkage is the tendency of genes to be inherited together because they are on the same chromosome
-Linked genes are inherits together unless crossing over occurs
-When crossing over takes place genetic recombination occurs
-Individuals can be referred to as recombinants
What happens when genes are linked? Depends on crossing over
Genes are more likely to cross over together when they are far apart
-% of recombinants can be used to estimate the relative distance between genes
-Frequency of crossing over together can be used to create a genetic map- a diagram showing the relative positions of genes
How many alleles can a gene have? Dozens
->2 alleles= multiple allelism
-May have dozens of single gene
-EX) Humans have 3 common alleles for a ABO blood types
-IA, IB, I
-Each codes for a versions of an enzyme that adds polysaccharides to the membrane of red blood cells
Are alleles always dominant of recessive? No codominance
-Alleles of genes are not always dominant or recessive
-Some alleles display codominance
-Neither is dominant of recessive
-Heterozygotes display the phenotype of both alleles
EX) ABO blood types
EX) Roan cattle
Are alleles always dominant of recessive? No, incomplete dominance
Incomplete dominance heterozygotes have an intermediate phenotype
EX) pure line plants with red flowers (RR) crossed to pure line plants with white flower(rr)
-Mendel would predict heterozygotes offspring with red flowers (Rr)
-However, Rr offspring have pink flowers
Does each gene affect just 1 trait? Not necessarily
-Some genes influence many traits= pleiotropic genes
-EX) Marfan syndrome
-Just a single gene is involved
-Mutations in the gene lead to a wide array of phenotypes
Are all traits determined by a gene? No, mot influenced by the environment too
The combines effect is referred to as gene- environment interaction
EX) Phenylketonuria, recessive for a gene that codes for an enzyme to break down phenylalanine to tyrosine» produces a serve mental handicap
-Individuals placed on a low-phenylalanine diet develop normally
Are all traits determined by a gene? No, several genes can affect one trait
The expression of many genes depends on the presence or absence of other genes: gene-gene interaction
-2+ genes control a single trait
EX) Comb shape in chickens
-R and P genes
-The R allele is expressed differently depending on which allele of P is present
Can Mendel’s Principles explain traits that don’t fall into distinct categories? Yes and no
Mendel worked with discrete traits
-Traits that vary continuously are called quantitative traits
EX) height or skin color
-Form a bell shaped curve
-Polygenic inheritance- many different genes adds a small amount to the trait
Applying Mendel’s Rules to Human Inheritance
-Mode of transmission describes a trait as autosomal or sex linked and the type of dominance
-Pedigrees can determine the mode of transmission for a given trait
-Dominant or recessive allele
-On a sex chromosome or autosome
What are genes made of?
Researchers knew that chromosomes were comprise of DNA and protein
-Most thought genes were made of proteins. Why?
-More complex
-More variable
(DNA is comprise of only 4 different nucleotides)
The Hershey-Chase experiment
How does the T2 virus infect the bacterium Escherichia Coli?
-This virus injects its genes into the cell
-These genes direct production of new virus particles
Hershey-Chase grew the virus in the presence of one of two radio activity isotopes
-32P, incorporated into DNA
-35S, incorporated into proteins
-Labeled viruses were used to infect E. coli cells
-Only the radioactive DNA was found inside the cells
-Therefore, genes must be composed of DNA
Hypothesis for DNA Replication: Semiconservative: parental strands separate and each is a template for a new strand
Each daughter has one old and one new strand
Hypothesis for DNA Replication: Conservative: The parental molecules serves as a templates for a entirely new molecule
1 daughter has both old strands, the other has new strand both new strands
Hypothesis for DNA Replication: Dispersive: The parent molecule is cut into sections
Each daughter had old and new DNA interspersed
Only the gene of a virus enter a host cell
- Start of infection
- Production of new virus particles
- End of infection
DNA synthesis: Catalyzed by DNA polymerase
-Several types
-Work only in one direction
-Add deoxyribonucleotides only to the 3 end of a growing DNA chain
-DNA synthesis always proceeds in the 5>3 direction
DNA Replication: Synthesis is Bidirectional
Origin of Replication. Replication bubble has 2 replication forks
How id the Helix opened and stabilized? DNA Helicase
Breaks hydrogen bonds between the 2 DNA strands to separate them
How is the Helix opened and stabilized? Single-strand DNA binding proteins(SSBPs)
Attach to the separated strands to prevent them from closing
How is the Helix opened and stabilized? Topoisomerase
Unwinding the DNA helix creates tension farther down the helix» topoisomerase cuts and rejoins the DNA to relieve this tension
Since DNA strands are antiparallel the synthesis process differs for each strand: TOWARD
The strand that is synthesized TOWARD the replication fork is the leading strand, or continuous strand: synthesized continuously 5>3
Since DNA strands are antiparallel the synthesis process differs for each strand: AWAY
The strand synthesized AWAY from the replication fork is the lagging strand or discontinuous strand: synthetized stepwise 5.3
