Meiosis and

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Chapter 23

• Meiosis and
• Sexual Reproduction

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Asexual Reproduction (review)

• Single parent gives rise to new offspring by
  mitotic cell division
• Each new individual receives a set of chromosomes
  identical to the parent chromosomes
• No variation of hereditary information

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(I) Sexual Reproduction

• Two parents give rise to new offspring by the
  fusion of nuclear materials from two different
  cells
• Offspring are not identical to the parent
• Variations exist, which increases the species
  ability to adapt to the changing environment

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(A) Gonads

• Sex organs
• 1. Males Testes
• 2. Females Ovaries

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(A) Gametes

• Sex cells
• 1. Males sperm
• 2. Females ovum

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(B) Fertilization

• The fusion of the nuclei of one sperm with one
  ova (egg) to produce a zygote
• Sperm (n) Ovum (n) 2n
• Sperm (23) Ovum (23) 46

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(C) Haploid

• Also known as monoploid
• Represented by n
• The gametes contain half the number of
  chromosomes
• Remember, gametes are sex cells that combine to
  form new offspring. Therefore gametes are
  haploid and once they fuse (combine), they form a
  zygote that is diploid

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(D) Diploid

• Contain the full number (set) of chromosomes
• Represented by 2n

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Homologous Chromosomes

• Pairs of similar chromosomes
• Humans have 46 chromosomes
• 22 pairs of homologous chromosomes
• 1 pair of sex chromosomes

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(II) Meiosis

• Known as Reduction Division
• Meiosis is a type of cell division in which the
  daughter cell receives only half the number of
  chromosomes present in the parent cell

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23 Pairs of chromosomes of a human cell

• The chromosomes labeled X and Y are the sex
  chromosomes
• XX female
• XY male

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(III) Stages of Meiosis

• In meiosis, each cell divides twice
• 1. The first stage is very similar to mitosis
• 2. The second stage is also like mitosis with
  the exception that there is
• NO replication of chromosomes in the second
  phase (meiosis 2)

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Phases of Meiosis
First Division Second Division
Prophase I Metaphase I Anaphase I Telophase I Prophase II Metaphase II Anaphase II Telophase II
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Key

• SS single-stranded
• DS double-stranded

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First Meiotic Division
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Prophase I

• Replication of SS chromosomes into DS chromosomes
  has already occurred.
• Each chromosome then pairs up with its
  homologous chromosome.
• This is known as synapsis.

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Prophase I

• Each chromosome pair consists of 4 chromatids and
  thus is a tetrad.
• Crossing over occurrs when segments of DNA are
  exchanged between the chromatids found in
  tetrads.

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Metaphase I

• Alignment of the tetrads (4 chromatids) in the
  middle of the cell (equatorial plane).

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Anaphase I

• The homologous chromosomes separate here.
• Chromrsomes are still double stranded.
• The process of separating the homologs is known
  as disjunction.

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Telophase I

• Cytokinesis occurs as in Mitosis however each of
  the daughter cells has half the of chromosome
  compared to the parent.
• After this point, the chromosomes WILL NOT
  replicate.
• Cells usually start prophase 2 immediately after
  telophase 1.

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SecondMeiotic Division
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Prophase II

• Chromosomes DO NOT replicate.
• DS chromosomes move towards the equatorial plane.

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Metaphase II

• Tetrads line up in the MIDDLE of the cell.
• This is sometimes called the equatorial plane or
  the metaphase plate.

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Anaphase II

• DS chromosomes separate.
• SS chromosomes move to opposite ends.

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Telophase II

• Both daughter cells undergo cytokinesis forming 4
  haploid cells.
• Thus, each cell has ½ the number of chromosomes
  as the parent.

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• Meiosis
• Diploid (2n) cells divide
• haploid (n) cells
• These cells mature into specialized reproductive
  cells (sperm and ova).

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(III) Sexual Reproduction

• in
• Animals

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(A) Reproductive System

• Gonads- specialized organs that produce gametes
• a) ovaries - female gonad
• - produce ova (egg)
• b) testes - male gonad
• - produce sperm cell

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(B) Hermaphrodite

• Contain both male and female reproductive
  structures

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(C) Gametogenesis

• Process by which gametes are produced in the
  gonads
• Two types
• Spermatogenesis
• Oogenesis

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Spermatogenesis

• Primary spermatocyte goes through first meiotic
  division and form two daughter cells of equal
  size
• Each daughter cell (secondary spermatocyte) goes
  through the second meiotic division forming four
  motile sperm cells of equal size

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Oogenesis

• The primary oocyte goes through first meiotic
  division and the cytoplasm of the cell is divided
  unequally
• The larger daughter cell is called the secondary
  oocyte and the smaller daughter cell is called
  the polar body
• During second meiotic division, 1 mature ovum is
  produced and 3 polar bodies

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Comparison of Ova and Sperm

• Egg cells contain stored food in the form of
  yolk.
• Egg cells are larger than sperm.
• Sperm are motile, egg cells sessile (cant move
  on their own).

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Fertilization

• Union of a haploid sperm nucleus with a haploid
  egg nucleus
• This results in a diploid zygote with the full
  number of chromosomes

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Two types of Fertilization

• Internal Fertilization
• External Fertilization

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Internal Fertilization

• Takes place inside the body of the female
• Less eggs are required
• Ex mammals and birds

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External Fertilization

• Eggs are fertilized outside the body of the
  female
• Large number of eggs are required
• This type of fertilization is found in an aquatic
  environment
• Ex fish, frogs, salamanders.

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