Meiotic Cell Division and Sexual Reproduction General Biology Name_____________________

What is Meiosis? Meiosis is a type of cell division in which sexually reproducing organisms produce sex cells that contain HALF of the normal number of chromosomes for that organism. At the time of fertilization, each genetically unique egg and sperm will provide half of the genetic instructions to form a unique offspring. Meiosis guarantees genetic variation in offspring created by sexual reproduction.

Asexual vs. Sexual Reproduction

Where does meiosis take place? Start with a cell that has the full number of chromosomes typical for this species – this is known as a DIPLOID cell. A diploid cell is one that contains one or more PAIRS of chromosomes, carrying paired genes that determine the traits o the organism. Meiosis occurs only in certain cells of the organism, such as the ovaries or testes in a human.

Paired chromosomes DNA replication occurs in preparation for meiosis, so the chromosomes are double armed, each DAC consisting of two identical sister chromatids. A matching pair of chromosomes is called a HOMOLOGOUS PAIR. The pair has a maternal homolog and a paternal homolog that were inherited from that organism’s mother and father. Cells with paired chromosomes are DIPLOID (2n). Cells without paired chromosomes are HAPLOID (n).

Homologous Pairs Same size Same shape Genetic instructions for Blood type A from mother Genetic instructions for Blood type B from father Same size Same shape Same pattern of bands when stained Have matching genes in the same locations BUT the specific form of each gene (called an ALLELE) may differ

Karyotyping Scientists can prepare a karyotype from a body cell of an adult or baby to look for chromosomal abnormalities (damaged, missing or extra chromosomes).

Meiosis Involves Two Cell Divisions During he first cell division (IPMATC-1) we start with a diploid cell that contains double armed chromosomes. The double armed chromosomes line up in pairs along the equator and they may swap some genes. Then the pairs are pulled apart. The cell divides into two haploid cells containing DAC. During the second cell division (IPMATC-2) we start with a haploid cell that contains double armed chromosomes. The DAC line up single file along the equator, then the sister chromatids are pulled apart. Each cell divides into two haploid cells containing single armed chromosomes.

Meiosis

Meiosis in Males (Spermatogenesis) One full cycle of meiosis produces FOUR haploid cells of equal size. In humans these cells will develop into functional sperm cells as they mature in the testes.

Meiosis in Females In humans the egg will mature in the ovary. One full cycle of meiosis produces FOUR haploid cells but they are NOT equal in size. Since the cytoplasm divides unequally, the result is one large egg cell and three small nonfunctioning cells called polar bodies. In humans the egg will mature in the ovary.

Meiosis as a Source of Variation SORTING OF GENES (“Independent Assortment”) The two members of each homologous pair may carry different alleles, so the way the different pairs randomly line up in relation to other pairs leads to many possible combinations in the sex cells that result.

Meiosis as a Source of Variation CROSSING OVER As homologous chromosomes pair up during Meiosis 1, they can “swap” or exchange genes. After separation each set is unique so there are no two sperm or egg cells, even from the same parent, that are alike

Meiosis as a Source of Variation RECOMBINATION - Each sperm and egg contain a unique combination of genetic instructions due to meiosis At fertilization the full species number of chromosomes is restored. This unique combination of thousands of genes at fertilization produces an offspring that is genetically similar (but not identical) to the parents.

When Meiosis Goes Wrong Trisomy = having three copies (instead of two) of a chromosome Ex. Down Syndrome (Trisomy 21) Monosomy = having one copy (instead of two) of a chromosome Ex. Turners Syndrome (Females inherit only one sex chromosome) Deletion = when part of a chromosome is missing Ex. Cri-du-Chat (deletion of part of Chromosome #5) Nondisjunction is the failure of chromosomes to separate properly during Meiosis 1 or 2. The resulting sex cells may have an extra or missing chromosome. Most of the time, the consequences are so severe that a resulting zygote cannot survive.

At Fertilization, two haploid gametes join to form a diploid ZYGOTE

In Humans…

MITOSIS MEIOSIS Body cells (somatic) Sex cells (gametes) 1  2 1  4   MITOSIS MEIOSIS Kinds of cells produced Body cells (somatic) Sex cells (gametes) Number of cells produced 1  2 1  4 Chromosome number of parent cell Diploid (2n=46) Chromosome number of daughter cells Haploid (n=23) Number of times the DNA is replicated 1 Number of cell divisions 2 Function 1) To form new cells during growth or to repair damaged cells. 2) To produce a new organism during asexual reproduction. To form gametes for sexual reproduction.