Meiosis and Sexual Reproduction

Bozeman Video—Cell Cycle,

Mitosis, & Meiosis http://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 Reproduction Involves

Meiosis

Gamete production

Fertilization

Produces genetic variation among offspring

SOMATIC VS GAMETE CELLS

AUTOSOMES VS.

SEX CHROMOSOMES

Homologous Chromosomes

Carry Different Alleles Cell has two of each chromosome

One chromosome in each pair from mother, other

from father

Paternal and maternal chromosomes carry

different alleles

Fig. 10-2, p.156

Homologous

Chromosomes

Sexual Reproduction

Shuffles Alleles

Through 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 Formation Gametes are sex cells (sperm, eggs)

Arise from germ cells

testes

ovaries

anther ovary

Figure 10-3

Page 156

Chromosome Number

Sum total of chromosomes in a cell

Germ 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 Divisions Two consecutive nuclear divisions

Meiosis I

Meiosis II

DNA is not duplicated between divisions

Four haploid nuclei form

Meiosis I

Each homologue in the

cell pairs with its partner,

then the partners

separate

p. 158

Meiosis II The 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 I Each 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 I Tetrads 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 I

Homologous

chromosomes segregate

to opposite poles

The sister chromatids

remain attached

Fig. 10-5, p. 158

Telophase I The chromosomes arrive

at opposite poles

Usually followed by

cytoplasmic division

Interkinesis (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 II Sister chromatids and

their centromeres

separate to become

independent

chromosomes at

opposite poles of each

cell

Fig. 10-5, p. 158

Telophase II and

Cytokinesis

The 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 I Anaphase I Metaphase 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 Over After crossing over, each chromosome contains

both maternal and paternal segments

Creates new allele combinations in offspring

Random Alignment Either 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)

Activator:

Discuss how unique you are as

an individual (in terms of

meiosis).

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

ROLES OF MITOSIS/MEIOSIS

IN LIFE CYCLES

sporophyte

meiosis diploid

fertilization

zygote

gametes

gametophytes

spores

haploid

Fig. 10-8a, p.162

Plant Life Cycle

multicelled

body

meiosis diploid

fertilization

zygote

gametes

haploid

Fig. 10-8b, p.162

Animal Life Cycle

FUNGAL AND ALGAL LIFE

CYCLE

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

Factors Contributing to

Variation among Offspring

Crossing over during prophase I

Random 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 Compared Mitosis

Functions

Asexual reproduction

Growth, repair

Occurs in somatic cells

Produces clones

(2 diploid daughter cells)

Meiosis Function

Sexual reproduction

Occurs in germ cells

Produces variable offspring

(4 haploid daughter cells)

Bozeman –Mitosis/Meiosis

Bead Simulation http://www.youtube.com/watch?v=zGVBAHAsjJM&f

eature=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 II

Anaphase 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 Video http://www.youtube.com/watch?v=eaf4j19_3Zg