1. Meiosis and chromosome number 2.Steps in meiosis 3.Source of genetic variation a.Independent...

1. Meiosis and chromosome number

2.Steps in meiosis

3.Source of genetic variationa.Independent alignment of homologuesb. Recombination

• Somatic cells are diploid.

• Gametes are haploid, with only one set of chromosomes

• human life cycle

• Meiosis creates gametes

• Mitosis of the zygote produces adult bodies

Figure 8.13

MEIOSIS FERTILIZATION

Haploid gametes (n = 23)

Egg cell

Sperm cell

Diploidzygote

(2n = 46)Multicellular

diploid adults (2n = 46)

Mitosis anddevelopment

Meiosis reduces the number of genomes from diploid to haploid

Figure 8.14, part 1

MEIOSIS I: Homologous chromosomes separate

INTERPHASE PROPHASE I METAPHASE I ANAPHASE I

Centrosomes(withcentriolepairs)

Nuclearenvelope

Chromatin

Sites of crossing over

Spindle

Sisterchromatids

Tetrad

Microtubules attached tokinetochore

Metaphaseplate

Centromere(with kinetochore)

Sister chromatidsremain attached

Homologouschromosomes separate

Steps in meiosis I

• In meiosis I, homologous chromosomes are paired

– While paired, they cross over and exchange genetic information (DNA)

– homologous pairs are then separated, and two daughter cells are produced

Figure 8.14, part 2

MEIOSIS II: Sister chromatids separate

TELOPHASE IAND CYTOKINESIS PROPHASE II METAPHASE II ANAPHASE II

Cleavagefurrow

Sister chromatidsseparate

TELOPHASE IIAND CYTOKINESIS

Haploiddaughter cellsforming

• Meiosis II is essentially the same as mitosis– sister chromatids of each chromosome separate

– result is four haploid daughter cells

MITOSIS MEIOSISDiploid

somatic cell

Diploid

gameteprecursor

2n 2n 1n 1n

1n 1n 1n 1n

division

duplication

haploiddiploid

• Each chromosome of a homologous pair comes from a different parent

– Each chromosome thus differs at many points from the other member of the pair

Homologous chromosomes carry different versions of genes (alleles) at corresponding loci

Figure 8.16

POSSIBILITY 1 POSSIBILITY 2

Two equally probable

arrangements of chromosomes at

metaphase I

Metaphase II

Gametes

Combination 1 Combination 2 Combination 3 Combination 4

Independent alignment of homologous chromosomes

• the exchange of corresponding segments between two homologous chromosomes

Crossing over further increases genetic variability

Figure 8.18A

TetradChaisma

Centromere

END OF INTERPHASE

PROPHASE I METAPHASE I ANAPHASE I

MEIOSIS I

Genetic recombination results from crossing over during prophase I of meiosis

TELOPHASE IIANAPHASE II

METAPHASE IIPROPHASE IITELOPHASE I

MEIOSIS

METAPHASE I METAPHASE I

TELOPHASE II

METAPHASE II

INDEPENDENT ASSORTMENT

polarbody

spermatogonium

primaryspermatocyte

secondaryspermatocyte

oogonium

primaryoocyte

secondaryoocyte

polar bodies(will be degraded)

spermatids

meiosis ll

meiosis l

SPERMATOGENESIS OOGENESISa b

• Abnormal chromosome count is a result of nondisjunction

– Either homologous pairs fail to separate during meiosis I

Accidents during meiosis can alter chromosome number

Figure 8.21A

Nondisjunctionin meiosis I

Normalmeiosis II

Gametes

n + 1 n + 1 n – 1 n – 1

Number of chromosomes

– Or sister chromatids fail to separate during meiosis II

Figure 8.21B

Normalmeiosis I

Nondisjunctionin meiosis II

Gametes

n + 1 n – 1 n n

Number of chromosomes

• Fertilization after nondisjunction in the mother results in a zygote with an extra chromosome

Figure 8.21C

Eggcell

Spermcell

n (normal)

Zygote2n + 1

• This karyotype shows three number 21 chromosomes

• An extra copy of chromosome 21 causes Down syndrome

Connection: An extra copy of chromosome 21 causes Down syndrome

Figure 8.20A, B

• The chance of having a Down syndrome child goes up with maternal age

Figure 8.20C

• Nondisjunction can also produce gametes with extra or missing sex chromosomes

– Unusual numbers of sex chromosomes upset the genetic balance less than an unusual number of autosomes

Connection: Abnormal numbers of sex chromosomes do not usually affect survival

Table 8.22

1. Meiosis and chromosome number 2.Steps in meiosis 3.Source of genetic variation a.Independent...

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