Chapter 13 Meiosis and Sexual Life Cycles. Overview: Hereditary Similarity and Variation Living organisms Are distinguished by their ability to reproduce

Chapter 13

Meiosis and Sexual Life Cycles

Overview: Hereditary Similarity and Variation

Living organisms Are distinguished by their ability to

reproduce their own kind Heredity

Is the transmission of traits from one generation to the next

Variation Shows that offspring differ somewhat in

appearance from parents and siblings

Concept 13.1: Offspring acquire genes from parents by inheriting chromosomes

Genetics Is the scientific study of heredity and

hereditary variation Genes

Are the units of heredity Are segments of DNA

Genes

• Genes are passed to the next generation through reproductive cells called gametes (sperm and eggs)

• Each gene has a specific location called a locus on a certain chromosome

• Most DNA is packaged into chromosomes

Comparison of asexual and sexual reproduction

In asexual reproduction One parent produces genetically identical

offspring by mitosis (clones) In sexual reproduction

Two parents give rise to offspring that have unique combinations of genes inherited from the two parents

Fig. 13-2

(a) Hydra (b) Redwoods

Parent

Bud

0.5 mm

Concept 13.2: Fertilization and meiosis alternate in sexual life cycles

A life cycle Is the generation-to-generation sequence of

stages in the reproductive history of an organism

In humans Each somatic cell has 46 chromosomes, made

up of two sets One set of chromosomes comes from each

parent

5 µmPair of homologouschromosomes

Centromere

Sistechromatids

Figure 13.3

A karyotype

Is an ordered, visual representation of the chromosomes in a cell

Fig. 13-3b

TECHNIQUE

Pair of homologousreplicated chromosomes

Centromere

Sisterchromatids

Metaphasechromosome

5 µm

Chromosome terminology

Homologous chromosomes Are the two chromosomes composing a

pair One from each parent Have the same inherited characteristics

for each locus Autosomes are homologous

chromosomes not directly involved in determining gender

Sex chromosomes

Are distinct from each other in their characteristics

Are represented as X and Y Determine the gender of the individual,

XX being female, XY being male

A diploid cell

The number of chromosomes in a single set is represented by n.

A diploid cell has two sets (2n) of each of its chromosomes

In a human has 46 chromosomes (2n = 46)

In a cell in which DNA synthesis has occurred

All the chromosomes are duplicated and thus each consists of two identical sister chromatids

Figure 13.4

Key

Maternal set ofchromosomes (n = 3)

Paternal set ofchromosomes (n = 3)

2n = 6

Two sister chromatidsof one replicatedchromosome

Two nonsisterchromatids ina homologous pair

Pair of homologouschromosomes(one from each set)

Centromere

A haploid cell

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

• For humans, the haploid number is 23 (n = 23)• Each set of 23 consists of 22 autosomes and a

single sex chromosome• In an unfertilized egg (ovum), the sex

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

either X or Y

Behavior of Chromosome Sets in the Human Life Cycle

At sexual maturity The ovaries and testes produce haploid

gametes by meiosis During fertilization

These gametes, sperm and ovum, fuse, forming a diploid zygote

The zygote Develops into an adult organism

Figure 13.5

Key

Haploid (n)

Diploid (2n)

Haploid gametes (n = 23)

Ovum (n)

SpermCell (n)

MEIOSIS FERTILIZATION

Ovary Testis Diploidzygote(2n = 46)

Mitosis anddevelopment

Multicellular diploidadults (2n = 46)

The human life cycle

The Variety of Sexual Life Cycles

The three main types of sexual life cycles Differ in the timing of meiosis and

fertilization

Textbook does not have specific examples but the photographic atlas in lab does

In animals

Gametes

Figure 13.6 A

Diploidmulticellular

organism

Key


n

n

n

2n2nZygote

Haploid

Diploid

Mitosis

(a) Animals

Meiosis occurs during gamete formation

Gametes are the only haploid cells


nn

n

nn

2n2n

Haploid multicellularorganism (gametophyte)

Mitosis Mitosis

SporesGametes

Mitosis

Zygote

Diploidmulticellularorganism(sporophyte)

(b) Plants and some algaeFigure 13.6 B

Plants and some algae

Exhibit an alternation of generations

The life cycle includes both diploid and haploid multicellular stages


n

n

n

n

n

2n

Haploid multicellularorganism

Mitosis Mitosis

Gametes

Zygote(c) Most fungi and some protistsFigure 13.6 C

In most fungi and some protists

Meiosis produces haploid cells that give rise to a haploid multicellular adult organism

The haploid adult carries out mitosis, producing cells that will become gametes

Depending on the type of life cycle, either haploid or diploid cells can divide by mitosis

However, only diploid cells can undergo meiosis

In all three life cycles, the halving and doubling of chromosomes contributes to genetic variation in offspring

Concept 13.3: Meiosis reduces the number of chromosome sets from diploid to haploid

Meiosis Takes place in two sets of divisions,

meiosis I and meiosis II

Results in four daughter cells with half as many chromosomes

Stages of Meiosis

Meiosis I Reduces the number of chromosomes

from diploid to haploid Called reduction division because it

halves the numbers of chromosome sets per cell

Meiosis II Produces four haploid daughter cells

BioFlix: MeiosisBioFlix: Meiosis

Fig. 13-7-1

Interphase

Homologous pair of chromosomesin diploid parent cell

Chromosomesreplicate

Homologous pair of replicated chromosomes

Sisterchromatids Diploid cell with

replicated chromosomes

Fig. 13-7-2

Interphase






Meiosis I

Homologouschromosomesseparate

1

Haploid cells withreplicated chromosomes

Fig. 13-7-3

Interphase






Meiosis I


1

Haploid cells withreplicated chromosomes

Meiosis II

2 Sister chromatidsseparate

Haploid cells with unreplicated chromosomes

Centrosomes(with centriole pairs)

Sisterchromatids

Chiasmata

Spindle

Tetrad

Nuclearenvelope

Chromatin

Centromere(with kinetochore)

Microtubuleattached tokinetochore

Tertads line up

Metaphaseplate


Sister chromatidsremain attached

Pairs of homologouschromosomes split up

Chromosomes duplicateHomologous chromosomes

(red and blue) pair and exchangesegments; 2n = 6 in this example

INTERPHASE MEIOSIS I: Separates homologous chromosomes

PROPHASE I METAPHASE I ANAPHASE I

Interphase and meiosis I

Figure 13.8

TELOPHASE I ANDCYTOKINESIS

PROPHASE II METAPHASE II ANAPHASE II TELOPHASE II ANDCYTOKINESIS

MEIOSIS II: Separates sister chromatids

Cleavagefurrow Sister chromatids

separate

Haploid daughter cellsforming

During another round of cell division, the sister chromatids finally separate;four haploid daughter cells result, containing single chromosomes

Two haploid cellsform; chromosomesare still doubleFigure 13.8

Telophase I, cytokinesis, and meiosis II

A Comparison of Mitosis and Meiosis

Meiosis and mitosis can be distinguished from mitosis by three events in Meiosis l1. Synapsis and crossing over

2. Tetrads on the metaphase plate

3. Separation of homologues

Fig. 13-9a

MITOSIS MEIOSIS

MEIOSIS I

Prophase I

Chiasma

Chromosomereplication

Homologouschromosomepair

Chromosomereplication

2n = 6

Parent cell

Prophase

Replicated chromosome

Metaphase Metaphase I

Anaphase ITelophase I

Haploid n = 3

Daughter cells ofmeiosis I

MEIOSIS II

Daughter cells of meiosis II

nnnn

2n2n

Daughter cellsof mitosis

AnaphaseTelophase

Concept 13.4: Genetic variation produced in sexual life cycles contributes to evolution

Reshuffling of genetic material in meiosis Produces genetic variation

In species that produce sexually The behavior of chromosomes during

meiosis and fertilization is responsible for most of the variation that arises each generation

Independent Assortment of Chromosomes

Homologous pairs of chromosomes Orient randomly at metaphase I of

meiosis In independent assortment

Each pair of chromosomes sorts its maternal and paternal homologues into daughter cells independently of the other pairs

Origins of Genetic Variation Among Offspring

• Three mechanisms contribute to genetic variation: Independent assortment of chromosomes Crossing over Random fertilization

Independent assortment

Figure 13.10

Key

Maternal set ofchromosomesPaternal set ofchromosomes

Possibility 1

Two equally probable arrangements ofchromosomes at

metaphase I

Possibility 2

Metaphase II

Daughtercells

Combination 1 Combination 2 Combination 3 Combination 4

Human variation

• The number of combinations possible when chromosomes assort independently into gametes is 2n, where n is the haploid number

• For humans (n = 23), there are more than 8 million (223) possible combinations of chromosomes

Crossing Over

Produces recombinant chromosomes that carry genes derived from two different parents

Figure 13.11

Prophase Iof meiosis

Nonsisterchromatids

Tetrad

Chiasma,site ofcrossingover

Metaphase I

Metaphase II

Daughtercells

Recombinantchromosomes

Random Fertilization

• Random fertilization adds to genetic variation because any sperm can fuse with any ovum (unfertilized egg)

• The fusion of two gametes (each with 8.4 million possible chromosome combinations from independent assortment) produces a zygote with any of about 70 trillion diploid combinations

Evolutionary Significance of Genetic Variation Within Populations

Genetic variation Is the raw material for evolution by

natural selection Random variation with independent

assortment and crossing over will produce a zygote with any of about

64 trillion diploid combinations (in humans)

Evolution

Mutations Are the original source of genetic

variation Sexual reproduction

Produces new combinations of variant genes, adding more genetic diversity

Selection pressure on variation that is heritable causes natural selection leading to evolution

Documents

Chapter 13 Meiosis and Sexual Life Cycles. Overview: Hereditary Similarity and Variation Living organisms Are distinguished by their ability to reproduce