1. Meiosis and chromosome number Life cycle and ploidy levels Steps in meiosis

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1. Meiosis and chromosome numberLife cycle and ploidy levels

2. Steps in meiosis

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

• Gametes are haploid, with only one set of chromosomes

• Somatic cells are diploid.

• The 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

• Chromosomes are duplicated before meiosis, then the cell divides twice to form four daughter cells.

Meiosis reduces the chromosome number 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 overSpindle

Sisterchromatids

Tetrad

Microtubules attached tokinetochore

Metaphaseplate

Centromere(with kinetochore)

Sister chromatidsremain attached

Homologouschromosomes separate

• In meiosis I, homologous chromosomes are paired– While paired, they cross over and

exchange genetic information

– 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 MEIOSISDiploidsomatic cell

Diploidgameteprecursor

2n 2n 1n 1n

1n 1n 1n 1n

division

duplication

haploiddiploid

Figure 8.15

MITOSIS MEIOSIS

PARENT CELL(before chromosome replication)

Site ofcrossing over MEIOSIS I

PROPHASE ITetrad formedby synapsis of homologous chromosomes

PROPHASE

Duplicatedchromosome(two sister chromatids)

METAPHASE

Chromosomereplication

2n = 4

ANAPHASETELOPHASE

Chromosomes align at the metaphase plate

Tetradsalign at themetaphase plate

METAPHASE I

ANAPHASE ITELOPHASE ISister chromatids

separate duringanaphase

Homologouschromosomesseparateduringanaphase I;sisterchromatids remain together

No further chromosomal replication; sister chromatids separate during anaphase II

Daughter cellsof mitosis

Daughter cells of meiosis II

MEIOSIS II

Daughtercells of

meiosis I

Haploidn = 2

n n n n

• 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

Genetic variation among offspring is a result of 1) Independent orientation of chromosomes in meiosis 2) random fertilization

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

Homologous chromosomes carry different versions of genes at corresponding loci

Figure 8.17A, B

Coat-color genes Eye-color genes

Brown Black

White Pink

Tetrad in parent cell(homologous pair of

duplicated chromosomes)

Chromosomes ofthe four gametes

• Crossing over is the exchange of corresponding segments between two homologous chromosomes

• Genetic recombination results from crossing over during prophase I of meiosis, which increases variation further

Crossing over further increases genetic variability

tetrad

Figure 8.18A

TetradChaisma

Centromere

• How crossing over leads to genetic recombination

Figure 8.18B

Tetrad(homologous pair ofchromosomes in synapsis)

Breakage of homologous chromatids

Joining of homologous chromatids

Chiasma

Separation of homologouschromosomes at anaphase I

Separation of chromatids atanaphase II and completion of meiosis

Parental type of chromosome

Recombinant chromosome

Recombinant chromosomeParental type of chromosome

Gametes of four genetic types

Coat-colorgenes

Eye-colorgenes

END OF INTERPHASE

PROPHASE I METAPHASE I ANAPHASE I

MEIOSIS I

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

8.21 Accidents during meiosis can alter chromosome number

Figure 8.21A

Nondisjunctionin meiosis I

Normalmeiosis II

Gametes

n + 1 n + 1 n – 1 n – 1Number 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 nNumber 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

8.20 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

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

Table 8.22

• Chromosome breakage can lead to rearrangements that can produce genetic disorders or cancer

– Four types of rearrangement are deletion, duplication, inversion, and translocation

8.23 Connection: Alterations of chromosome structure can cause birth defects and cancer

Figure 8.23A, B

Deletion

Duplication

Inversion

Homologouschromosomes

Reciprocaltranslocatio

Nonhomologouschromosomes

• Translocation

Figure 8.23Bx

• Chromosomal changes in a somatic cell can cause cancer

Figure 8.23C

Chromosome 9

– A chromosomal translocation in the bone marrow is associated with chronic myelogenous leukemia

Chromosome 22Reciprocaltranslocation

“Philadelphia chromosome”

Activated cancer-causing gene

1. Meiosis and chromosome number Life cycle and ploidy levels Steps in meiosis

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