meiosis

Review Every human cell (except 1 type) has 46 chromosomes = Diploid (2n) number. 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?

MEIOSIS Diploid (2n)  haploid (n) The form of cell division by which gametes, with half the number of chromosomes are produced. Diploid (2n)  haploid (n) 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

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

GAMETOGENESIS

OOGENESIS

SPERMATOGENESIS

This produces a fertilized egg = Zygote!

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

Genetic variation what are the three sources? 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 Crossing over can occur between “nonsister” chromatids Segments of the chromatids break and reattach to the other chromatid

Independent assortment describes how different genes independently separate from one another when reproductive cells develop. the combinations of traits in the offspring do not always match the combinations of traits in the parent Assort randomly at the middle of the cell

What can go wrong? 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.

INTERPHASE Centrioles Chromatin duplicates

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

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

PROPHASE 1 Homologous chromosomes pair up

PROPHASE 1 …. and crossing over may occur.

PROPHASE 1 centrioles begin building the spindle

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

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

ANAPHASE 1 Homologous pairs of chromosomes are pulled apart.

ANAPHASE 1 Sister chromatids remain attached at the centromere

ANAPHASE 1

TELOPHASE 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

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

Meiosis II Meiosis II is very much like Mitosis

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

PROPHASE 2 the nuclear membrane breaks down

PROPHASE 2

PROPHASE 2 centrioles begin building the spindle

PROPHASE 2

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

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

ANAPHASE 2

TELOPHASE 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

CYTOKINESIS 2 Cytokinesis occurs Four different, haploid cells are formed

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

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

Turner’s syndrome 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

Klinefelter Syndrome 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

Review of terms 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

Control of the cell cycle and Stem cells

Normal control of the cell cycle Gene Segment of DNA that controls the production of proteins Enzyme type of protein in all living things that changes the rate of reactions Production of enzymes is directed by genes Cyclin protein that controls the cell cycle and is activates the enzymes

Loss of Control of the cell cycle How? Failure to produce enzymes Overproduction of enzymes Production of the wrong enzyme -What can happen? - Cancer: a malignant growth resulting from uncontrolled cell division.

Cancer: a mistake in the cell cycle Results from changes in genes that control the cell cycle Malignant Tumor Mass of cancerous cells, can deprive normal cells of nutrients Metastasis Cancer cells spread through bloodstream to other organs of the body

Causes and prevention Causes Prevention Genetics Environmental factors ( where you live, cigarette smoke, UV radiation, viral infections) Prevention Healthy lifestyle Diet, avoiding alcohol, drugs, smoking Protecting from the sun Cancer screening (pap smear, colonoscopy, mammogram)

Stem cells Embryonic stem cells: unspecialized cells that have the potential to become a wide variety of specialized cells. Produce every tissue in the body Found in embryos Adult Stem Cells -found in the umbilical cord and bone marrow. - can produce some different types of cells but not all Example: bone marrow stem cells can only produce different types of blood cells.

Potential uses for stem cells Cures for diseases Reverse paralysis caused by spinal cord injury Organ regeneration