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
2
2
X
• DNA content = 2C
1
X
• Ploidy = 2n
1
Diploid cell, n=3, AFTER DNA replication
• 3 Homologous Pairs
• One homologous
chromosome from each
parent = TWO SISTER
CHROMATIDS
22
• DNA content = 4C
22
XX
XX
• Ploidy = 2n
11
11
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
Chiasmata
• Homologous
chromosomes pair
• Synaptonemal complex
(proteins) attaches
homologs
– “synapsis”
• Homologs form tetrad
Tetrad
Meiosis I – late Prophase I
Chiasmata
• Chromosomes cross
over, form “chiasmata”
Spindle fiber
• Exchange of DNA
between homologs
occurs at chiasma
• Spindles form and attach
to kinetochores as in
mitosis
Tetrad
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 chromosomes
separate
Sister chromatids remain
paired
Meiosis II - Summary
Sister chromatids separate
Haploid daughter cells result
Origins of Genetic Variation
1.
Independent Assortment of Chromosomes
•
2.
Crossing over
•
3.
Recombination among chromosomes
Recombination within chromosomes
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
DNA Replication
# Cell Divisions
# Daughter cells
“Ploidy” of daughters
Synapsis of homologs?
Crossing-over
(recombination)
Biological Purpose
Mitosis
Interphase
1
2
2n (diploid)
No
No
Duplicate cells
faithfully
Meiosis
Interphase I
2
4
n (haploid)
Yes
Yes
Generate
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