Every human cell (except 1 type) has 46 chromosomes = Diploid (2n) number.Review Since we don’t want more then 46 chromosomes in our cells, then the gametes must have HALF the normal # of chromosomes= HAPLOID (n)

a little more….. So how do we make these cells? Would mitosis work?

The form of cell division by which gametes, with half the number of chromosomes are produced. Diploid (2n)  haploid (n) MEIOSIS Meiosis is sexual reproduction TWO divisions (meiosis I and meiosis II

Sex cells divide to produce gametes (sperm or egg) The new cells are called Haploid, they have half the number of chromosomes Meiosis is similar to mitosis with some chromosomal differences

 It only happens in the GONADS (testes and ovaries)  Male: Spermatogenesis  Female: Oogenesis

GAMETOGENESIS

OOGENESIS

SPERMATOGENESIS

This produces a fertilized egg = Zygote!

 Meiosis needs two divisions!  Main difference between mitosis and meiosis:  Meiosis produces 4 new gametes!  We need 4 gametes every division for GENETIC VARIATION!

1. Crossing Over  Occurs during Prophase I 2. Independent Assortment  Occurs during Metaphase I 3. Random Fertilization  Sperm joins with the egg cell

 Crossing over can occur between “nonsister” chromatids  Segments of the chromatids break and reattach to the other chromatid

 describes how different genes independently separate from one another when reproductive cells develop.  the combinations of traits in the offspring of his crosses did not always match the combinations of traits in the parents

 Non disjunction is the failure of chromosome pairs to separate properly during meiosis stage 1 or stage 2  This could arise from a failure of homologous chromosomes to separate in meiosis I, or the failure of sister chromatids to separate during meiosis II or mitosis. The result of this error is a cell with an imbalance of chromosomes.

Chromatin duplicates CentriolesINTERPHASE

chromatin becomes chromosomes PROPHASE 1 the nuclear membrane disintegrates homologous chromosomes pair up- crossing over may occur

the chromosomes condense centrioles move to opposite poles of cell & begin making the spindle. the nuclear membrane breaks down PROPHASE 1

Homologous chromosomes pair up

…. and crossing over may occur. PROPHASE 1

centrioles begin building the spindle PROPHASE 1

METAPHASE 1 Pairs of homologous chromosomes line-up at the center of the cell

Each pair is assorted independently So this pair could have assorted like this METAPHASE 1 So this pair could have assorted like this

Homologous pairs of chromosomes are pulled apart. ANAPHASE 1

Sister chromatids remain attached at the centromere

ANAPHASE 1

new nuclear membrane forms around each group of chromosomes forming haploid nuclei. TELOPHASE 1

new nuclear membrane forms around each group of chromosomes TELOPHASE 1

cytokinesis occurs TELOPHASE 1

Now, BOTH haploid cells will begin Meiosis II CYTOKINESIS 1 in this division, sister chromatids will separate

 Meiosis II is very much like Mitosis

the nuclear membrane breaks down PROPHASE 2 centrioles move to opposite poles of the parent cell

the nuclear membrane breaks down PROPHASE 2

centrioles begin building the spindle PROPHASE 2

The doubled chromosomes move to the equator of the cell METAPHASE 2

sister chromatids are pulled apart (like in mitosis) ANAPHASE 2

new nuclear membrane forms around each group of chromosomes cytokinesis occurs TELOPHASE 2

the spindle breaks down TELOPHASE 2

A new nuclear membrane forms around each group of chromosomes TELOPHASE 2

Cytokinesis occurs Four different, haploid cells are formed CYTOKINESIS 2

1. Down Syndrome 2. Turner’s Syndrome 3. Klinefelter’s Syndrome

 Trisomy: having an extra or 3 rd chromosome  Many down syndrome cases can be identified by having an extra chromosome

 Women and girls with turner’s, have only one X chromosome  This is an example of monosomy  Symptoms include lack of ovarian development, short stature, webbed neck, arms that turn slightly at the elbow

 Men and boys with Klinefelter have a Y chromosome and 2 X chromosomes.  This is an example of trisomy  Men and boys are usually tall, and lack secondary sex characteristics such as underarm and facial hair

 Diploid – number of chromosomes in a somatic cell- 46  Haploid/ monoploid- number of chromosomes in a gamete- 23  Homologous chromosome- same size and shape  Gametes- reproductive cells- sex cells  Zygote- the first cells as a result of fertilization of the egg and sperm cell- has a diploid number of chromosomes  Gonads- the organs that make gametes