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Meiosis & Sexual Reproduction. Sexual Reproduction. Involves: Meiosis Gamete production Fertilization Produces genetic variation among offspring. Sexual Reproduction Shuffles Alleles. offspring inherit new combinations of alleles, leading to variations in traits

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Presentation Transcript
sexual reproduction
Sexual Reproduction
  • Involves:

Meiosis

Gamete production

Fertilization

  • Produces genetic variation among offspring
sexual reproduction shuffles alleles
Sexual Reproduction Shuffles Alleles
  • offspring inherit new combinations of alleles, leading to variations in traits
  • variation in traits is the basis for evolutionary change
variation
Variation
  • Sexual reproduction results in greater variation among offspring than does asexual reproduction.
  • Two parents give rise to offspring that have unique combinations of genes.
  • Offspring vary genetically from their siblings and from both parents
meiosis
Meiosis
  • function: production of gametes
  • where: gonads of the mature adult; ovaries & testes.
  • two consecutive nuclear divisions (Meiosis I & Meiosis II)
  • DNA is NOT replicated during interkinesis
  • result: 4 haploid (1n) daughter cells
slide6
Meiosis reduces the chromosome number from diploid to haploid.

Meiosis, like mitosis, is preceded by chromosome duplication…

…but, in meiosis the cell divides twice to form four daughter cells.

meiosis i
Meiosis I
  • starts with synapsis, the pairing of homologous chromosomes
  • crossing over homologous chromosomes exchange corresponding segments.
  • each homologous pair separates producing two daughter cells, each with one set of chromosomes
meiosis i stages

Prophase I

Metaphase I

Anaphase I

Telophase I

Meiosis I - Stages
slide9

MEIOSIS I: Homologous chromosomes separate

INTERPHASE PROPHASE I METAPHASE I ANAPHASE I

Sister chromatids remain attached

Microtubulesattached to kinetochore

Centrosomes (with centriole pairs)

Metaphaseplate

Sites of crossing over

Spindle

Centromere(with kinetochore)

Homologouschromosomes separate

Sisterchromatids

Nuclearenvelope

Tetrad

Chromatin

  • The stages of meiosis

Figure 8.14 (Part 1)

what are homologous chromosomes
What are homologous chromosomes?
  • Each chromosome can be distinguished by its size, position of the centromere, and by pattern of staining with dyes.
  • Homologous chromosomes carry genes that control the same traits.
slide12

TEM 2,200

Tetrad

Crossing over increases genetic variability.

Genetic recombination results from crossing over during prophase I of meiosis and increases variation still further.

slide13

MEIOSIS II: Sister chromatids separate

TELOPHASE IAND CYTOKINESIS

TELOPHASE IIAND CYTOKINESIS

PROPHASE II METAPHASE II ANAPHASE II

Cleavagefurrow

Sister chromatidsseparate

Haploid daughter cellsforming

Figure 8.14 (Part 2)

meiosis ii stages

Prophase II

Metaphase II

Anaphase II

Telophase II

Meiosis II - Stages
fertilization
Fertilization

is the union of two haploid gametes; one from each parent.

Ex: in humans the egg and sperm each contain 23 chromosomes so after fertilization a zygote will have 46 chromosomes.