AP BiologyChapter 10

Meiosis

Key Concepts:

Sexual reproduction entails meiosis, gamete formation, and fertilization

Germ cells of male and female animals produce gametes

Diploid cells have two of each type of chromosome, one from each parent

Meiosis divides the chromosome number in half

Key Concepts:

Gametes are haploid (n)

The union of gametes at fertilization results in a diploid number (2n)

During meiosis, chromosomes may exchange random segments by crossing over

Meiosis leads to variations in traits among offspring

Comparison of Asexual and Sexual Reproduction

Asexual

Single parent offspring inherit same genes as parent (number and type)

Sexual Produces genetic variation among offspring

Involves

Meiosis

Gamete formation

Fertilization

Chromosomes come in pairs, like

socks

Onehomolog

Homologouschromosomes(unreplicated)

Homologous Chromosomes

Cell has two of each chromosome

One chromosome in each pair from ♀ (mom) and one from ♂ (dad)

Paternal and maternal chromosomes carry the same genes but different alleles.

Sexual Reproduction Shuffles Alleles Through sexual reproduction,

offspring inherit new combinations of alleles, which leads to variations in traits.

This variation is traits is the basis for evolutionary change

Gamete Formation Gametes are sex cells (sperm, eggs) Arise from germ cells

anther ovary

Germ cells are diploid (2n) Gametes are haploid (n) Meiosis halves chromosome

number

Chromosome Number

Sum total of chromosomes in a

cell

Germ cells are diploid (2n)

Gametes are haploid (n)

Meiosis halves chromosome

numberHuman Karyotype

Meiosis: Two Divisions

Two consecutive nuclear

divisions Meiosis I

Meiosis II

DNA is not duplicated

between divisions

Four haploid nuclei form

A Chromosome Refresher…

Germ Cell at Interphase

A diploid cell (2n)

DNA duplicated

Ready for first division of meiosis

Meiosis I

Telophase IAnaphase IMetaphase I

spindle equator

one pair of homologous chromosomes

newly forming microtubules

Prophase I

Meiosis I Stepped Art

Meiosis I - Overview

Meiosis I

Prophase I Homologs pair

Crossing over occurs

Spindle forms

Nuclear envelope disappears

Metaphase I Pairs of homologous chromosomes are

at spindle equator

Random arrangement

Crossing over

Crossing Over

Effect of Crossing Over After crossing over, each

chromosome contains both maternal and paternal segments

Creates new allele combinations in offspring

Either the maternal of paternal member of a homologous pair can end up at either pole

The chromosome in a gamete are a mix of chromosomes from the two parents

Meiosis I – continued… Anaphase I

Duplicated chromosomes

pulled apart from homologue

toward opposite spindle poles

Sister chromatids remain attached

Telophase I Chromosomes arrive at poles

Usually followed by cytokinesis

Cells divides into two cells

Meiosis II - overview NO DNA duplication between I and II Sister chromatids are separated

Result is four haploid daughter cells

Meiosis II Prophase II

New spindle formed

Chromosomes move towards equator

Metaphase II Duplicated

chromosomes positioned at the equator

Anaphase II Sister chromatids are

moved apart

Telophase II Four nuclei with

haploid chromosome number (n) resulting in four cells

1 2 3

combinations possible

or

or

or

Stepped Art

Possible outcomes of the random alignment of only three pairs of chromosomes at metaphase I (without crossing over!!)

As a result of random alignment, the number of possible combinations in a gamete is 2n, where n=# of chromosome types

Random alignment

Results of Recombination

Oogenesis

Spermatogenesis

Fertilization

Male and female gametes unite and

nuclei fuse

Fusion of two haploid nuclei

produces diploid nucleus in the

zygote

Which two gametes unite is random Adds to variation among offspring

In Conclusion

Life cycle of sexually reproduction includes meiosis, gamete formation, and fertilization

A germ cell (2n) has two of each type of chromosome inherited from each parent. Each pair is homologous.

Chromosomes are duplicated in interphase.

Meiosis consists of two divisions

In Conclusion In meiosis I, each chromosome

moves away from homologous partner. In meiosis II, sister chromatids are separated

Meiosis produces haploid cells Crossing over, random allocation of

chromosomes, and the chance of gametes meeting at fertilization contribute to variation in traits among offspring