Post on 18-Dec-2015
Comparison of Asexual and Sexual Reproduction
• In asexual reproduction
– One parent produces genetically identical offspring by mitosis
Figure 13.2
Parent
Bud
0.5 mm
• In sexual reproduction:
– Two parents give rise to offspring that have unique combinations of genes inherited from the two parents.
• Somatic (nonreproductive cells) – are diploid. Body cells. Human somatic cells have 46 chromosomes.
• Gametes (reproductive cells) – are haploid. Human sperm and ova have 23 chromosomes.
Types of Cells
• During fertilization:
–These gametes, sperm and ovum, fuse, forming a diploid zygote.
• The zygote
–Develops into an adult organism.
Why is the number of chromosomes in gametes ½ that of somatic cells?
• If not, after 10 generations the 46 chromosomes in human cells would increase to over 46 X 210.
Meiosis
• A process of cell division in which the number of chromosomes in certain cells is halved during gamete formation.
• A reduction division.
Figure 13.5
Key
Haploid (n)
Diploid (2n)
Haploid gametes (n = 23)
Ovum (n)
SpermCell (n)
MEIOSIS FERTILIZATION
Ovary Testis Diploidzygote(2n = 46)
Mitosis anddevelopment
Multicellular diploidadults (2n = 46)
The Human Life Cycle
The Variety of Sexual Life Cycles
• The three main types of sexual life cycles:–Differ in the timing of meiosis and
fertilization
• In animals
– Meiosis occurs during gamete formation
– Gametes are the only haploid cells
Gametes
Figure 13.6 A
Diploidmulticellular
organism
Key
MEIOSIS FERTILIZATION
n
n
n
2n2nZygote
Haploid
Diploid
Mitosis
(a) Animals
MEIOSIS FERTILIZATION
nn
n
nn
2n2n
Haploid multicellularorganism (gametophyte)
Mitosis Mitosis
SporesGametes
Mitosis
Zygote
Diploidmulticellularorganism(sporophyte)
(b) Plants and some algaeFigure 13.6 B
• Plants and some algae:– Exhibit an alternation of generations.– The life cycle includes both diploid and haploid
multicellular stages.
MEIOSIS FERTILIZATION
nn
n
n
n
2n
Haploid multicellularorganism
Mitosis Mitosis
Gametes
Zygote(c) Most fungi and some protistsFigure 13.6 C
• In most fungi and some protists:– Meiosis produces haploid cells that give rise to
a haploid multicellular adult organism– The haploid adult carries out mitosis, producing
cells that will become gametes
Sexual Life Cycle• Reproduction that alternates
between fertilization (a diploid state) and meiosis (a haploid state) – sexual reproduction.
• Sexual reproduction – provides DNA from both parents.
Some Sexual Life Cycles• Unicellular Protists – individual cells
function as gametesundergo meiosis and fuse with others.
• Plants- may produce haploid cells undergo mitosis and produce a multicellular haploid organism.
• Animals- set aside germ cells undergo meiosis and become haploid gametes.
The Stages of Meiosis• An overview of meiosis
Figure 13.7
Interphase
Homologous pairof chromosomesin diploid parent cell
Chromosomesreplicate
Homologous pair of replicated chromosomes
Sisterchromatids Diploid cell with
replicatedchromosomes
1
2
Homologous chromosomes separate
Haploid cells withreplicated chromosomes
Sister chromatids separate
Haploid cells with unreplicated chromosomes
Meiosis I
Meiosis II
Centrosomes(with centriole pairs)
Sisterchromatids
Chiasmata
Spindle
Tetrad
Nuclearenvelope
Chromatin
Centromere(with kinetochore)
Microtubuleattached tokinetochore
Tertads line up
Metaphaseplate
Homologouschromosomesseparate
Sister chromatidsremain attached
Pairs of homologouschromosomes split up
Chromosomes duplicate
Homologous chromosomes(red and blue) pair and exchangesegments; 2n = 6 in this example
INTERPHASE MEIOSIS I: Separates homologous chromosomes
PROPHASE I METAPHASE I ANAPHASE I
Interphase and Meiosis I
Figure 13.8
TELOPHASE I ANDCYTOKINESIS
PROPHASE II METAPHASE II ANAPHASE II TELOPHASE II ANDCYTOKINESIS
MEIOSIS II: Separates sister chromatids
Cleavagefurrow Sister chromatids
separate
Haploid daughter cellsforming
During another round of cell division, the sister chromatids finally separate;four haploid daughter cells result, containing single chromosomes
Two haploid cellsform; chromosomesare still doubleFigure 13.8
Telophase I, Cytokinesis, and Meiosis II
Meiosis• Two nuclear divisions.
• Labeled PMAT I and PMAT II.
• Prophase I is very different in meiosis than in mitosis.
Prophase I• The duplicated homologous chromosomes
condense and the ends of chromatids attach to the nuclear envelope.
• Homologous pairs align next to each other.• The chromatids of one homologue align in
precise register with the chromatids of the other in a process called – synapsis (zipping up a zipper).
Prophase I• The DNA of chromatids unwind and
base pair with the complementary strand on the the other homologue.
• The crossed-over sister chromatids form an X-shaped structure – chiasma.
• DNA from one homologue sister chromatid is exchanged with the other – Crossing Over.
Prophase I
• The chiasmata hold the two pairs of sister chromatids together at the ends – Terminal Chiasmata.
• The homologous chromatids have exchanged DNA.
Metaphase I• Terminal chiasmata are still formed.• The nuclear membrane breaks down and
spindle fibers are forming.• Only one side of each centromere faces
outward toward the growing spindle fiber.• The fibers can attach to kinetochore proteins
on only one side of each centromere.
Metaphase I
• Alignment of each pair on the metaphase plate is random.
• Either homologue may be situated toward a given pole.
Anaphase I• Spindle fibers contract and break the
chiasmata and pull the centromeres toward the poles.
• The entire centromere of the homologue proceeds to one pole. (Different than mitosis).
• Each pole receives a haploid number of chromosomes, one member of each homologous pair.
Anaphase I
• Because the orientation of the homologous chromosomes on the metaphase plate is random, genes on separate chromosomes are inherited independently – Independent Assortment.
Telophase I• Individual chromosomes cluster at the
poles.• Each chromosome is made of two
chromatids that are not identical because of crossing over that occurred in Prophase I.
• Cytokinesis may or may not occur after Telophase I. Nuclei may not reform.
The Second Meiotic Division• No DNA replication before this
division (Different than mitosis).• Undergo a simple mitotic
division with the products from Telophase I.
• Results are four haploid cells.• Nuclei are reorganized.
Figure 13.9
MITOSIS MEIOSIS
Prophase
Duplicated chromosome(two sister chromatids)
Chromosomereplication
Chromosomereplication
Parent cell(before chromosome replication)
Chiasma (site ofcrossing over)
MEIOSIS I
Prophase I
Tetrad formed bysynapsis of homologouschromosomes
Metaphase
Chromosomespositioned at themetaphase plate
Tetradspositioned at themetaphase plate
Metaphase I
Anaphase ITelophase I
Haploidn = 3
MEIOSIS II
Daughtercells of
meiosis I
Homologuesseparateduringanaphase I;sisterchromatidsremain together
Daughter cells of meiosis II
n n n n
Sister chromatids separate during anaphase II
AnaphaseTelophase
Sister chromatidsseparate duringanaphase
2n 2n
Daughter cellsof mitosis
2n = 6
Comparison of Mitosis and Meiosis
A Comparison of Mitosis and MeiosisMeiosis and mitosis can be distinguished from mitosis by three events in Meiosis l:
• Synapsis and crossing over- Homologous chromosomes physically connect and exchange genetic information.
• Tetrads on the metaphase plate-– At metaphase I of meiosis, paired homologous chromosomes (tetrads) are positioned on the
metaphase plates• Separation of homologues:
– At anaphase I of meiosis, homologous pairs move toward opposite poles of the cell– In anaphase II of meiosis, the sister chromatids separate
Concept 13.4: Genetic variation produced in sexual life cycles contributes to evolution
3 Sources of Genetic Variation:1. Reshuffling of genetic material in meiosis-
Crossing Over.2. Homologous pairs of chromosomes are
oriented randomly at metaphase - Independent Assortment.
3. Random Fertilization of Gametes- Will produce a zygote with any of about 64 trillion diploid combinations
Crossing Over• Crossing over:
– Produces recombinant chromosomes that carry genes derived from two different parents
Figure 13.11
Prophase Iof meiosis
Nonsisterchromatids
Tetrad
Chiasma,site ofcrossingover
Metaphase I
Metaphase II
Daughtercells
Recombinantchromosomes
Independent Assortment– Each pair of chromosomes sorts its maternal and paternal homologues
into daughter cells independently of the other pairs
Figure 13.10
Key
Maternal set ofchromosomes
Paternal set ofchromosomes
Possibility 1
Two equally probable arrangements ofchromosomes at
metaphase I
Possibility 2
Metaphase II
Daughtercells
Combination 1 Combination 2 Combination 3 Combination 4
Evolutionary Significance of Genetic Variation Within Populations
• Genetic variation:-Is the raw material for evolution by natural selection
• Mutations:– Are the original source of genetic variation
• Sexual reproduction:– Produces new combinations of variant genes, adding more genetic
diversity
The Fate of Haploid Cells
n
Gametes
nn
sperm ova
Undergo Mitosis Multicellular
Haploid Plants
Haploid Fungi
Oogenesis Spermatogenesis
Oogonium (female germ cell, 2n) Spermatogonium (male germ cell, 2n)
Germ cells committed to
meiosisPrimary Oocyte (2n) Primary Spermatocyte (2n)
First Meiotic Division
Secondary Ooctye (n)
First Polar Body (n)
Secondary Spermatocyte
(n)
Secondary Spermatocyte
(n)Second Meiotic Division
Ovum (n) Second Polar Body (n)
Four Spermatids (n)
Four Spermatozoa (n)
___Before Ovulation____
___After Fertilization____
Cloning to Save an Endangered Species
Enculeated Cow Egg
Main Idea
Frozen Bateng Cells
Bateng Nucleus
Surrogate Cow Cloned
Bateng
What animal group hasn’t been cloned…yet?
Primates
Why?