R E D U C I N G T H E C H R O M O S O M E N U M B E R

MEIOSIS

MEIOSIS• Chromosomes carry the DNA, our genetic code that creates

all the characteristics of our body.

• Human’s have 46 chromosomes, goldfish have 94 chromosomes and fruit flies have 8 chromosomes.

• Now if two human’s mate, why do we not have 92 chromosomes in the offspring?

• Well a little thing called Meiosis happens in our reproduction cells – sperm and eggs.

MEIOSIS• As we have learned, our body has cells that divide. These

cells are called somatic cells. This process is called Mitosis.

• Now, human’s somatic cells have 23 pairs of chromosomes for a total of 46 chromosomes.

• Our sex cells are called gametes.

• Male gametes are called sperm, and female gametes are called eggs.

MEIOSIS• Meiosis only happens in cells that produce gametes.

• Each gametes is then able to produce half of the genetic material for the offspring.

• Cells with half of the chromosomes are called haploid and are symbolized with a single n.

• Cells that have a complete set of chromosomes are called diploid symbolized as 2n.

MEIOSIS

During Meiosis Diploid cells (2n) Haploid cells (n)

• If human body cells have 46 chromosomes, then gametes (sperm and egg) each have 23 chromosomes.

HOMOLOGOUS CHROMOSOMES• A sperm will have 23 chromosomes and

an egg will have 23 chromosomes• When the sperm and egg meet, the 23

chromosomes will pair up with a corresponding chromosome, this is called homologous chromosomes.

• Example: one sperm chromosome carries the DNA for eye colour, it will pair up with the egg chromosome that has the DNA for eye colour.

• Thus each parent will contribute one half (n) (haploid) of the chromosomes to create a diploid (2n) somatic cell with the chromosome for eye colour.

PRIOR TO MEIOSIS

Spindle fibers

Sister Chromatids

Centrioles

Plasma Membrane

Centromere

Nuclear membrane

Recall this image

PRIOR TO MEIOSIS - INTERPHASE

• Recall Chromosomes are what carry our DNA.

• Before Meiosis begins DNA replication occurs in a process called Interphase.

• Each Duplicated Chromosome contains two (2) sister chromatids which are attached at their centromeres.

MEIOSIS I

• There are four (4) phases in Meiosis I:

• Prophase I • Metaphase I• Anaphase I• Telophase I

PROPHASE I

• Is the longest and most complex stage.

• Chromatids condense and shorten

• Nuclear membrane disintegrates

• During prophase the homologous chromosomes pair up (synapsis) and the non-sister chromatids exchange DNA.

WHY IS CROSSING OVER IMPORTANT?

• During crossing over segments of non-sister chromatids break and reattach to the other chromatid.

Tetrad

Non-sister chromatids

WHY IS CROSSING OVER IMPORTANT?

• Multiple crossovers create an infinite number of genetic possibilities for just one gamete.

• Variation is important and crossing over creates variation.

PROPHASE I

centriolesspindle fiber

METAPHASE I

• Shortest phase in Meiosis

• Pairs of homologous chromosomes lines up at the equator of the cell

• Independent assortment occurs.

INDEPENDENT ASSORTMENT

• The process of random segregation and assortment of homologous chromosomes during metaphase resulting in the production of genetically unique gametes.

METAPHASE I

Equator

OR

Equator

ANAPHASE I

• Homologous chromosomes separate and are pulled to opposite ends (poles) of the cell.

• This ensures that each new cell will receive only one chromosome from each homologous chromosome.

• Unlike in mitosis the sister chromatids remain attached.

ANAPHASE I

TELOPHASE I

• One chromosome from each homologous pair is at each pole of the cell

CYTOKINESIS

• The cytoplasm divides via cytokinesis to yield two new daughter cells.

MEIOSIS II

• Meiosis II has four (4) stages as well, and this occurs after Meiosis I:• Prophase II• Metaphase II• Anaphase II• Telophase II

PROPHASE II

• There is one chromosome of each homologous pair in each cell. • Chromosomes relax and condense again

METAPHASE II

• The chromosomes line up at the equator of the cell

Equator

ANAPHASE II• Chromosome breaks apart at the

centromere and sister chromatids move to opposite poles of the cell.• Once the chromosomes separate, each

sister chromatids is considered to be a chromosome.

TELOPHASE II

• Spindle fibres begin to breakdown, and a nuclear membrane forms around each set of chromosomes • Cytoplasm begins to divide

CYTOKINESIS

• Each sex cell divides into 2 producing 4 haploid daughter cells

• 4 gametes are produced – sperm or eggs

THE STAGES OF MEIOSIS

ASSIGNMENT

• Page 42 in Workbook • Textbook page 78 – 1-11, 14 and 17

Note, question 7, do not answer the fruit fly part.