Chapter 10: Meiosis and Sexual Reproduction (Outline) Reduction in Chromosome Number

Homologous Pairs Genetic Recombination

Crossing-Over Independent Assortment Fertilization

Phases of Meiosis Meiosis I Meiosis II

Meiosis Compared to Mitosis Human Life Cycle

Meiosis: Halving the Chromosome Number

Special type of cell division Used only for sexual reproduction Halves the chromosome number prior to

fertilization Parents diploid (2n) Meiosis produces haploid gametes (1n) Gametes fuse in fertilization to form diploid zygote Becomes the next diploid generation

If gametes were not haploid the number of chromosomes would double itself in each generation

Homologous Pairs ofChromosomes

In diploid body cells, chromosomes occur in pairs

Diploid cells have two of each type

Human cells have 46 chromosomes in 23 homologous pairs

Homologous Chromosomes Paired chromosomes in somatic cells

Similar in size, shape and position of their centromeres

Carry information about the same genetic traits (not always the same information)

When stained, they show similar banding patterns

Homologous Chromosomes

Homologous Pairs ofChromosomes

Homologous chromosomes have genes controlling the same trait at the same position Each gene occurs in duplicate, why?

The variants that exist for a gene are called alleles

An individual may have: Identical alleles for a specific gene on both homologs

(homozygous for the trait), or

A maternal allele that differs from the corresponding paternal allele (heterozygous for the trait)

Overview of Meiosis

Meiosis requires 2 nuclear divisions and produces 4 haploid daughter cells

Cells are diploid at beginning of meiosis Pairs of chromosomes are called homologues Meiosis I

Homologues line up side by side at equator-synapsis

Synapsis results in a bivalent When pairs separate, each daughter cell

receives one member of the pair Cells are now haploid

Overview of Meiosis (cont.) Meiosis II

No replication of DNA occurs in this division, why? Centromeres divide & sister chromatids migrate to

opposite poles to become individual chromosomes Each of the four daughter cells produced has the

haploid chromosome number and each chromosome is composed of one chromatid

In plants, daughter cells are haploid spores that germinate to haploid generation; gametes produced by mitosis

In animals, daughter cells are gametes (i.e. sperm or eggs)

Meiosis Overview

Genetic Variation

Meiosis helps ensure genetic recombination

In a changing environment, asexual reproduction might be disadvantageous

Sexual reproduction might give offspring better chance of survival

Meiosis brings about genetic variation in two key ways:

Crossing-over

Independent assortment

Crossing-Over Exchange of genetic material between nonsister

chromatids of a bivalent during meiosis I

At synapsis, a nucleoprotein lattice appears between homologues

Holds homologues together and aligns DNA of nonsister chromatids

Allows crossing-over to occur

Homologues are held together by chiasmata

Homologues then separate and are distributed to different daughter cells

Synapsis and crossing over

Independent Assortment

Independent assortment: When homologues align at the metaphase plate:

They separate in a random manner The maternal or paternal homologue may be

oriented toward either pole of mother cell Causes random mixing of blocks of alleles into

gametes

Fertilization

Gametes produced by one person are genetically different those produced by another person

When gametes fuse at fertilization: Chromosomes donated by the parents are combined In humans, (223)2 = 70,368,744,000,000 chromosomally

different zygotes are possible

If crossing-over occurs only once (423)2, or 4,951,760,200,000,000,000,000,000,000

genetically different zygotes are possible Remember, crossing-over can occur several

times in each chromosome!

Significance of Genetic Variation

Asexual reproduction produces genetically identical clones

Sexual reproduction produces genetic variety Asexual reproduction is advantageous when

environment is stable However, if environment changes, genetic

variability introduced by sexual reproduction may be advantageous

Phases of Meiosis I

Prophase I

Each chromosome is internally duplicated (consists of two identical sister chromatids)

Homologous chromosomes (maternal homologue and paternal homologue) align side by side (synapsis)

Synapsis results in association of four chromatids (a tetrad)

Paired homologous chromosomes exchange genetic material (crossing-over)

Phases of Meiosis I

Metaphase I

Homologous pairs (bivalents) or tetrads arranged onto the metaphase plate independently

The centrioles are at opposite poles of the cell

Spindle fibers from one pole of the cell attach to one duplicated chromosome of each pair (seen as sister chromatids)

Phases of Meiosis I

Anaphase I

Synapsis breaks up

Homologous chromosomes separate from one another and move towards opposite poles

Each pole randomly receives a maternal or paternal chromosome from each homologous pair

Each is still an internally duplicate chromosome with two chromatids

Phases of Meiosis I

Telophase I Daughter cells have one internally duplicate

chromosome from each homologous pair

One (internally duplicate) chromosome of each type (1n, haploid)

Nuclear envelope may reorganize, and cytokinesis may take place

Interkinesis Similar to mitotic interphase but shorter

No replication of DNA, why?

Meiosis I

Phases of Meiosis II:Similar to Mitosis

Prophase II – Chromosomes condense Metaphase II – chromosomes align at

metaphase plate Anaphase II

Centromere dissolves Sister chromatids separate and move to opposite

poles (daughter chromosome) Telophase II and Cytokinesis II

Four haploid cells All genetically unique

Meiosis II

Overview of Meiosis I & II

Contrasting Mitosis and Meiosis

Several fundamental differences between the two processes include: Meiosis requires two nuclear divisions, but mitosis

requires one nuclear division

Meiosis produces four daughter cells, but mitosis results in two daughter cells following cytokinesis

In meiosis, daughter cells are haploid, whereas mitosis preserves chromosome number

In meiosis, daughter cells are genetically different from parent and each other, but mitosis results in daughter cells that are genetically identical to parent and to each other

Meiosis vs. Mitosis Occurrence

Meiosis occurs only at certain times in the life cycle of sexually reproducing organisms

In humans, meiosis occurs in reproductive organs and produces gametes

Mitosis is more common since it occurs in all tissues during growth & repair

Process Meiosis I compared to Mitosis

Meiosis II compared to Mitosis

Meiosis I Compared to Mitosis

Meiosis II Compared to Mitosis

Life Cycle Basics:Plants Life cycle – reproductive events that occur from one

generation to the next similar generation Haploid multicellular gametophyte alternate with

diploid multicellular sporophyte Mosses are haploid most of their

life cycle In fungi and most algae, only the

zygote is diploid In plants, algae and fungi,

gametes are produced by haploid individuals

Life Cycle Basics:Animals In animals, somatic cells are diploid and

multiply by mitosis – the only haploid cells produced are gametes

Gametes develop when germ line cells undergo meiosis

Gametogenesis is the formation of gametes Spermatogenesis (male gametogenesis) forms

four haploid sperm cells for each cell that enters meiosis

Oogenesis (female gametogenesis) forms one egg cell (ovum) for every cell that enters meiosis, plus polar bodies

The Human Life Cycle

A sperm and egg fuse at fertilization

Results in a zygote Undergoes mitosis

Results in multicellular embryo As a result of mitosis, each

somatic cell in body Has same number of

chromosomes as zygote Has genetic makeup

determined when zygote was formed

Oogenesis in Humans

Ovaries contain oogonia that produce primary oocytes during fetal development

Primary oocyte continue to develop at onset of puberty and divide through meiosis I into two cells

One of these cells (secondary oocyte) receive most of the cytoplasm; the other polar body may divide or disintegrate

Secondary oocyte begins meiosis II but stops at metaphase II

Then leaves the ovary and enters the oviduct If sperm enters, meiosis II continues & another polar

body forms

Spermatogenesis in Humans

Spermatogenesis takes place within the testes

Stem cells within the testes (spermatogonia) become primary spermatocytes; undergo spermatogenesis

Meiosis produces haploid secondary spermatocytes (meiosis I) and haploid spermatids (meiosis II)

Four spermatids are produced from the original primary spermatocyte – each differentiates into a mature sperm (spermatozoa)

Gametogenesis in Mammals