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
