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Meiosis. 11.4. Homologous Chromosomes. Homologous chromosomes : pairs of chromosomes with genes for the same traits, but they can have different information about those traits. Humans. Humans have 46 chromosomes or 23 pairs of homologous chromosomes.

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homologous chromosomes
Homologous Chromosomes
  • Homologous chromosomes: pairs of chromosomes with genes for the same traits, but they can have different information about those traits
humans
Humans
  • Humans have 46 chromosomes or 23 pairs of homologous chromosomes.
  • Somatic(body) cells contain 46 chromosomes.
  • 46 is the diploid number for humans.

- Diploid means that a cell has both homologs for each chromosome

  • Somatic cells reproduce by mitosis
slide4

½ the chromosomes in a cell come from Mom, ½ the chromosomes in a cell come from Dad

reproductive cells
Reproductive Cells
  • Gametes(eggs and sperm) are called haploid because they contain one set of chromosomes.
  • Eggs also called Ovum and sperm contain 23 chromosomes.
  • @Eggs and sperm are produced by meiosis- a process that reduces chromosomes by half@
what is meiosis
What is Meiosis?
  • A type of cell division where ONE diploid cell (46 chromosomes) become FOUR haploid cells(23 chromosomes each).
  • The cells produced by meiosis are gametes (sex cells)
who does it
Who does it?
  • Plants
  • Fungi
  • Animals (including you)
    • Males start meiosis at puberty.
    • Females start meiosis in utero.
where does it occur
Where does it occur?
  • In your gonads
    • Females- the ovary
    • Males-the testicles
why does it occur
Why does it occur?
  • Genetic Variation
    • Mixes genes from Mom and Dad to create a unique baby
  • @Reduces the chromosome number by half@
  • Ensures that each cell produced has one copy from each homologous pair of chromosomes
  • 23 in sperm + 23 in egg unite (fertilization) leads to a 46 zygote(baby)
meiosis creates the cells needed for sexual reproduction
Meiosis creates the cells needed for sexual reproduction
  • Two haploid gametes join to form one

diploid cell.

- the new diploid cell (fertilized egg)

is called a zygote

how does it occur
How does it occur?
  • In two stages called Meiosis I and Meiosis II.
  • Each stage contains 4 distinct phases

prophase

metaphase

anaphase

teleophase

telophase is followed by the division of cytoplasm known as cytokinesis.

prophase 1
Prophase 1
  • Homologouschromosomes

join to form a tetrad

-chromosomes in the tetrad

cross over and exchange

genes

slide15

@Crossing over produces new

genetic combinations@

- It is another way to increase

genetic diversity

metaphase 1
Metaphase 1
  • During Metaphase 1, the tetrads

(groups of 4 homologous chromosomes)

line up down the center of the cell

- they assort independently

-Chromosomes from each

parent line up independently

of each other

anaphase i
Anaphase I
  • During anaphase I, spindle fibers pull each homologous chromosome pair toward opposite ends of the cell.
  • When anaphase I is complete, the separated chromosomes cluster at opposite ends of the cell.
telophase i and cytokinesis
Telophase I and Cytokinesis
  • During telophase I, a nuclear membrane forms around each cluster of chromosomes.
  • Cytokinesis follows telophase I, forming two new cells.
meiosis ii
Meiosis II
  • @The two cells produced by meiosis I now enter a second meiotic division. @
  • Unlike the first division, neither cell goes through a round of chromosome replication before entering meiosis II.
prophase ii
Prophase II
  • As the cells enter prophase II, their chromosomes—each consisting of two chromatids—become visible.
  • The chromosomes do not pair to form tetrads, because the homologous pairs were already separated during meiosis I.
metaphase ii
Metaphase II
  • During metaphase of meiosis II, chromosomes line up in the center of each cell.
anaphase ii
Anaphase II
  • As the cell enters anaphase, the paired chromatids separate.
telophase ii and cytokinesis
Telophase II, and Cytokinesis
  • These four daughter cells now contain the haploid number (N)—just two chromosomes each.
independent assortment
Independent Assortment
  • @Independent assortment increases genetic diversity@

- There are 23 pairs of homologous chromosomes

in humans

- independent assortment ensures that there are

223 possible combinations

223= 8,388,608 combinations

- When two gametes join to form the zygote in fertilization the possible number of combinations is

223 X 223 = 64 trillion

how is mitosis different from meiosis
How is mitosis different from meiosis?
  • Mitosis creates 2 daughter cells with same amount of chromosomes
  • Meiosis creates 4 daughter cells with half amount of chromosomes
  • Meiosis goes through 2 cycles