Sexual Sexual Reproduction:Reproduction:

MeiosiMeiosiss

What is Sexual What is Sexual Reproduction?Reproduction?

Combines genetic material from two Combines genetic material from two parents to produce a new individualparents to produce a new individual

Creates an increase in variation among Creates an increase in variation among the offspringthe offspring

Populations can adapt to changing Populations can adapt to changing environmentsenvironments

The world is more enjoyable with The world is more enjoyable with variationvariation

Costly – it takes twice as many Costly – it takes twice as many individuals to produce an offspringindividuals to produce an offspring

Genetic material (DNA) is found in chromosomes

Each chromosome has a “twin” that:• contains the same genes • codes for the same traits • has the same shape • is the same size• Has the same location of the centromere • Has the same banding pattern when stained

One chromosome came from the father, the other came from the mother

These twins are called homologous.

Homologous Chromosomes

Homologous Chromosomes

Homologous Chromosomes

Cells that have only one of each chromosome are called Haploid (n).

This is half of the usual amount of DNA.

Cells that have two of each chromosome are called Diploid (2n).

This is the normal condition for most cells.

N =

1 – 2

9

3

6

7

10

631

13

23

24

39

Homologous Homologous ChromosomesChromosomesHumans have 23 pairs of chromosomes –

22 pairs of autosomes and one pair of sex chromosomes

In females they are all homologous

In males, the autosomes

are homologous, but the sex

chromosomes are not Karyotype – arranges chromosomes in homologous

pairs

Special Special Considerations:Considerations:

1.1. If a sperm containing 46 chromosomes If a sperm containing 46 chromosomes fused with an egg containing 46 fused with an egg containing 46 chromosomes, how many chromosomes would chromosomes, how many chromosomes would the resulting fertilized egg contain? the resulting fertilized egg contain?

2.2. In order to produce a fertilized egg with In order to produce a fertilized egg with the appropriate number of chromosomes (46), the appropriate number of chromosomes (46), how many chromosomes should each sperm how many chromosomes should each sperm and egg have?and egg have?

MeiosisMeiosis

Purpose: Purpose: Divide the nucleus to reduce the Divide the nucleus to reduce the

number of chromosomes in halfnumber of chromosomes in half Ensure that each new cell has a Ensure that each new cell has a

complete set of chromosomescomplete set of chromosomes Create gametesCreate gametes Also called reduction divisionAlso called reduction division

Human Human Life Life

CycleCycleMost of our cells are diploid (2n)

Reproductive cells called gametes are haploid (n)

They are produced in gonads (ovary and testes) from diploid gametocytes

Gametes

Meiosis I and Meiosis II

Two Stages of Meiosis

In meiosis I the homologous

chromosomes are separated

In meiosis II the sister chromatids

are separated

Four cells are produced, each with one set of

chromosomes (n).

A cell normally has two sets of chromosomes

(2n)

Meiosis IThe phases of meiosis I are similar to mitosis:

Prophase I, Metaphase I, Anaphase I, Telophase I

The formation of tetrads is called synapsis

Dyads are sister chromatids attached by the centromere

Prophase 1Prophase 1•chromosomes condense•homologs pair up forming tetrads (four chromatids are lined up)•Nuclear envelope fragments •Crossing-over can occur in this stage.

What is Crossing Over?

During synapsis of prophase I:

Homologous chromosomes tangle and trade parts of their DNA

Variation increases

Genes that are usually linked (on the same

chromosome) can become separated

Homologous chromosomes line up at the equatorial plate.

Meiosis 1

Metaphase 1Metaphase 1

Homologous pairs separate Sister chromatids remain together.

Meiosis 1

Anaphase 1Anaphase 1

Meiosis 1

Telophase 1Telophase 1

Two daughter cells are formed Each daughter contains only one chromosome of the homologous pair (haploid).

Meiosis IIAfter a phase called interkinesis (similar to G2), both cells formed in

meiosis I go through phases again

Prophase II, Metaphase II, Anaphase II, Telophase II

Inte

rkin

esis

chromosomes condense, nuclear envelope fragments

Meiosis 2

Prophase IIProphase II

Chromosomes align at the equatorial plate.

Meiosis 2

Metaphase IIMetaphase II

Sister chromatids separate

Meiosis 2

Anaphase IIAnaphase II

Cell division is complete. Four haploid daughter cells are produced.

Meiosis 2

Telophase IITelophase II

Meiosis I in an Animal Cell

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Homologous pairsalign at the metaphaseplate.

Metaphase I

DNA Replication

Homologous chromosomes separate, pulled to opposite poles by centromeric spindle fibers.

Anaphase I

Daughter cells haveone chromosome fromeach homologous pair.

Telophase I

Homologous chromosomespair during synapsis.

Prophase I

Meiosis I2n = 4

Chromosomes stillconsist oftwo chromatids.

Interkinesis

n = 2

Meiosis II in an animal

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Metaphase IIChromosomes alignat the metaphaseplate.

Anaphase IIDaughter chromosomesmove toward the poles.

Telophase IISpindle disappears,nuclei form, and cytokinesis takesplace.

Meosis II

Prophase IICells have onechromosomefrom each homologous pair.

n = 2 n = 2

Daughter CellsMeiosis resultsin four haploiddaughter cells.

n = 2 n = 2

Nondisjunction

Disjunction is the separation of tetrads or sister chromatids

Nondisjunction- no separation occurs

Turner Syndrome

XO

Klinefelter Syndrome

XXY

Down Syndrome

(trisomy 21)

Four cells not identical to parent

Two cells genetically identical to parent cell

2n 2n

n

2nn

Meiosis vs.

Mitosis

Mitosis Meiosis

One division Two divisions

Two daughter cells per cycle

Four daughter cells per cycle

Daughter cells genetically identical

Daughter cells genetically different

Same chromosome number as parents

Chromosome number half that of parents

Occurs in somatic cells Occurs in gametocytes

Throughout life cycleCompleted after sexual

maturityUsed in growth, repair, asexual reproduction

Sexual reproduction, new gene combinations