Science: Meiosis

Gametes

These are sex cells, and they contain half the regular number of chromosomes (haploid number - n). Examples: sperm cells (male gamete - produced in the male gonad, the testes) and ova pr ovum (female gamete - produced in the female gonad, the ovary). "N" is the number of chromosomes in a set. Polyploid cells have more than two chromosomes in each set. They undergo meiosis.

Monosomy X (Turner's syndrome)

1:5000 live births; the only viable monosomy in humans - women with Turner's have only 45 chromosomes!!! XO individuals are genetically female, however, they do not mature sexually during puberty and are sterile. Short stature and normal intelligence. (98% of these fetuses die before birth)

Klinefelter syndrome

47, XXY males. Male sex organs; unusually small testes, sterile. Breast enlargement and other feminine body characteristics. Normal intelligence. B. 47, XYY males: Individuals are somewhat taller than average and often have below normal intelligence. At one time (~1970s), it was thought that these men were likely to be criminally aggressive, but this hypothesis has been disproven over time. 3. Trisomy X: 47, XXX females. 1:1000 live births - healthy and fertile - usually cannot be distinguished from normal female except by karyotype

Ultimate Goal of Meiosis

The ultimate goal is to allow for sexual reproduction by producing haploid daughter cells, or gametes. A zygote will eventually result from fertilization. Main functions: 1. increasing variation: crossing over, independent assortment. 2. Keeping chromosomes from doubling and re-doubling.

Meiosis II

No interphase occurs because there is no more DNA replication, so it's very short. Meiosis II is similar to mitosis. The four phases of mitosis II: Prophase II - It's the same as prophase in mitosis. Metaphase II - it's the same as metaphase in mitosis (sister chromatids align at the metaphase plate. Anaphase II - It's the same as anaphase in mitosis (sister chromatids separate). Telophase II - It's the same as telophase in mitosis (nuclei form as the nuclear envelope reappears and cytokinesis occurs and four non identical haploid (n) daughter cells (gametes) are produced).

Down Syndrome

People with Down syndrome are 47, 21+. Down syndrome affects 1:700 children and alters the child's phenotype either moderately or severely: characteristic facial features, short stature; heart defects susceptibility to respiratory disease, shorter lifespan prone to developing early Alzheimer's and leukemia often sexually underdeveloped and sterile, usually some degree of mental retardation. Down Syndrome is correlated with age of mother but can also be the result of nondisjunction of the father's chromosome 21.

Meiosis

The process by which gametes with half the number of chromosomes are produced, diploid cells are reduced to haploid cells. If meiosis did not occur, the chromosome number in each new generation would double, and the offspring would not be able to survive with the extra chromosomes. It is two cell divisions, but dividing 3 times technically (meiosis I and meiosis II) but with only one duplication of chromosomes. In males, it is spermatogenesis (primary and secondary spermatocytes - produces 4 sperm). In females, it is oogenesis (primary and secondary oocytes - produces one egg, other 3 polar bodies die).

Somatic Cells

They are the body cells, and they contain the normal number of chromosomes (diploid number - 2n). These cells are also called autosomes. Examples: skin cells and nerve cells.

Mitosis vs. Meiosis

They make a different amount of cells (mi - 2, me - 4) different kinds of cells (mi - diploid, me - haploid) number of divisions (mi - 1, me - 2 (technically 3)) number of chromosomes (mi - 46, me - 23) meiosis is also different in males and females, but mitosis is the same for everyone. no synapsis in mitosis

Edward's syndrome (trisomy 18)

almost every organ system affected 1:10,000 live births. Children with full Trisomy 18 generally do not live more than a few months.

Monosomes and Trisomies

Monosomy - one pair of chromosomes are missing a chromosome. Example: Monosomy 23 - only has one chromosome in the 23rd set of chromosomes (Turner's Syndrome (X)) Trisomy - one pair of chromosomes has an extra chromosome. Example: Trisomy 18 - has an extra chromosome in the 18rd set (Edward's Syndrome). Example: Trisomy 21 - has an extra chromosome in the 21st set (Down's Syndrome). Example: Trisomy 23 - has an extra chromosome in the 23rd set (XXY) (Klinefelter's Syndrome).

What's the relationship of chromosomes and DNA?

Chromosomes are made of DNA.

Telophase I

Each pole now has a haploid set of chromosomes. Cytokinesis occurs and two haploid cells are formed.

Homologous Chromosomes

Homologous chromosomes are a pair of chromosomes (maternal and paternal) that are similar in shape and size. Homologous pairs (tetrads) carry genes controlling the same inherited traits. Each locus (position of a gene) is in the same position on homologues. Humans have 23 pairs of homologous chromosomes (22 autosomes, and 1 pair of sex chromosomes). Each homologous chromosome has 4 chromatids. We have 23 pairs of homologous chromosomes. Autosomes are sets 1-22, and they are the code for most of the offspring's traits. Sex chromosomes are the 23rd set, and they are the code for the sex of the offspring. XX = female, XY = male.

Anaphase I

Homologous chromosomes separate and move towards the poles. Sister chromatids remain attached at their centromeres.

Diploid Cells and Diploid Number

If an organism has the diploid number (2n), it has two matching homologues per set, one from the mother with the mother's DNA, and one from the father with the father's DNA. Most organisms are diploids. Humans have 23 sets of chromosomes, with 46 total chromosomes (per cell), so a human's diploid number is 46. Each species has a set chromosome number. Examples: a pineapple has 25 sets of chromosomes, a fruit fly has 4 sets of chromosomes, a dog has 39 sets of chromosomes, and a hedgehog has 45 sets of chromosome.

Fertilization

In ovulation, ovum is released from the ovary and transported to an area where fertilization occurs. Fertilization is the joining of the sperm and ovum, and it occurs in the fallopian tube in humans . The result of fertilization is a zygote, or a fertilized egg. The fusion of a sperm (from the father, 23 chromosomes, haploid cell) and an egg (from the mother, 23 chromosomes, haploid cell) forms a zygote (46 chromosomes, diploid cell).

Karyotype

It is a picture of an individual's chromosomes. Making a karyotype can help analyze amniocentesis.

Non-Disjunction

It is the failure of homologous chromosomes, or sister chromatids, to separate during meiosis. Nondisjunction results in the production of zygotes with abnormal chromosome numbers. An abnormal chromosome number (abnormal amount of DNA) is damaging to the offspring.

Four Phases of Meiosis I

It reduces the chromosome number by ½ . Four phases: Prophase I Metaphase I Anaphase I Telophase I

Amniocentesis

It's a procedure a pregnant woman can have in order to detect some genetic disorders such as nondisjunction. Amniotic fluid is sampled to be used for chromosomal studies in amniocentesis.

Interphase I

It's similar to mitosis interphase. Chromosomes replicate in S phase. Each duplicated chromosome is two identical sister chromatids attached at their centromeres. Centrioles also replicate. Nucleus and nucleolus are visible.

Metaphase I

It's the shortest phase Tetrads align on the metaphase plate. Independent assortment occurs - the orientation of homologous pair to the poles is random, allowing for variation. The formula is 2n. For example 2n = 4 is n = 2. So, 22 = 4 combinations.

Prophase I

Longest and complex phase. 90% of meiosis is spent in prophase I spindle fibers form, centrioles migrate to the poles, and nuclear membrane dissolves. Chromosomes condense, and synapsis occurs (when homologous chromosomes come together to form a tetrad. Tetrad - a pair of homologous chromosomes, or four chromatids (sister and nonsister chromatids). Crossing over occurs, and it is when segments of non-sister chromatids break and reattach to the other chromatid. The chiasmata (chiasma) are the sites of crossing over. It creates variation in the offspring's traits. Example: a cells with 20 chromosomes (40 chromatids) at the beginning of meiosis would produce a cell with 10 chromosomes.