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Cell cycle Need to getthe basics ofmitosis first

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1) PROPHASE -

chromosomescondense

Mitotic spindle formsAt end, nuclear

envelope starts to break down

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2) METAPHASE -chromosomes line up

in center of cell,formingMETAPHASEPLATE

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3) ANAPHASE - sisterchromatids detach

from each other, moveto opposite ends of cell

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4) TELOPHASE -spindle disappears,

nuclear envelopesreform, the cell divides

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Sexual reproduction the key to long term survival of a species

This is cloning of cells

It means cells can separate DNA double helices usingthe spindle

BUT

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Fig. 13-4

Key

Maternal set ofchromosomes ( n = 3)

Paternal set ofchromosomes ( n = 3)

2n = 6

Centromere

Two sister chromatidsof one replicatedchromosome

Two nonsisterchromatids ina homologous pair

Pair of homologouschromosomes(one from each set)

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But diploid plus diploid would be4-ploid.

Somehow we must reduce the genetic material inthe egg and sperm to ! the diploid level (1n,

haploid)

haploid + haploid = diploid (1n + 1n = 2n)

The spindle provides a mechanism for separatingcopies of the genetic material (compare mitosis)

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A gamete (sperm or egg) contains a single set ofchromosomes, and is haploid (n)

For humans, the haploid number is 23 ( n = 23)

Each set of 23 consists of 22 autosomes and a singlesex chromosome In an unfertilized egg (ovum), the sex chromosome

is X In a sperm cell, the sex chromosome may be either

X or Y

Copyright 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings

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Fig. 13-5Key

Haploid ( n ) Diploid (2 n )

Haploid gametes ( n = 23 )

Egg ( n )

Sperm ( n )

MEIOSIS FERTILIZATION

Ovary Testis

Diploidzygote

(2n = 46)

Mitosis anddevelopment

Multicellular diploidadults (2 n = 46)

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Figure 13.9a

Prophase

Duplicatedchromosome

MITOSIS

Chromosomeduplication

Parent cell

2 n 6

Metaphase

AnaphaseTelophase

2 n 2 n

Daughter cellsof mitosis

MEIOSIS

MEIOSIS I

MEIOSIS II

Prophase I

Metaphase I

Anaphase I Telophase I

Haploidn 3

Chiasma

Chromosomeduplication Homologous

chromosome pair

Daughtercells of

meiosis I

Daughter cells of meiosis II

n n n n

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Origins of Genetic Variation Among Offspring The behavior of chromosomes during meiosis

and fertilization is responsible for most of the

variation that arises in each generation Three mechanisms contribute to genetic

variation Independent assortment of chromosomes

Crossing over Random fertilization

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Figure 13.10-3

Possibility 1 Possibility 2

Two equally probablearrangements ofchromosomes at

metaphase I

Metaphase II

Daughtercells

Combination 1 Combination 2 Combination 3 Combination 4

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Crossing Over

Crossing over produces recombinantchromosomes , which combine DNA inheritedfrom each parent

Crossing over begins very early in prophase I,as homologous chromosomes pair up gene bygene

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In crossing over, homologous portions of twononsister chromatids trade places

Crossing over contributes to genetic variation

by combining DNA from two parents into asingle chromosome

Chiasma

Centromere

TEM

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Figure 13.11-5Prophase I of meiosis

Nonsister chromatidsheld togetherduring synapsis

Pair of homologs

Chiasma

Centromere

TEMAnaphase I

Anaphase II

Daughtercells

Recombinant chromosomes

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Random Fertilization

Random fertilization adds to genetic variationbecause any sperm can fuse with any ovum(unfertilized egg)

The fusion of two gametes (each with 8.4million possible chromosome combinationsfrom independent assortment) produces azygote with any of about 70 trillion diploid

combinations

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Crossing over adds even more variation Each zygote has a unique genetic identity

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Terms

Homologous chromosomeCrossing overMeiosisMitosisGameteHaploidDiploid

Sister chromatid Non-sister chromatid

centromereMeiosis I

Meiosis IISpindle

Chiasmata