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Meiosis. Learning the Basics…. Chromosomes contain genetic information Human somatic cells have 46 chromosomes Divided into 23 pairs of homologous chromosomes , or homologs ; one set of 23 from mother, one set of 23 from father. Specific traits are coded for in genes

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learning the basics
Learning the Basics…
  • Chromosomes contain genetic information
  • Human somatic cells have 46 chromosomes
  • Divided into 23 pairs of homologous chromosomes, or homologs;
    • one set of 23 from mother, one set of 23 from father.
  • Specific traits are coded for in genes
    • Example: eye colour, hair colour, etc
slide3
Each of your parents contribute to genes towards a specific trait, but they are different forms of the same gene, called alleles.
    • Mom’s gene codes for blue eyes
    • Dad’s gene codes for green eyes
    • Both are alleles for the “eye colour gene”
recall
Recall:
  • Mitosis involves cell division that ensures both daughter cells receive a full set of chromosomes (diploid cells)
  • Sexual reproduction involves the “merging” of two gametes (sex cells) together (eg. sperm and egg)

Twice the chromosomes = New species?!?

meiosis1
Meiosis:
  • Division process that prevents this “doubling” of genetic material from occurring.
  • Creates gametes with half the number of chromosomes (haploid cells)
  • Fertilization is the fusion of 2 gametes (male + female) to get back to the diploid number
stages of meiosis
Stages of Meiosis
  • Prior to the start of meiosis, cell undergoes S Phase (chromosome replication)
  • Meiosis has two rounds of cell division:
    • Meiosis I
    • Meiosis II
  • Each round is divided into 4 sub phases:
    • Prophase, Metaphase, Anaphase, Telophase
prophase i
Prophase I

Tetrad of a homologous pair

  • Chromosomes condense & shorten, visible
  • Spindle fibresform
  • Centrioles moving towards poles of the cell
  • Homologous chromosomes form a tetrad made of 4 chromatids
    • Process of forming a tetrad is called synapsis
prophase i1
Prophase I
  • In the tetrad, chromatids often break at the end and swap places with the sister chromatid
  • This process is called recombination or crossing over and allows for genetic variation
  • The location of recombination is called the chiasma
recombination crossing over
Recombination/Crossing Over:

Homologous pair of chromosomes(tetrad)

metaphase i
Metaphase I
  • Tetrads line up along equator randomly

= Independent Assortment

  • Spindle fibres attach to the pair of sister chromatids
anaphase i
Anaphase I
  • The sister chromatids do NOT separate here (different than mitosis)
  • Instead, the pairs of chromosomes move apart to opposite poles
telophase i
Telophase I
  • Chromosomes condense slightly, nuclear membrane may form
  • Cytokinesis occurs forming two genetically different daughter cells
  • Each daughter cell has half the number of chromosomes as the parent cell (haploid - n)
slide15

Spindle

fibres

Nucleus

Nuclear

envelope

Prophase I

(early)

(diploid)

Prophase I

(late)

(diploid)

Metaphase I

(diploid)

Anaphase I

(diploid)

Telophase I

(diploid)

Meiosis I

meiosis ii1
Meiosis II
  • Similar to mitosis, but no duplication of chromosomes during a very short interphase (no G1 or S phase)
  • Each chromosome (made of two "mixed" chromatids) lines up at equator, centromeres split, and each chromosome is pulled to opposite poles
  • End result – four haploid cells
prophase ii
Prophase II
  • Nuclear envelope begins to break down
  • Spindle fibres begin to form
  • Centrioles begin to move to poles
metaphase ii
Metaphase II
  • Chromosomes align along equator of cell.
  • Spindle fibres attach to centromeres of sister chromatids
anaphase ii
Anaphase II
  • Spindle fibres contract and pull sister chromatids apart
telophase ii
Telophase II
  • Nuclear envelope assembles
  • Chromosomes decondense
  • Spindle disappears
  • Cytokinesis divides each cell into two
results of meiosis
Results of Meiosis:
  • 4 haploid gamete cells
  • 1 copy of each chromosome
  • 1 allele of each gene
  • Different combinations of alleles for different genes along the chromosome
slide23

Four

Non-identical

haploid

daughter cells

Prophase II

(haploid)

Metaphase II

(haploid)

Anaphase II

(haploid)

Telophase II

(haploid)

Meiosis II

slide24

VIDEO http://www.youtube.com/watch?v=kVMb4Js99tA

Mitosis

Meiosis

Number of divisions

1

2

Number of daughter cells

4

2

Yes

Genetically identical?

No

Chromosome #

Same as parent

Half of parent

Where

Somatic cells

Germline cells

When

Throughout life

At sexual maturity

Role

Growth and repair

Sexual reproduction