Chapter 10Meiosis and Sexual Reproduction

Bozeman Video—Cell Cycle, Mitosis, & Meiosishttp://www.youtube.com/watch?v=2aVnN4RePyI

Impacts, Issues: Why Sex Asexual reproduction is easier and faster

Sexual reproduction can be an alternative adaption in changing environments

Asexual Reproduction Single parent produces offspring

All offspring are genetically identical to one another and to parent

Sexual ReproductionInvolves

MeiosisGamete productionFertilization

Produces genetic variation among offspring

SOMATIC VS GAMETE CELLS

AUTOSOMES VS. SEX CHROMOSOMES

Homologous Chromosomes Carry Different AllelesCell has two of each chromosome One chromosome in each pair from mother,

other from fatherPaternal and maternal chromosomes carry

different alleles

Fig. 10-2, p.156

Homologous Chromosomes

Sexual Reproduction Shuffles AllelesThrough sexual reproduction, offspring inherit

new combinations of alleles, which leads to variations in traits

This variation in traits is the basis for evolutionary change

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

testes

ovaries

anther ovary

Figure 10-3Page 156

Chromosome NumberSum total of chromosomes in a cellGerm cells are diploid (2n)Gametes are haploid (n)Meiosis halves chromosome number

Fig. 10-4, p.157

1 2 3 4 5

6 7 8 9 10 11 12

13 14 15 16 17 18

19 20 21 22 XX (or XY)

Human Karyotype

Meiosis: Two DivisionsTwo consecutive nuclear divisions

Meiosis IMeiosis II

DNA is not duplicated between divisionsFour haploid nuclei form

Meiosis I

Each homologue in the cell pairs with its partner,

then the partners separate

p. 158

Meiosis IIThe two sister chromatids of each duplicated

chromosome are separated from each other

one chromosome (duplicated)

two chromosomes (unduplicated)

p. 158

Meiosis I - Stages

Prophase I Metaphase I Anaphase I Telophase I

Prophase IEach duplicated

chromosome pairs with homologue

Homologues swap segments(THIS IS KNOWN AS CROSSING OVER WHICH OCCURS AT A SITE CALLED THE CHIASMATA)

Each chromosome becomes attached to spindle

Longest phase of meiosis

Fig. 10-5, p. 158

Metaphase ITetrads are aligned

on the metaphase plate

Chromosomes are pushed and pulled into the middle of cell

The spindle is fully formed

Fig. 10-5, p. 158

Anaphase IHomologous

chromosomes segregate to opposite poles

The sister chromatids remain attached

Fig. 10-5, p. 158

Telophase IThe chromosomes arrive

at opposite polesUsually followed by

cytoplasmic divisionInterkinesis (reforming of

the nuclear membrane)may occur before Meiosis II but no more DNA duplication

Fig. 10-5, p. 158

Prophase II

Microtubules attach to the kinetochores of the duplicated chromosomes

If interkinesis happened, the nuclear membrane redisappears

Fig. 10-5, p. 158

Metaphase II

Duplicated chromosomes line up singly at the spindle equator, midway between the poles

Fig. 10-5, p. 158

Anaphase IISister chromatids

and their centromeres separate to become independent chromosomes at opposite poles of each cell

Fig. 10-5, p. 158

Telophase II and CytokinesisThe chromosomes

have arrived at opposite ends of the cell

A nuclear envelope forms around each set of chromosomes

Four haploid cells

Fig. 10-5, p. 158

Telophase IAnaphase IMetaphase I

spindle equator

one pair of homologous chromosomes

newly forming microtubules

Prophase I

Meiosis I

Fig. 10-5a, p.158

Stepped Art

Meiosis II

Prophase II Metaphase II Anaphase II Telophase II

Fig. 10-5b, p.159

Stepped Art

Crossing Over•Each chromosome becomes zippered to its homologue•All four chromatids are closely aligned•Nonsister chromosomes exchange segments

Effect of Crossing OverAfter crossing over, each chromosome

contains both maternal and paternal segments

Creates new allele combinations in offspring

Random AlignmentEither the maternal or paternal member of a

homologous pair can end up at either pole

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

Possible Chromosome Combinations As a result of random alignment, the number of

possible combinations of chromosomes in a gamete is:

2n

(n is number of chromosome types)

Bozeman Video--Meiosishttp://www.youtube.com/watch?v=rB_8dTuh73c

ROLES OF MITOSIS/MEIOSIS IN LIFE CYCLES

sporophyte

meiosisdiploidfertilization

zygote

gametes

gametophytes

sporeshaploid

Fig. 10-8a, p.162

Plant Life Cycle

multicelledbody

meiosisdiploidfertilization

zygote

gametes

haploid

Fig. 10-8b, p.162

Animal Life Cycle

FUNGAL AND ALGAL LIFE CYCLE

FertilizationMale and female gametes unite and nuclei

fuseFusion of two haploid nuclei produces diploid

nucleus in the zygoteWhich two gametes unite is random

Adds to variation among offspring

Factors Contributing to Variation among Offspring

Crossing over during prophase IRandom alignment of chromosomes at metaphase I

(AKA Law of Independent Assortment of Chromosomes)Random combination of gametes at fertilization (1 in 8

million possible egg combinations x 1 in 8 million posssible sperm combinations = 1 in 64 trillion possible zygote

Natural Selection-increases frequency of reproductively favorable traits

Mitosis & Meiosis ComparedMitosisFunctions

Asexual reproduction

Growth, repair Occurs in somatic

cellsProduces clones

MeiosisFunction

Sexual reproduction

Occurs in germ cells

Produces variable offspring

Bozeman –Mitosis/Meiosis Bead Simulationhttp://www.youtube.com/watch?v=zGVBAHAsjJM&feature=c4-overview&playnext=1&list=TLZldufdv0wDU

Prophase vs. Prophase I Prophase (Mitosis)

Homologous pairs do not interact with each other

Prophase I (Meiosis) Homologous pairs become zippered together

and crossing over occurs

Anaphase, Anaphase I, and Anaphase IIAnaphase I (Meiosis)

Homologous chromosomes separate from each other

Anaphase/Anaphase II (Mitosis/Meiosis)Sister chromatids of a chromosome separate

from each other

Comparison of Mitosis and Meiosis

Meiosis Square Dance Videohttp://www.youtube.com/watch?v=eaf4j19_3Zg