Cell Division (Cellular Reproduction) Part 3 of 4

SEXUAL REPRODUCTION

The exchange of genetic material between two cells is a sexual union.

Cells exchange or mix their genetic material together, producing unique, yet related cells.

The exchange is accomplished by a sexual union of 2 cells that separate after the genetic exchange occurs.

The process of genetic-recombination between 2 cells is know as CONJUGATION.

GAMATES are the sex cells found in multicellular sexual organisms. There are 2 types of sex cells, MALE and FEMALE.

Sex cells divide in a special process called MEIOSIS.

Meiosis is a longer process than mitosis.

Meiosis only occurs in the germ cells (gametes) of sexually reproducing organisms.

Meiosis is a process of cellular division where the genetic material is reduced by half in the newly created cells.

Sex cells have half the number of chromosomes as the DIPLOID CELL (all cells that have a full number of chromosomes are called diploid) or parent cell and are called HAPLOID CELLS.

Meiosis forms 4 haploid cells that contain half as many chromosomes as the parent cells.

When each parent donates half of the chromosomes, the offspring will have the full number of needed chromosomes.

In sexual reproduction, genetic material from the male and female are put together, a process called FERTILIZATION, to produce a new cell, called a ZYGOTE (a diploid cell that has the full number of needed chromosomes). The new cell must have the same number of chromosomes as each parent. Meiosis ensures that each parent donates only half of the needed chromosomes.

Meiosis allows for genetic variability among organisms.

There are 2 divisions in Meiosis and each includes 4 stages; Prophase, Metaphase, Anaphase and Telophase.

The 2 divisions separate attached chromosomes and produce a total of 4 haploid nuclei.

The actions of Interphase, during the cell cycle are the same for sex cells. DNA replication only occurs once during the Interphase before Meiosis I.

MEIOSIS I
Meiosis I is call the reduction division stage because it separates HOMOLOGOUS chromosomes(matched copies of chromosomes) into 2 different daughter nuclei.

1. Prophase I
– The chromosomes begin to coil-up and condense.
– The homologous chromosomes move next to each other, a process called SYNAPSIS. This action forms a four-part structure called a TETRAD, consisting of 2 sets of sister CHROMTIDS (a chromatid is one copy of a doubled chromosome).
– At the time of the synapsis, genetic material may recombine into a new arrangement, a process called GENETIC RECOMBINATION.
– Also during synapsis the chromatids may exchange genetic material, a process called CROSSING OVER. This is another way that the genes are shuffled, ensuring that each offspring has a random combination of traits from both parents.

2. Metaphase I
– The homologous pairs prepare for separation.
– The homologous pairs line-up at the METAPHASE PLATE/equatorial plane (the center of the cell).
– Spindle fibers help move the chromatids into position and attach to the sister chromatids.

3. Anaphase I
– One chromatid from each of the homologous pairs moves toward a separate pole.
– The attached spindle fibers pull the chromatid toward the poles.
– The homologous pairs separate with sister chromatids remaining together.
– As a result each daughter cell will have half the number of chromosomes of its parent cell.

4. Telophase I
– During Telophase I there is some uncoiling of the chromosomes. The nuclear membrane reappears and cytokinesis produces 2 daughter cells.
– 2 new daughter cells are formed, with each daughter cell containing only one chromosome of the homologous pair.
– The 2 new daughter cells each have half the number of chromosomes of the original parent cell.
– Even though the chromosome number is halved, there is still double the amount of chromosomes necessary for the final product. This is why the cell undergoes the 4 stages; prophase, metaphase, anaphase and telophase, one more time in MEIOSIS II.
– Meiosis II reduces the sister chromatid to a single chromosome.

INTERKENSIS
A brief period of rest called INTERKENSIS separates Meiosis I from Meiosis II.

It is sometimes called INTERPHASE II, however no DNA is replicated, because the chromosomes are already doubled.

MEIOSIS II
The 2nd stages of Meiosis are called; Prophase II, Metaphase II, Anaphase II and Telophase II.

These 4 stages ensure that each of the 4 cells created, has a single copy of each chromosome.

Sister chromatids become separated into different nuclei during Meiosis II.

1. Prophase II
– The chromosomes condense and move toward the equatorial plane (the center of the cell), where the centromeres will attach to the spindle fibers.

2. Metaphase II
– The chromosomes line-up along the metaphase plate/equatorial plane.
– The chromosomes attach to the spindle fibers and are moved into position at the center of the cell.

3. Anaphase II
– The centromeres split and the sister chromatids move toward opposite poles.

4. Telophase II
– The chromosomes unwind, the nuclear membrane reforms and the cell divides.
– Cytokinesis ends the process with 4 separate haploid cells forming.
– Both cells from the beginning of Meiosis II were products of a single cell that began at the start of Meiosis I. Since both of these cells divided again, the end result of Meiosis is: From one cell we get 4 cells.
– Each of the 4 cells is a haploid cell (a cell with half the number of chromosomes).

Advertisements