Genetics Slides for Test 1

Five Critical Experimental Innovations

1. Controlled Crosses Between Plants 2. Pure-Breeding Strains to Begin Experimental Crosses 3. Selection of Single Traits with Dichotomous Phenotypes 4. Quantification of Results 5. Further Experimental Crosses

Consistent Results of These Experiments

1. Dominance of one phenotype over the other in the F1 generation 2. Re-emergence of the recessive phenotype in the F2 generations 3. A ratio of approximately 3:1 (dominant:recessive) among F2 phenotypes

Types of Microtubules in Cells

1. Kinetochore microtubules embed in the kinetochore at the centromere of each chromatid, and are responsible for chromosome movement 2. Polar microtubules extend toward the opposite pole of the centrosome and contribute to cell elongation and cell stability 3. Astral microtubules grow toward the membrane of the cell, and contribute to cell stability

Mendel's Approach Followed the Modern Scientific Method

1. Make initial observations about a phenomenon or process 2. Formulate a testable hypothesis 3. Design a controlled experiment to test the hypothesis 4. Collect data from the experiment 5. Interpret the experimental results, comparing them to those expected under the hypothesis 6. Draw a conclusion and reformulate the hypothesis if necessary

The Central Dogma of Biology

DNA replication----transcription--->RNA----translation--->protein

Independent Assortment of Alleles from the RrGg × RrGg Cross

Mendel predicted that alleles of each locus unite at random to produce the F2 generating • round, yellow R-G- (¾)(¾) = 9/16 • round, green R-gg (¾)(¼) = 3/16 • wrinkled, yellow rrG- (¼)(¾) = 3/16 • wrinkled, green rrgg (¼)(¼) = 1/16

An Aid to Prediction of Gamete Frequency

The forked-line diagram is used to determine gamete genotypes and frequencies

Probability Calculations in Genetics

• A Punnett square can be used to determine the phenotypes expected in a cross, such as a monohybrid or dihybrid cross • The independence of the genes in the cross gives a quicker way to predict this • multiply the probability of the phenotype at one locus (¾ or ¼) by the probability of the phenotype at the second locus (also ¾ or ¼), third locus, fourth locus, etc. • This can be applied to any number of genes in a cross

Prediction of the Results of Genetic Crosses

• A Punnett square is used to illustrate the random union of all types of gametes produced by the parents in a cross • In the case of the dihybrid crosses performed by Mendel, the Punnett square illustrates the observed phenotypic ratio in the F2 • 9/16 have both dominant traits • 3/16 have one dominant and one recessive trait • 3/16 have one recessive and one dominant trait • 1/16 have both recessive traits

Construction of a Binomial Expansion Formula

• A binomial expansion contains two variables; p, the frequency of one outcome, and q, the frequency of the alternative outcome (p and q may or may not be equal, depending on the type of outcome) • (p + q) = 1, because there are only two outcomes • We expand the equation by the power of n, where n = the number of successive events: (p + q)^n

Eukaryotic Chromosomes Are Organized as Chromatin

• A eukaryotic chromosome has one DNA double helix, associated with a diverse array of proteins • The DNA and associated proteins of a chromosome are called chromatin • Proteins that organize chromosomes are essential and provide a mechanism for condensation, segregation, and organization of chromosomes

Dihybrid and Trihybrid Crosses Reveal the Independent Assortment of Alleles

• All seven traits Mendel studied using monohybrid crosses showed the same pattern of inheritance, which is explained by the law of segregation • Mendel also studied the inheritance of two or more traits simultaneously • Led to Mendel's second law, the law of independent assortment

Hypothesis Testing by Test-Cross Analysis

• Based on his segregation hypothesis, Mendel posited that half of the gametes of heterozygous F1 individuals would carry the dominant allele and half the recessive • He tested this by crossing suspected heterozygous individuals with homozygous recessive individuals from a pure-breeding stock (i.e., a test cross) • If hypothesis supported, he predicted that 50% of offspring will have the dominant trait and 50% the recessive

Mendel's First Law

• Based on his theory of particulate inheritance, Mendel formulated the law of segregation (Mendel's first law) • The hypothesis describes the particulate nature of inheritance, the separation of alleles into gametes, and the random union of gametes to produce progeny in predictable proportions

Metaphase Chromosomes

• By the end of prometaphase, kinetochore microtubules are bound to each kinetochore • Metaphase chromosomes are 10,000-fold condensed compared to the onset of prophase; these chromosomes are pulled toward each centrosome by the kinetochore microtubules • The opposing forces align the chromosomes along the metaphase plate

Stages of the Cell Cycle

• Cell division is regulated by control of the cell cycle, a cycle of DNA replication and division • Cell cycles of all eukaryotes are similar • The two principal phases of the cell cycle are M phase, the short time during which the cells divide and a longer interphase, the time between M phases

Chromosomes During Mitosis

• Cells at the beginning and the end of mitosis are diploid (2n) • Progressive condensation of chromosomes begins in prophase and reaches a maximum in metaphase • Centromeres, specialized sequences where sister chromatids are joined together, become visible in prophase; centromeres bind protein complexes called kinetochores

Metaphase I

• Chiasmata between homologs are dissolved, completing crossing over • Homologs align on opposite sides of the metaphase plate • Kinetochore microtubules from opposite poles attach to both sister chromatids of a different homolog

Higher-Order Chromatin Structure

• Chromatin exists in a 30-nm fiber or more condensed state during interphase and becomes maximally condensed during metaphase of mitosis • Interphase chromosomes have variably sized loops of 30-nm fibers that form a 300-nm fiber • Chromosome shape depends on the chromosome scaffold, composed of filamentous, non-histone proteins

Higher-Order Chromatin Condensation

• Chromatin loops of 20 to 100 kb are anchored to the chromosome scaffold by nonhistone proteins at sites called MARs (matrix attachment regions) • The radial loop-scaffold model suggests that the loops gather into "rosettes" and are further compressed by non-histone proteins • Metaphase chromatin is compacted 250-fold compared to the 300-nm fiber

Roles of Higher-Order Chromatin Condensation

• Chromosome compaction allows for efficient separation of chromosomes during mitosis • Humans: ~6,000,000,000 base pairs among 46 chromosomes; ~6 ft of DNA per nucleus • The chromatin loops formed during condensation play a role in the regulation of gene expression • Active transcription takes place in chromatin loops • Transcription increases with distance from MARs • Larger loops more transcription than smaller loops

Prophase I: Leptotene and Zygotene

• Chromosome condensation begins in leptotene stage • Meiotic spindle forms as microtubules extend out from centrosomes • Nuclear envelope disintegrates during zygotene • Homologous chromosomes undergo synapsis (alignment) • Synaptonemal complex is formed between homologs • Synaptonemal complex: a tri-layer, protein bridge structure that tightly binds non-sister chromatids of homologous chromosomes • Non-sister chromatids: chromatids belonging to different members of a homologous pair

Prophase I: Pachytene

• Chromosome condensation continues in pachytene • Paired homologs are called tetrads due to the four visible chromatids • Recombination nodules can be seen at intervals in the synaptonemal complex • Aggregates of enzymes and proteins needed for crossing over between homologs

Prophase I: Diplotene

• Chromosomes continue to condense in diplotene, and the synaptonemal complex begins to dissolve • Homologs pull apart slightly, revealing chiasmata,at locations where crossing over has occurred • Cohesin protein is present between sister chromatids, to resist the pulling forces of kinetochore microtubules

Diploid/haploid

• Diploid - two sets of chromosomes (2n) • Diploid number is the total number of chromosomes (humans: 2n = 46) • Haploid - one set of chromosomes (n) • Haploid number is the number of chromosomes in one set, which is also the number of pairs (humans: n = 23)

Interphase

• During the Gap 1 (G1) phase of interphase, all proteins needed for normal cell function are transcribed and translated; the duration of G1 varies • DNA is replicated during S phase or synthesis phase, which follows G1 • A small number of cells enter G0 after G1; cells in G0 never progress through the cell cycle • During S phase, DNA replication results in doubling of the DNA in each nucleus • Two sister chromatids are produced for each chromosome • The completion of S phase leads into G2 or Gap 2 phase, during which the cells prepare for division

Chromatin Composition

• Each chromosome is approximately half DNA and half protein • About half of the proteins are histone proteins, small basic proteins that tightly bind DNA • The remaining proteins, the non-histone proteins, are very diverse and perform a variety of tasks in the nucleus

Selection of Single Traits with Dichotomous Phenotypes

• Each of the traits Mendel chose had just two possible phenotypes, or dichotomous forms • Easily distinguished from one another with no intermediate phenotypes • Early in his experiments, he worked with an additional trait, but discontinued it when he noticed a connection with another trait

Consistent Inheritance Patterns across All Traits Studied by Mendel

• F1 plants are crossed or allowed to self-fertilize in a monohybrid cross • Cross in which the two individuals are both heterozygous for a gene (G/g × G/g) • A 3:1 phenotypic ratio is predicted for the F2 produced by a monohybrid cross • A 1:2:1 genotypic ratio is also predicted (¼ G/G, ½ G/g, ¼ g/g)

Example of Conditional Probability

• For cross Gg × Gg, what is the probability that the yellow-seeded progeny are heterozygous? • Yellow-seeded offspring make up ¾ of the offspring, with two possible genotypes: GG and Gg • As the yellow-seeded offspring cannot be gg, there is a 2/3 chance they are Gg and a 1/3 chance they are GG

Binomial Expansion Formula — Example

• For families with three children, predict the proportions with each possible combination of boys and girls • p = probability of a boy = 0.5; q = probability of a girl = 0.5 • Binomial expansion: (p + q)^3= p^3 + 3p^2q + 3pq^2+ q^3 • p^3 = 0.125 (3 boys); 3p^2q = 0.375 (2 boys, 1 girl); 3pq^2= 0.375 (1 boy, 2 girls); q^3= 0.125 (3 girls)

Cell Cycle Checkpoints

• Genetically controlled signals drive the cell cycle • Cell cycle checkpoints are monitored by protein interactions for readiness to progress to the next stage • One common mechanism consists of a protein kinase joined with a cyclin protein

Anaphase I

• Homologs separate and are pulled to opposite poles of the cell • Sister chromatids firmly attached by cohesin

The Sum Rule

• If more than one outcome will satisfy the conditions of the probability question, the individual probabilities are summed to find the joint probability of occurrence of any two or more equivalent events • Also called the addition rule • Example: rolling one die and getting either a 1 or 6 • P = (1/6)+(1/6) = 1/3

The Product Rule

• If two or more events are independent of one another, the likelihood of their simultaneous or consecutive occurrence is the product of their individual probabilities • Also called the multiplication rule • Example: rolling two die and getting two 6s • P = (1/6)*(1/6) = 1/36

Application of Binomial Expansion to Progeny Phenotypes

• In a self-fertilized Gg pea plant, give the proportion of yellow and green peas in pods with six peas each • p = probability of yellow peas = 0.75; q = probability of green peas = 0.25

Segregation

• In an organism that is genotype Aa, the homologs bearing A and a separate from one another during anaphase I • At the end of meiosis, two gametes have the A allele and two have a • This generates the 1:1 ratio predicted by the law of segregation

Cytokinesis

• In animal cells, a contractile ring of actin creates a cleavage furrow around the circumference of the cell; this pinches the cell in two • In plants, a new cell wall is constructed along the cellular midline • In both, cytokinesis divided the cytoplasm and organelles between the daughter cells

Chromosome Distribution

• In animal cells, two centrosomes appear, which migrate to form the opposite poles of the dividing cell • Centrosomes are the source of spindle fiber microtubules; microtubules have a minus (-) end at the centrosome and a plus (+) end that grows away from the centrosome • The spindle fibers emanate from the centrosomes in a pattern called the aster

Homozygous and Heterozygous Individuals

• In pure-breeding individuals, like Mendel's parent plants, the two alleles for a trait are identical • These are called homozygous (AA & aa) • F1 plants had different alleles and are called heterozygous (Aa)

Completion of Cell Division

• In telophase, nuclear membranes reassemble around the chromosomes at each pole • Decondensation returns chromosomes to their diffuse interphase state • Two identical nuclei occupy the elongated cell, which will divide into two daughter cells by cytokinesis

Independent Assortment

• Independent assortment of alleles is illustrated by behavior of two pairs of homologs during meiosis • For organism with genotype AaBb, two equally likely arrangements of paired homologs can occur • One yields gametes AB and ab whereas the other produces gametes Ab and aB; • These four gamete combinations occur with equal likelihood

Prophase I: Diakinesis

• Kinetochore microtubules move synapsed chromosome pairs toward the metaphase plate • Homologs align side by side

Substages of M Phase

• M phase is divided into • Prophase • Prometaphase • Metaphase • Anaphase • Telophase • M phase accomplishes karyokinesis, partitioning of DNA into daughter cell nuclei, and cytokinesis, the partitioning of the cytoplasm

Multicellular Eukaryotes Mainly Reproduce Sexually

• Males and females have distinct reproductive tissues and structures • Mating requires the production of haploid gametes from both parents • The union of haploid gametes produces diploid offspring

Meiosis vs. Mitosis

• Many features of meiosis are similar or identical to mitosis (e.g., interphase) • Meiosis is distinguished from mitosis on the basis of events during M phase and the end-production of four haploid gametes • Meiotic interphase is followed by two division stages called meiosis I and meiosis II with no DNA replication between them

Stages of Meiosis I

• Meiosis I is divided into prophase I, metaphase I, anaphase I, and telophase I • Pairing and recombination of homologs takes place in prophase I (the longest phase) • Prophase I is subdivided into five stages: leptotene, zygotene, pachytene, diplotene, and diakinesis

Meiosis I and II

• Meiosis I: pairs of homologous chromosomes (homologs), each composed of two sister chromatid, separate from one another • Meiosis II: sister chromatids separate from one another to produce four haploid gametes, each with one chromosome from the original diploid pair

Meiosis II

• Meiosis II divides each haploid daughter cell into two haploid cells by separating sister chromatids from one another • The process is similar to mitosis in a haploid cell • End result: four genetically distinct haploid cells, each with one chromosome of a homologous pair

Quantification of Results

• Mendel counted the number of progeny plants of each type and counted offspring from each cross • He identified patterns in his results, such as the consistent ratios between phenotypes • The ratios he detected are the foundation for his laws of heredity

Gregor Mendel Discovered the Basic Principles of Genetic Transmission

• Mendel entered the priesthood as a route to higher education • He studied the natural sciences as preparation to become a teacher, but he did not complete his teaching examinations • He decided to pursue his interest in natural science by studying heredity in peas

Results of Self-Fertilization Experiments Agreed with Predictions

• Mendel expected that homozygous F2 plants should produce progeny with the dominant phenotype only • He expected that heterozygous F2 plants would generate a 3:1 ratio of dominant:recessive phenotype among their progeny • These predictions were confirmed by experimental results

Replicate, Reciprocal, and Test Crosses

• Mendel made many replicate crosses, producing hundreds or thousands of progeny, by repeating each cross several times • He performed reciprocal crosses, in which the same genotypes are crossed, but the sexes of the parents are reversed • He also performed test crosses

Mendel's Approach

• Mendel obtained 34 different varieties of peas from local suppliers and examined the characteristics of each • He identified 14 strains representing seven specific traits each with two forms (phenotypes) that could be easily distinguished • He worked with these strains for 5 years, determining how each character was inherited

Mendel's Work Was Not Appreciated at First

• Mendel published his findings in 1866 • Scientists of the day did not appreciate the significance of Mendel's experiments • In 1900, his work was "rediscovered" and a revolution in biology was launched

Probability Theory Predicts Mendelian Ratios

• Mendel recognized that chance is the principle underlying the segregation of alleles for a given gene and the independent assortment of alleles of genes at different loci • Four rules of probability theory describe and predict the outcome of genetic events

Pure-Breeding Strains to Begin Experimental Crosses

• Mendel took 2 years prior to beginning his experiments to establish pure-breeding (or true-breeding) strains • Strains that consistently produce the same phenotype • Accomplished by inbreeding • Each experiment began with crosses between two pure-breeding parental generation plants (P generation) that produced offspring called F1(first filial generation)

Monohybrid Crosses Reveal the Segregation of Alleles

• Mendel's carefully planned and executed experiments and quantitative analysis disproved the blending theory of heredity and produced a new theory • All traits Mendel studied gave consistent results • Pure-breeding parental strains were cross-fertilized to produce an F1 generation; these were self-fertilized or cross-fertilized to produce F2 • All of the F1 generation had the same phenotype

Mendel's Modern Experimental Approach

• Mendel's experimental approach was in part influenced by his training in physics and math • His success stemmed from counting the individuals with each trait in his experiments • Quantitative rather than qualitative • He chose an organism, the pea, that was easy to work with and had many available varieties

The Blending Theory of Inheritance

• Mendel's experiments tested the blending theory of heredity • Blending theory viewed traits in offspring as a mixture of parental traits (intermediate) • Under this theory, a black cat and a white cat would produce gray kittens, and the black and white traits would never reappear if the gray kittens were crossed to each other

Hypothesis Testing by F2 Self-Fertilization

• Mendel's hypothesis predicts that F2 plants with the dominant phenotype can be homozygous or heterozygous • The heterozygous state (2/3) is twice as likely as the homozygous state (1/3) • He used a self-fertilization experiment to test the predictions of the hypothesis

Evidence of Particulate Inheritance and Rejection of the Blending Theory

• Mendel's results rejected the blending theory of heredity • Phenotypes not intermediate (F1 all dominant phenotype) • Recessive phenotype reappears in F2 • Mendel proposed the theory of particulate inheritance • Posits that plants carry two discrete hereditary units for each trait (i.e., particles) • One particle received via the egg and the other via pollen • Parents pass one of their two particles to offspring

Mitosis & the Cell Cycle

• Mitosis is the process of cell division that produces two genetically identical daughter cells from one original parental cell • Cell division is regulated by control of the cell cycle, a cycle of DNA replication and division • The two principal phases of the cell cycle are M phase, the short time during which the cells divide and a longer interphase, the time between M phases

Mitosis Divides Somatic Cells

• Mitosis is the process of cell division that produces two genetically identical daughter cells from one original parental cell • It is precisely controlled to prevent an excess or insufficient number of cells

Cell Division

• Mitosis produces two identical daughter cells that are exact replicas of the parental cell • Occurs in somatic cells (non-reproductive), which are most of the cells in the body • Meiosis produces daughter cells (gametes) that have half the number of chromosomes as the parental cell • Occurs in germ-line or reproductive cells, which are in gonads • Gametes are not identical to one another

Mitosis Produces Identical Daughter Cells

• Mitosis separates replicated copies of sister chromatids into identical nuclei, forming two genetically identical daughter cells • The diploid number of chromosomes (2n) is maintained throughout the cell cycle

Mitosis & the Cell Cycle

• Mitosis: cell division that produces two genetically identical daughter cells from one parental cell • Cell division is regulated by the cell cycle • Cell cycle has two principal phases: M phase and Interphase

Cell Cycle Mutations and Cancer

• Normal cells proliferate only when needed, in response to signals from growth factors • Also responsive to neighboring cells; growth is moderated to serve the best interests of the organism • Cancer is characterized by out-of-control proliferation of cells that invade and displace normal cells • Due to loss of control over cell cycle

Controlled Crosses Between Plants

• Pea plants are capable of self-fertilization and cross-fertilization, which can be done artificially • Self-fertilization occurs naturally • Cross-fertilization involves removing the anthers from a flower and introducing pollen of the desired type with a small brush

Meiosis

• Produces gametes that have half the number of chromosomes as the original cell (i.e., they're haploid) • Gametes are produced from germ-line or reproductive cells. • Cells located in gonads • Gametes are not identical to one another

Mitosis

• Produces two identical daughter cells that are exact replicas of the parental cell • Occurs in somatic cells (non-reproductive), which are most of the cells in the body • Chromosomes are usually present in pairs

Cyclins and Cdks

• Protein kinases are activated by association with cyclins and so are called cyclin-dependent kinases (Cdks) • Protein kinases present continuously but cyclins cycle • Multiple cyclin and Cdks form a variety of protein complexes • Cdk/cyclin complexes catalyze phosphorylation of target proteins • Relative abundance of various complexes at different checkpoints signals transition from one stage to the next

Further Experimental Crosses

• Pure-breeding parental plants, when crossed, produce F1 that all have the same phenotype • F1 offspring that are crossed to one another produce the next generation of offspring, the second filial generation (F2) • These can be crossed to produce the third filial generation (F3) and so on, as needed

Meiosis Produces Gametes for Sexual Reproduction

• Reproduction can be divided into two broad categories: • Asexual reproduction: reproduce without mating and have genetically identical offspring (clones) • Sexual reproduction: gametes are produced, unite during fertilization, and offspring share 50% with each parent

The Mechanistic Basis of Mendelian Ratios

• Separation of homologs and sister chromatid in meiosis constitutes the mechanical basis of Mendel's laws • Law of Segregation • For Aa, 1:1 ratio of A to a • Law of Independent Assortment • For AaBb, AB = aB = Ab = aB

Chromosomes

• Sex chromosomes determine sex and differ between genders • Autosomes: chromosomes that are not sex chromosomes, which are most of them

Anaphase

• Sister chromatids separate at anaphase and begin to move toward opposite poles in the cell • In anaphase A the sister chromatids separate due to the enzyme separase cleaving Scc1, the central component of cohesin • The separation of sister chromatids is called chromosome disjunction • During anaphase B, polar microtubules extend in length • This causes the cell to take on an elongated shape • The altered shape facilitates cytokinesis at the end of telophase, leading to formation of two daughter cells

Binomial Probability

• Some questions involve predicting the likelihood of a series of events for which there are exactly two possible outcomes • We use binomial probability calculations to answer this type of question • It expands the binomial expression to reflect the number of outcome combinations and the probability of each

Telophase I and Cytokinesis

• Telophase I: nuclear membranes reform around the separated haploid sets of chromosomes • Cytokinesis follows telophase I and divides the cytoplasm to create two haploid cells • Meiosis I is called the reductional division because the ploidy of the daughter cells is half compared to the original diploid parent cell

Higher Levels of Chromatin Compaction

• The 10-nm fiber is not observed under normal cellular conditions • Instead, a 30-nm fiber (6 times more condensed) is observed • The 30-nm fiber forms when the 10-nm fiber coils into a solenoid structure, with 6-8 nucleosomes per turn and histone H1 stabilizing the solenoid

Mendel's Second Law

• The 9:3:3:1 ratios generated in Mendel's dihybrid crosses illustrate Mendel's second law, also known as the law of independent assortment • The law states that during gamete formation the segregation of alleles at one locus is independent of the segregation of alleles at another locus. • Within the 9:3:3:1 ratio, Mendel recognized two 3:1 ratios for each trait

Chromatin Structure

• The DNA wrapped around the core particle (histone octamer) is called core DNA • Electron micrographs of DNA in its least condensed state show a 10-nm fiber, or "beads-on-a-string" morphology-the "beads" are the nucleosomes • The "string" between nucleosomes is linker DNA • Length fairly constant within species but variable among species (range: 13 - 110 bp)

Segregation of Alleles

• The Punnett Square method of diagramming a genetic cross is a simple tool of genetic analysis • The alleles (in gametes) carried by one parent are arranged along the top of the square and those of the other parent, down the side • The results expected from fusion of the gametes are placed within the square

The Cyclin D1 Gene Is a Proto-Oncogene

• The gene cyclin D1 leads to formation of the cyclin D1-Cdk4 complex that stimulates the cell cycle to enter S phase • Cyclin D1 is a proto-oncogene: a gene that when expressed stimulates cell cycle progression • Mutation in proto-oncogene that results in cancer changes designation to oncogene

Alleles

• The hereditary particles referred to in the theory are alleles • Together the two alleles for each trait determine the phenotype of the individual • Mendel used letters as symbols to represent the alleles for each trait

Mendel Compared Predicted Outcomes to Actual Results from Trihybrid Crosses

• The number of gamete genotypes can be expressed as 2n, where n = number of genes • In a trihybrid cross, eight different gametes are possible, each equally likely (1/8) • Using the ¾, ¼ expected phenotype frequencies for each individual trait, an expected phenotypic ratio for F2 progeny can be generated

Conditional Probability

• The product and sum rules are used before a cross is made, in order to predict the likelihood of certain outcomes • Conditional probability involves questions asked after a cross has been made and is applied when information about the outcome modifies the probability calculation

The Retinoblastoma Protein

• The retinoblastoma protein, pRB, is a target of cyclin D1-Cdk4, which is active at the G1-S checkpoint • In normal cells, pRB binds a transcriptional activator, E2F; the resulting complex blocks cell cycle progression (G1 to S) • Cyclin D1-Cdk4 phosphorylates pRB, which frees E2F, which then activates genes needed for S phase

Sister Chromatid Cohesion

• The tension created by the pull of the kinetochore microtubules is balanced by sister chromatid cohesion • The protein cohesin localizes between sister chromatids and holds them together, preventing their premature separation • Cohesin is a 4-subunit protein that coats sister chromatids along their entire length, with the greatest concentration at the centromeres

Dominant and Recessive Traits

• The trait shown by the F1 offspring was called the dominant phenotype (e.g., yellow seeds) • The trait that was not apparent in the F1 was called the recessive phenotype (e.g., green seeds) • When F1 were crossed, 75% of the F2 had the dominant trait, but the recessive trait reappeared in the other 25%

Histones & Nucleosomes

• There are five types of histone proteins: H1, H2A, H2B, H3, and H4; they are highly conserved among eukaryotes • Two molecules each of histones H2A, H2B, H3, and H4 form an octamer • A span of DNA ∼146 bp long wraps around each octamer to form a nucleosome • The nucleosome is the first level of DNA condensation, and compacts the DNA about sevenfold

Meiosis I

• Three hallmark events occur in Meiosis I 1. Homologous chromosome pairing to form a tetrad (4) 2. Crossing over between homologous chromosomes (i.e., recombination) 3. Segregation of homologous chromosomes, which reduces chromosomes to the haploid number

Dihybrid-Cross Analysis of Two Genes

• To study the simultaneous transmission of two traits, Mendel made dihybrid crosses between organisms that differed for two traits • He began each cross with pure-breeding lines (e.g., RRGG and rrgg) and produced F1 that were heterozygous for both traits (e.g., RrGg). • If assortment is random, four gametes should be equally likely in the F1 (e.g., RG, Rg, rG, rg)

Testing Independent Assortment using Trihybrid Crosses

• To test his hypothesis about independent assortment further, Mendel performed trihybridcrosses • Trihybrid cross involved three traits: round vs. wrinkled peas, yellow vs. green peas, and purple vs. white flowers • The cross was: RRGGPP × rrggpp; F1 were RrGgPp

Testing Independent Assortment

• To test his hypothesis about independent assortment, Mendel performed test-cross analysis • He predicted that F1 seeds were dihybrid with genotype RrGr • If correct, then crossing them to a plant with genotype rrgg will yield four offspring phenotypes in equal frequencies

Mutations Related to Cancer Development

• Two kinds of mutations alter cyclin D1-Cdk4 and pRB interactions • First, some mutations increase the number of copies of cyclin D1 • High levels of cyclin D1 interact with Cdk4 to promote uncontrolled entry into S phase, due to constant phosphorylation of pRB • Second, some mutations affect RB1 gene producing a pRB that binds weakly or not at all to E2F • Can cause uncontrolled entry into S phase due to constant availability of E2F, which activates genes needed for progression to S phase • Several types of cancers are associated with RB1 mutations, including retinoblastoma, and bladder, lung, bone, and breast cancers

The RB1 Gene Is a Tumor Suppressor Gene

• Unphosphorylated pRB acts like a brake on the cell cycle, preventing progression to S phase • One of many proteins known as tumor suppressors because of their role in blocking the cell cycle • The gene RB1, which produces pRB, is therefore a tumor suppressor gene