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MEIOSIS, THE BASIS OF SEXUAL REPRODUCTION

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The life cycle of a multicellular organism is the sequence of ... A human karyotype. Homologous. pair of. chromosomes. in diploid. parent cell. Homologous ... – PowerPoint PPT presentation

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Title: MEIOSIS, THE BASIS OF SEXUAL REPRODUCTION


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MEIOSIS, THE BASIS OF SEXUAL REPRODUCTION
  • Sexual reproduction depends on
  • Meiosis
  • Fertilization

3
Gametes and the Life Cycle of a Sexual Organism
Haploid gametes (n 23)
  • The life cycle of a multicellular organism is the
    sequence of stages leading from the adults of one
    generation to the adults of the next

Egg cell
Sperm cell
Meiosis
Fertilization
Diploid zygote (2n 46)
Multicellular diploid adults (2n 46)
Mitosis and development
4
  • Different organisms of the same species have the
    same number and types of chromosomes
  • Humans have in all somatic cells
  • Two different sex chromosomes, X and Y
    (these count as a pair of sex chromosomes)
  • 22 pairs of matching chromosomes, called
    autosomes
  • Total of 23 pairs or 46 chromosomes

5
  • Humans are diploid organisms
  • All cells of the body contain two sets of
    chromosomes diploid (2n).
  • Cell produced by the ovaries and testes are
    haploid (1n) , having only one set of chromosomes.

Gametes sperm and egg cells, haploid
6
Homologous Chromosomes
Pair of homologous chromosomes
Centromere
Sister chromatids
  • Homologous chromosomes are matching pairs of
    chromosomes

A human karyotype
7
Homologous chromosomes separate
1
2
Chromosomes duplicate
3
Sister chromatids separate
Sister chromatids
Homologous pair of duplicated chromosomes
Homologous pair of chromosomes in diploid parent
cell
Meiosis II
Interphase before meiosis
Meiosis I
8
The Process of Meiosis
Interphase
  • Haploid gametes are produced in diploid organisms
  • Two consecutive divisions occur, meiosis I and
    meiosis II, preceded by interphase

Centrosomes (with centriole pairs)
Nuclear envelope
Chromatin
Chromosomes duplicate
9
  • Meiosis I

Meiosis I Homologous chromosomes separate
Telophase I and Cytokinesis
Anaphase I
Prophase I
Metaphase I
Microtubules attached to Chromosomes
Sister chromatids remain attached
Cleavage furrow
Sites of crossing over
Spindle
Sister chromatids
Tetrad
Centromere
Tetrads line up
Homologous chromosomes pair and exchange segments
Two haploid cells form chromosomes are still
double
Pairs of homologous chromosomes split up
10
  • Meiosis II

Meiosis II Sister chromatids separate
Telophase II and Cytokinesis
Prophase II
Anaphase II
Metaphase II
Sister chromatids separate
Haploid daughter cells forming
During another round of cell division, the sister
chromatids finally separate four haploid
daughter cells result, containing single
chromosomes
11
Review Comparing Mitosis and Meiosis
Mitosis
Meiosis
Parent cell (before chromosome duplication)
Site of crossing over
Duplicated chromosome (two sister chromatids)
Meiosis I
Chromosome duplication
Chromosome duplication
Tetrad formed by pairing of homologous chromosome
s
Pairing of homologous chromosomes
Prophase
Prophase I
2n 4
Tetrads align at the middle of the cell
Chromosomes align at the middle of the cell
Metaphase
Metaphase I
Anaphase Telophase
Anaphase I Telophase I
Homologous chromosomes separate during anaphase I
sister chromatids remain together
Sister chromatids separate during anaphase
Haploid n 2
Daughter cells of meiosis I
2n
2n
Daughter cells of mitosis
No further chromosomal duplication sister chromat
ids separate during anaphase II
Meiosis II
n
n
n
n
Daughter cells of meiosis II
12
The Origins of Genetic Variation
  • Offspring of sexual reproduction are genetically
    different from their parents and from one another

Why?
  • Independent Assortment of Chromosomes
  • Crossing Over

13
  • 1. Independent assortment every chromosome pair
    orients independently of the others during meiosis

Possibility 1
Possibility 2
Metaphase of meiosis I
Metaphase of meiosis II
Gametes
Combination a
Combination b
Combination c
Combination d
14
Tetrad
Prophase I of meiosis
  • 2. Crossing over

Chiasma, site of crossing over
  • Homologous chromosomes exchange genetic
    information
  • Genetic recombination occurs

Metaphase I
Spindle microtubules
Metaphase II
Gametes
Recombinant chromosomes
15
Random Fertilization
  • each egg cell produced and each sperm cell
    produced will be
  • slightly different genetically
  • which egg cell gets fertilized and which sperm
    does
  • the fertilizing is entirely random

16
  • Sex chromosomes

Male
Female
Somatic cells
44 XX
44 XY
  • Are designated X and Y
  • Determine an individuals sex

22 X
22 Y
22 X
Sperm
Egg
44 XY
44 XX
Female
Male
17
Sex-Linked Genes
  • Other genes located on sex chromosomes
    can influence the inheritance of certain traits
  • Were discovered during studies on fruit flies

(a)
(b)
18
Sex-Linked Disorders in Humans
  • A number of human conditions result from
    sex-linked (X-linked) genes
  • These are genes that are found on the
    X chromosomes

19
  • Red-green color blindness
  • Is characterized by a malfunction of
    light-sensitive cells in the eyes

Figure 9.30
20
  • Hemophilia
  • a blood-clotting disease

21
  • Duchenne muscular dystrophy
  • Is characterized by a progressive weakening and
    loss of muscle tissue
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