Outline for today’s lecture (Ch. 13)

• Sexual and asexual life cycles

• Meiosis

• Origins of Genetic Variation

– Independent assortment

– Crossing over (“recombination”)

Heredity

• Transmission of traits between generations

• Molecular basis of heredity is DNA replication

• Gene is a specific segment of DNA

• Physical location on the chromosome is called a genetic LOCUS (plural = “loci”)

– e.g., the “eye-color locus”, Adh locus

Asexual Life Cycles

• Single (diploid) individual is the parent

• Parent passes copies of ALL its genes to its offspring (reproduces “clonally”)

• Various mechanisms

– Mitotic cell division in unicellular Eukaryotes

– Vegetative reproduction, e.g., plant cuttings, hydra budding

– Parthenogenesis

Sexual Life Cycles

• Two (diploid) parents give rise to offspring

• Offspring differ genetically from their parents and their siblings

• GAMETES are haploid reproductive cells that transmit genes across generations

Sexual Life Cycles

• Key Point: Sexual reproduction → Genetic variation

• MOST eukaryotes reproduce sexually at least sometimes

• Most prokaryotes (e.g., bacteria) exchange genes at least occasionally

Sexual Life Cycles – Human Example

• 46 Chromosomes

• 22 Homologous pairs, called “autosomes”

– Same length– Same centromere

position– Same sequence (+/-)– SAME GENES!!

Sexual Life Cycles – Human Example

• One pair of “sex chromosomes”– i.e., “sex-determining gene(s)”

reside on these chromosomes

• Females are XX

• Males are XY

• Only small region of homology (= same genes) between X, Y X Y

Schematic drawing of a chromosome

Diploid cell, n=3 BEFORE DNA replication

• 3 Homologous Pairs– 2 autosomes

– 1 sex chromosome (XX)

• One homologous chromosome from each parent

• DNA content = 2C

• Ploidy = 2n

X

2

1

X

1

2

Diploid cell, n=3, AFTER DNA replication

• 3 Homologous Pairs

• One homologous chromosome from each parent = TWO SISTER CHROMATIDS

• DNA content = 4C

• Ploidy = 2n

XXXX

1111

2222

Sexual Life Cycles - animals

• Free-living stage is diploid

• Gametes formed by meiosis

• Haploid gametes merge genomes to form diploid zygote (“syngamy”)

Sexual Life Cycles - Plants

• Diploid sporophyte forms haploid spores by meiosis

• Spores form gametophyte by mitosis

• Gametophyte forms gametes by mitosis

• Gametes merge to form diploid zygote

Sexual Life Cycles - Fungi

• Free-living, multicellular organism is haploid

• Gametes formed by mitosis

• Gametes merge to form diploid zygote

• Zygote undergoes meiosis to form haploid cells

Meiosis

• RECALL: Function of MITOSIS is to faithfully replicate the parental genome in each daughter cell with no change in information content

• Function of MEIOSIS is to produce haploid cells from diploid cells

• Necessary for the formation of gametes

• Necessary for sexual reproduction

Meiosis – an overview

• Interphase 1 –

– Begin with two homologous chromosomes,

– DNA content = 2C

– Ploidy = 2n (diploid)

Meiosis – an overview

• Interphase 1 –

– Chromosomes replicate

– DNA content = 4C

– Ploidy = 2n

Meiosis – an overview• “Meiosis I”

– Homologous chromosomes separate

– Cell Division #1

– Result is TWO haploid (ploidy = n) cells with TWO SISTER CHROMATIDS of one of the two homologs

Meiosis – an overview

• “Meiosis II”

– Sister chromatids separate– Cell Division # 2

– Result is FOUR haploid daughter cells, each with an unreplicated chromosome (= 1C)

– Half as many chromosomes as the parent cell

Meiosis I – early Prophase I

• Homologous chromosomes pair

• Synaptonemal complex (proteins) attaches homologs

– “synapsis”

• Homologs form tetrad

Tetrad

Chiasmata

Meiosis I – late Prophase I

• Chromosomes cross over, form “chiasmata”

• Exchange of DNA between homologs occurs at chiasma

• Spindles form and attach to kinetochores as in mitosis

Tetrad

ChiasmataSpindle fiber

Meiosis I – Metaphase I

• Chromosomes lined up on metaphase plate in homologous pairs

• Spindles from one pole attach to one chromosome of each pair

• Spindles from the other pole attach to the other chromosome of the pair

Kinetochore

Meiosis I – Anaphase I

• Homologous chromosomes separate and move along spindle fibers toward pole

• Sister chromatids remain attached at centromeres

• Note that recombination has occurred!

Meiosis I – Telophase and cytokinesis

• Homologous chromosomes reach (opposite) poles

• Each pole has complete haploid complement of chromosomes

• Each chromosome consists of two sister chromatids

Meiosis II – Prophase II

• Spindle forms

• Chromosomes move toward metaphase plate

Meiosis II – Metaphase II

• Chromosomes reach metaphase plate, as in mitosis

• Kinetochores of sister chromatids attach to spindle fibers from opposite poles

Meiosis II – Anaphase II

• Centromeres of sister chromatids separate

• Sister chromatids move toward opposite poles

Meiosis II – Telophase and cytokinesis

• Mechanism as before

• Note that now FOUR HAPLOID DAUGHTER CELLS formed from each parent cell

• Note that some chromosomes are recombinant, some are not

Meiosis I - Summary

Chiasma (site of crossing-over)

Tetrad formed by synapsis of homologs

Meiosis I - Summary

Tetrads align at metaphase plate

Meiosis I - Summary

Homologous chromosomesseparate

Sister chromatids remain paired

Meiosis II - Summary

Sister chromatids separate

Haploid daughter cells result

Origins of Genetic Variation

1. Independent Assortment of Chromosomes

• Recombination among chromosomes

2. Crossing over

• Recombination within chromosomes

3. Random fertilization

Independent Assortment of Chromosomes

Independent Assortment of Chromosomes

• Number of possible combinations of chromosomes within a gamete

– Two homologs A, B: Mom = A1B1, Dad = A2B2

• Four combinations: A1B1, A1B2, A2B1, A2B2

– Three homologs: Mom = A1B1C1, Dad = A2B2C2• Eight combinations:

A1B1C1, A1B1C2, A1B2C1, A1B2C2, A2B1C1, A2B2C1, A2B1C2, A2B2C2

– n homologs: 2n combinations

Crossing-over – Recombination within chromosomes

• Averages ≥ 2 per chromosome per meiosis in humans, flies

• If no crossing-over, genes on same chromosomes would always be inherited together

Crossing-over – Recombination within chromosomes

Human genome has ~20K genes. Suppose each gene assorts independently. How many combinations?

Review: Mitosis vs. Meiosis

Event Mitosis Meiosis

DNA Replication InterphaseInterphase I

# Cell Divisions 1 2# Daughter cells 2 4“Ploidy” of daughters 2n (diploid) n (haploid)Synapsis of homologs? No YesCrossing-over No Yes

(recombination)Biological Purpose Duplicate cells Generate

faithfully gametes

Meiosis, Genetic variation, and Evolution

• Role of segregation

• Role of crossing-over

• What about LIMITS to evolution?

– E.g., body size

For Thursday: Introduction to Mendelian Genetics

• Read Chapter 14 through p. 260