11-4 MEIOSIS PG 275 (May take a test on this section as a stand alone test!)

Video Clip Intro

The sex cells or gametes that carry the genetic information are formed by a special kind of cell division known as MEIOSIS.

The number of chromosomes, the structures that contain the genetic information, is reduced by half during meiotic division.

By fertilization the full number of chromosomes is restored.

These 2 processes Meiosis and fertilization- allow for infinite variety in the selection and recombination of genetic traits.

From Mendel's genetics we know that an organism inherits a single copy of each gene from each of their parents.

These 2 copies are segregated from one another during the formation of gametes.

EX: fruit fly...each body or somatic cell of a fruit fly contains 8 chromosomes.

If chromosomes were not separated the offspring would have 16 chromosomes…their offspring would have 32 and so on.

Drosophila melanogaster

CHROMOSOME NUMBER PG 275

The chromosomes of the fruit fly can be divided into 2 sets…4 chromosomes from the male parent and 4 chromosomes from the female parent.

Each chromosome in the male set has a corresponding chromosome in the female set.

Video Clip-Homologous Chromosomes (will need to retrieve from f:drive) Will need to retrieve from f: drive….

HOMOLOGOUS- a description of chromosomes that occur in pairs; having a corresponding structure…the chromosomes are called HOMOLOGS.

EX: Each of the chromosomes from the male parent have a corresponding chromosome from the female parent.

A cell that contains both sets of homologous chromosomes are said to be DIPLOID.

A diploid cell contains 2 complete sets of chromosomes and 2 complete sets of genes.The diploid number is sometimes represented by 2N…so for the Drosophila 2N= 8.

DIPLOID- "two sets"…a cell that contains both sets of homologous chromosomes.All of an organisms cells (except for sex cells) contain 2 alleles for a given trait.

Gametes or sex cells contain only a single set of genes because alleles are separated during the process of gamete formation.

The GAMETES of Sexually reproducing organisms contain a single set of chromosomes and genes.

HAPLOID- a cell that contains a single set of chromosomes..represented by the symbol N.Drosophila, N =4…

“THE PHASES OF MEIOSIS”

Video Clip… Video: Meiosis (2-3 min.)

MEIOSIS- a process of reduction division in which the number of chromosomes per cell is cut in half and homologous chromosomes that exist in a diploid cell are separated.

In most organisms meiosis takes place in 2 stages..1 st & 2 nd meiotic divisions.

The phases of meiosis are very different from the phases of MITOSIS.

Video clip to retrieve from f: drive

SEE FIG page 276…

MEIOSIS I….

Special cells in reproductive organs undergo a round of DNA replication…this resembles mitosis but it is not the same!

In Prophase 1 of meiosis, each chromosome seeks out its corresponding homologous chromosome to form a special structure called a tetrad.

There are 4 chromatids in a tetrad;

In a process called “CROSSING OVER” homologous chromosomes may exchange portions of their chromatids, an exchange of genes that produces new combinations of genes…see figure page 276

Video Clip: Crossing over (Will need to retrieve from f: drive

METAPHASE 1

Metaphase I Tetrads (rather than individual chromosomes) line up in the center of the cell. ANAPHASE I The homologous chromosomes separate and (telophase 1) two new cells are formed.

The 2 new cells have sets of chromosomes that are different from the parent cells and different from each other.

MEIOSIS II….

The 2 cells produced by meiosis I enter meiosis II.In the second meiotic division the cells do not under go DNA replication so each cell’s chromosomes contains 2 chromatids.

In METAPHASE II of meiosis, : chromosomes line up in the middle of the cell. In ANAPHASE II they separate…each of the 4 daughter cells receives 2 chromatids.

The daughter cells contain the HAPLOID number = 2 chromosomes each.

*The amount of genetic material has been reduced and the combinations of chromosomes in each gamete have been made at random.

Video clip: has to be retrieved from f: drive Phases of meiosis

“MEIOSIS & GENETICS”

Meiosis I results in segregation and independent assortment.

GAMETE FORMATION…

In males the haploid gametes produced by meiosis are called SPERM.Pollen grains contain haploid sperm cells.

The female gamete is called an egg in animals and an OVULE in higher plants.

In females the cell divisions are uneven, and the egg or ovule gets most of the cytoplasm…see figure pg 278;

3 other cells called polar bodies are produced in the female during meiosis..they are called polar bodies.

Video: Meiosis Square Dance 10 minute video

“COMPARING MITOSIS AND MEIOSIS”

Mitosis results in the production of 2 genetically identical cells.

A diploid cell divides and gives rise to 2 diploid daughter cells that are genetically identical to each other and identical to the original parent cell.

Meiosis begins with a diploid cell but produces 4 haploid cells that are different from the original diploid cell and different from one another.WHY?

Homologous chromosomes are separated during the 1 st meiotic division and crossing over occurs…giving new gene combinations on the chromosomes.

*MITOSIS results in the production of 2 genetically identical diploid cells.

*MEIOSIS results in the production of 4 genetically DIFFERENT haploid cells.

TEST CH 11