Meiosis and Sexual Life Cycles

Chapter 13

A. P. Biology

Mr. Knowles

Liberty Senior High School

Mitosis- Review

Mutations

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.

Leewenhoek’s Early Ideas of Sperm

• 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

Gametes Fuse to Form a Zygote

n

n

2nSyngamy

• During fertilization:

–These gametes, sperm and ovum, fuse, forming a diploid zygote.

• The zygote

–Develops into an adult organism.

Fertilization or Syngamy

Zygote Undergoes Mitosis

Zygote is Diploid (2n)

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.

Interphase I

Prophase I

Synapsis in Prophase I

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.

Crossing Over During Prophase I

Prophase I

• The chiasmata hold the two pairs of sister chromatids together at the ends – Terminal Chiasmata.

• The homologous chromatids have exchanged DNA.

Prophase I

Metaphase I

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.

Metaphase I

Anaphase I

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

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 1

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.

Telophase I

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.

Prophase II

Metaphase II

Metaphase II

Anaphase II

Anaphase II

Telophase II

Telophase II

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

Gametogenesis

Meiotic divisions that result in gametes.

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____

Seminiferous Tubule

Mature Sperm

Spermatids

Primary Spermatocytes

Rat Sperm

Fertilization or Syngamy

Zygote is Diploid (2n)

Cloning to Save an Endangered Species

Enculeated Cow Egg

Main Idea

Frozen Bateng Cells

Bateng Nucleus

Surrogate Cow Cloned

Bateng

Microtubules and Motor Proteins-Spindle Fibers

Cloned Animals as of 4-04

What animal group hasn’t been cloned…yet?

Primates

Why?

Can haploid cells develop into functioning animals or plants?

Bees Do It!

How to make a bee colony?

Queen, (2n)

Female Worker,

(2n)

Male Drone, (1n)

Meiosis

Fertilized Egg, (2n)

Unfertilized Egg, (1n)

Mitosis w/o Cytokinesis