Bellringer 12/02 What does AIDS and HIV stand for? What are some possible ways to get HIV? REMINDERS : HW #14 due Friday, Notebook check this week, LAB FEE anytime, 15 week progress reports next Monday! Pass your old bellringer sheets forward!

Copy this in your notebook… Name of Exchange 1______________________ Name of Exchange 2______________________ Name of Exchange 3______________________ Testing status: Positive or Negative ____________ Patient 0 = the original, first carrier of the virus Patient 0 Name? _________________________

Bellringer Wed, 12/03 What is meiosis? True or False: A muscle cell performs meiosis Fill in the blank: The two sex cells in humans are: In Males_________ and In Females__________ REMINDERS : HW # 14 due Fri, Notebook check this week, 15 wk progress reports Mon, LAB FEE anytime!

Meiosis – A Source of Original Distinction Why do you share some but not all characters of each parent? What are the rules of this sharing game? At all levels, the answers lie in meiosis. Meiosis- for sexual reproduction, special, two-step cell division only found in sex cells (female- egg and male- sperm)

Reminders, Review and Something New Chromosomes = condensed DNA, they form when cell is ready to divide Chromosomes = condensed DNA, they form when cell is ready to divide – 46 in Humans, 23 pairs Made up of 2 halves held together in the center by the centromereMade up of 2 halves held together in the center by the centromere Sister chromatids- exact copies of a chromosome held together by a centromere (this is what we see in the S phase when DNA is copied) Sister chromatids- exact copies of a chromosome held together by a centromere (this is what we see in the S phase when DNA is copied) Chromatin- loose DNA Chromatin- loose DNA Genes- sections of a chromosome responsible for one trait Genes- sections of a chromosome responsible for one trait Genome- A cell’s complete set of an organism’s genetic material (chromosomes and DNA) Genome- A cell’s complete set of an organism’s genetic material (chromosomes and DNA)

Boy or Girl? The Y Chromosome “Decides” X chromosome Y chromosome

Both male and female have 46 chromosomes 23-maternal (mother) and 23- paternal (from father) BUT- when it comes to the sex chromosomes (#23) THE MALE sperm decides… Male: XY Female: XX

Meiosis performs 2 functions- 1) Meiosis takes 1 cell with two copies of every chromosome (diploid) and makes 4 cells with a single copy of every chromosome (haploid). This is a good idea if you’re going to combine two cells to make a new organism. This trick is accomplished by halving chromosome number first before conception. So… In meiosis, one diploid cells produces four haploid cells.

2) Meiosis scrambles the specific forms of each gene (parts of a chromosome) that each sex cell (egg or sperm) receives. This makes for a lot of genetic diversity. This trick is accomplished through independent assortment and crossing-over. We’ll find out what these mean in a second!

Why do we need meiosis again? Meiosis is necessary to halve the number of chromosomes going into the sex cells!Meiosis is necessary to halve the number of chromosomes going into the sex cells! Why halve the chromosomes in gametes (sperm and egg)? At fertilization the male and female sex cells will provide ½ of the chromosomes each – so the offspring has half the genes from both parents making one complete set of its own!At fertilization the male and female sex cells will provide ½ of the chromosomes each – so the offspring has half the genes from both parents making one complete set of its own!

Meiosis Parent cell – chromosome pair Chromosomes copied 1 st division - pairs split 2 nd division – produces 4 gamete cells with ½ the original no. of chromosomes

Meiosis – PMAT I and II Parent cell 4 gametes 1 st division 2 nd division

Stages of Meiosis: PMAT I and II The key is Reduction Division- as the cell divides, it reduces the number of chromosomes in half. Reminder : Diploid parent cells (46 chromosomes, or 23 X2) to Haploid daughter cells (23 chromosomes)

Meiosis I : Separates Homologous Chromosomes Interphase Interphase –Each of the chromosomes replicate –The result is two genetically identical sister chromatids which remain attached at their centromeres- like mitosis!

Prophase I During this phase each pair of chromatids don’t move to the equator alone, they match up with their homologous pair and fasten together (synapsis) in a group of four called a tetrad. Exception: x and y!During this phase each pair of chromatids don’t move to the equator alone, they match up with their homologous pair and fasten together (synapsis) in a group of four called a tetrad. Exception: x and y! Extremely IMPORTANT!!! It is during this phase that crossing over can occur. Extremely IMPORTANT!!! It is during this phase that crossing over can occur. Crossing Over= the exchange of segments during synapsis. Crossing Over= the exchange of segments during synapsis.

Crossing Over

Metaphase I The chromosomes line up at the equator attached by their centromeres to spindle fibers from centrioles.The chromosomes line up at the equator attached by their centromeres to spindle fibers from centrioles. –Still in homologous pairs! – REMINDER : What are homologous pairs? homologous chromosomes sister chromatids sister chromatids

Anaphase I The spindle guides the movement of the chromosomes toward the polesThe spindle guides the movement of the chromosomes toward the poles –Sister chromatids remain attached –Move as a unit towards the same pole The homologous chromosome moves toward the opposite poleThe homologous chromosome moves toward the opposite pole –Contrasts mitosis – chromosomes appear as individuals instead of pairs (meiosis)

Telophase I This is the end of the first meiotic cell division.This is the end of the first meiotic cell division. The cytoplasm divides, forming two new daughter cells.The cytoplasm divides, forming two new daughter cells. Each of the newly formed cells has half the number of the parent cell’s chromosomes, but each chromosome is already replicated ready for the second meiotic cell divisionEach of the newly formed cells has half the number of the parent cell’s chromosomes, but each chromosome is already replicated ready for the second meiotic cell division

Cytokinesis Occurs simultaneously with telophase IOccurs simultaneously with telophase I –Forms 2 daughter cells Plant cells – cell platePlant cells – cell plate Animal cells – cleavage furrowsAnimal cells – cleavage furrows NO FURTHER REPLICATION OF GENETIC MATERIAL PRIOR TO THE SECOND DIVISION OF MEIOSIS. NO NEW INTERPHASE!!! NO FURTHER REPLICATION OF GENETIC MATERIAL PRIOR TO THE SECOND DIVISION OF MEIOSIS. NO NEW INTERPHASE!!!

Figure 13.7 The stages of meiotic cell division: Meiosis I

Meiosis II : Separates sister chromatids Proceeds similar to mitosisProceeds similar to mitosis REMINDER: THERE IS NO INTERPHASE II ! REMINDER: THERE IS NO INTERPHASE II !

Prophase II Each of the daughter cells forms a spindle, and the double stranded chromosomes move toward the equatorEach of the daughter cells forms a spindle, and the double stranded chromosomes move toward the equator

Metaphase II The chromosomes are positioned on the metaphase plate in a mitosis-like fashionThe chromosomes are positioned on the metaphase plate in a mitosis-like fashion

Anaphase II The centromeres of sister chromatids finally separateThe centromeres of sister chromatids finally separate The sister chromatids of each pair move toward opposite polesThe sister chromatids of each pair move toward opposite poles –Now individual chromosomes

Telophase II and Cytokinesis Nuclei form at opposite poles of the cell and cytokinesis occursNuclei form at opposite poles of the cell and cytokinesis occurs After completion of cytokinesis there are four daughter cellsAfter completion of cytokinesis there are four daughter cells –All are haploid (n)

The stages of meiotic cell division: Meiosis II

One Way Meiosis Makes Lots of Different Sex Cells (Gametes) – Independent Assortment Independent assortment produces 2 n distinct gametes, where n = the number of unique chromosomes. That’s a lot of diversity by this mechanism alone. In humans, n = 23 and 2 23 = 6,000,0000.

Another Way Meiosis Makes Lots of Different Sex Cells – Crossing-Over Crossing-over multiplies the already huge number of different gamete types produced by independent assortment.

The Key Difference Between Mitosis and Meiosis is the Way Chromosomes Uniquely Pair and Align in Meiosis Mitosis The first (and distinguishing) division of meiosis

Boy or Girl? The Y Chromosome “Decides”

Meiosis – division error Chromosome pair

Meiosis error - fertilization Should the gamete with the chromosome pair be fertilized then the offspring will not be ‘normal’. In humans this often occurs with the 21 st pair – producing a child with Downs Syndrome

21 trisomy – Downs Syndrome Can you see the extra 21 st chromosome? Is this person male or female?