Meiosis Chapter 8

Chromosome Number Organisms have tens of thousands of genes that determine individual traits. The more closely related two organisms are, the more genes they’ll have in common Genes are lined up on chromosomes that can hold thousands of genes. In body cells of animals and most plants, chromosomes occur in pairs. One chromosome in the pair came from the male parent and one came from the female parent. These pairs are called homologous chromosomes – each pair has genes for the same traits A cell with two of each kind of chromosome is called diploid (2n).

Chromosome Number Organisms produce gametes (sex cells) that contain one of each kind of chromosome. A cell with only one of each kind of chromosome is called haploid (n). Sex cells have one of each kind of chromosome so that when they combine (as egg and sperm do during fertilization), the resulting cell is diploid. Each species has a specific number of chromosomes. Humans have 23 pairs (46 total) Fruit Flies have 4 pairs (8 total) Dogs have 39 pairs (78 total)

Human Chromosomes Humans have 23 pairs of chromosomes (46 total) 22 pairs of autosomes 1 pair of sex chromosomes Half of each pair came from one parent and half came from the other parent

Meiosis: Why? Mitosis divides one diploid cell to form two diploid cells For example: A human cell with 46 chromosomes divides to form two cells with 46 chromosomes. If each parent were to pass on a diploid cell to the offspring, that offspring would then have 4 copies of each chromosome 46 chromosomes from each parent would yield a 92 chromosome offspring Meiosis allows for two divisions to divide a one diploid cell into four haploid cells.

Meiosis: Where and Who? Meiosis takes place in the gonads (sexual organs) For humans, these are the ovaries and testes The process of meiosis produces egg and sperm cells Two gametes come together by fertilization The haploid sperm and egg join to form a diploid zygote

Meiosis Phases Meiosis I Meiosis II Prophase I Metaphase I Anaphase I Telophase I Meiosis II Prophase II Metaphase II Anaphase II Telophase II

Interphase Before Meiosis (just like before Mitosis) the cell must prepare for division: Cells increase in size DNA is replicated Necessary proteins and RNA are synthesized During this phase, chromosomes are not yet visible.

Meiosis: Prophase I Chromosomes become visible Nuclear envelope disappears Centrioles head to opposite poles and spindle forms Homologous chromosomes (one pair of sister chromatids from the mother and one from the father) pair up to form a tetrad The tetrad pairs up so tightly that crossing over occurs

Crossing Over

Meiosis: Metaphase I Spindle fibers attach to the centromeres Tetrads line up along the equator (or middle of the cell) Note that homologous chromosomes line up together along the equator in Meiosis where in Mitosis, they lined up independently to one another.

Meiosis: Anaphase I Homologous chromosomes separate and head to opposite ends of the cell Centromeres DO NOT split – Sister chromatids will stay together until the next division

Meiosis: Telophase I and Cytokinesis Spindle is broken down Chromosomes uncoil Cytoplasm divides into two cells

Meiosis: Prophase II Chromosomes become visible Spindle forms If nuclear membrane reformed after Telophase I, it will break down now

Meiosis: Metaphase II Spindle pulls the sister chromatids to the middle of the cell where they line up along the equator in random order (just as they did during Mitosis)

Meiosis: Anaphase II Centromere of each sister chromosome splits and each sister chromatid heads for an opposite pole

Meiosis: Telophase II and Cytokinesis Nuclei reform (nuclear envelope reappears) Spindle breaks down Chromosomes uncoil Cytoplasm divides into a total of four haploid cells that will become gametes Each cell contains one chromosome from each homologous pair Let’s See it!

Variability Meiosis has a large role in maintaining variability in a species. Through sexual reproduction, offspring are not simply replicas of one organism but a genetic combination of two organisms Crossing over during Prophase I insures that a parent organism can pass on different gametes each time it reproduces, creating a variety of offspring.

Chromosomal Mutations Chromosomal mutations can happen when chromosomes break and do not repair correctly. pg. 306

Chromosomal Mutations Errors can also occur during Meiosis. Sometimes the homologous chromosomes do not separate properly – this is called nondisjunction This results in gametes with either an extra copy of a chromosome or no copy at all. Normal Example Nondisjunction Examples

Types of Nondisjunction Remember: In normal fertilization, a zygote would get one copy of a chromosome from each parent resulting in one pair of each type of chromosome (humans: 23 pairs) Monosomy – when the zygote gets a copy of a chromosome from only one parent so it is missing one chromosome Most zygotes with monosomy do not survive One exception is the case of Turner’s Syndrome Females have only one X chromosome instead of two These people will still have female sexual characteristics but they will generally be underdeveloped

Types of Nondisjunction Trisomy – In this case, the zygote gets one copy of a chromosome from one parent and two copies from the other parent resulting in three copies rather than the normal two copies. Down Syndrome (Trisomy 21) – This person has three copies of the 21st chromosome. This can lead to mental retardation, susceptibility to certain illness or diseases, and a shorter life span Klinefelter's syndrome (XXY) – This person has two copies of the X chromosome as well as a copy of the Y chromosome. This person will be male but may suffer from underdeveloped testicles and infertility.

Identifying Chromosomal Disorders To determine whether or not an organism has the proper number of each chromosome, one can look at a karyotype To make a karyotype a photograph is taken of the paired chromosomes during metaphase These pairs are cut out and arranged in a chart according to length and location of centromere Once arranged, it is easy to see if there are any extra or missing chromosomes This individual has an extra Y chromosome