Meiosis
Reproduction
When living things reproduce sexually, each parent contributes a cell to make a new organism. Sperm and egg join together, combining their genetic information. Sperm and egg need to be made in a special way, a unique kind of cell division that cuts the genetic information in half
this is called meiosis, and it occurs in the gonad
Life Cycle
In sexually reproducing organisms, cells alternate between having the full amount of genetic materials and having half the amount of genetic material. In humans, for example, body cells, called somatic cells, have twice the genetic material of cells that produce in sexual reproduction, called gametes. Body cells produce gametes by meiosis, and then gametes combine by fertilization to create somatic cells, creating a life cycle.
Somatic cells undergo meiosis to produce gametes: male gametes are called sperm, and female gametes are called eggs.
Sperm and eggs join together in fertilization, creating a first cell called a zygote.
The zygote divides by mitosis to create a multicellular organism.
Counting chromosomes
Gametes have half the genetic material, or half the number of chromosomes, as somatic cells. One complete set of chromosomes is called the haploid number of chromosomes, and is represented by the letter N.
Gametes have one set of chromosomes, making them haploid (1N or N). Somatic cells usually have two sets of chromosomes, making them diploid
Human cells have 23 different kinds of chromosomes. Human gametes have 23 chromosomes because they have one complete set. Human somatic cells have 46 chromosomes because they have two complete sets (2 × 23 = 46) The number of chromosomes sets a cell has is called its ploidy
Homologous Chromosomes
Diploid cells have matching pairs of chromosomes, called homologous chromosomes. For example, when sperm and egg join together to form a human zygote, each gamete contributes on set of 23 chromosomes. Each of the 23 chromosomes is unique and contains genes for certain traits.
homologous chromosomes are a pair of chromosomes that contain the same genes as each other—get one of each pair from mom and the other from dad
Different kinds of chromosomes: recognizing differences
chromosome length: chromosomes can be very different lengths
position of the centromere
staining pattern
the 22 pairs that are exactly the same in men and women are called autosomal chromosomes. Sex chromosomes are different between men and women—-men have an X and a Y, women have two X chromosomes
Meoisis
Following the Plan
Cells that are going to undergo meiosis receive a signal that causes them to leave g1 phase of interphase and enter S phase
During S phase, cells copy all chromosomes, creating replicated chromosomes that have identical sister chromatids attached at the centromere
the cells proceed through G2 and into meiosis
during meiosis, chromosomes are moved around by the spindle
The chromosomes are moved and sorted in very similar ways as they are in mitosis and the same terms, prophase, metaphase, anaphase, and telophase—describe these movements
Overview of meiosis
goal is to cut amount of DNA in half to make haploid gametes
the purpose of meiosis 1 is to separate pairs of homologous chromosomes
the purpose of meiosis 2 is to separate sister chromatids
Meiosis 1
Prophase 1
nuclear membrane breaks down
the nucleoli disappear
homologous chromosomes synapse—stick together, forming a tetrad that allows for crossing over to occur, resulting in genetic variation—tetrad has four arms
chromosomes condense and become visible
spindle attaches to the chromosomes
metaphase 1
pairs lined up in the middle of the cell
anaphase 1
homologous chromosomes separated from each other, and one from each pair goes to opposite sides of the cell
telophase 1
nuclear membranes form in some organisms, creating two nuclei—now haploid
spindle breaks down
cytokinesis occurs
cytokinesis occurs
Meiosis 2
prophase 2
spindle forms in each cell, nuclear membranes break down
metaphase 2:
chromosomes lined up in the middle of the cell
anaphase 2
sister chromatids are separated and move to different ends of the cell
telophase 2
chromosomes uncoil
spindle breaks down
nuclear membranes reform
nucleoli reappear
cytokinesis occurs in both cells, four cells form. In females of some species, only one of the new gametes will become an egg. Other three may break down or become tissue that supports the egg
Crossing Over
When homologous chromosomes synapse, or pair up, during prophase 1 on meiosis, little bits of DNA are switched between the two homologous chromosomes in a process called crossing over
*crossing over helps increase genetic diversity
Types:
Homologous chromosomes are attached all along their length. Proteins called the synaptonemal complex act like glue to stick the homologous chromosomes together
Proteins make small cuts in the DNA backbone of the homologous chromosomes
Proteins reseal the breaks in the DNA backbone, attaching pieces of one homologous chromosome to the other
Why Two Divisions?
genetic variability that results from sexual reproduction makes it more likely that a species will survive
crossing over between homologous chromosomes in prophase 1 creates new patterns of genetic information in the chromosomes
every cell that undergoes meiosis sorts its homologous chromosomes during anaphase 1 independently from all other cells in the gonads undergoing meiosis. Every gamete gets a complete set of chromosomes, but each gamete can have a slightly different assortment of which homologous chromosome it got from each pair
sexual reproduction requires that gametes from one parent combine with gametes from another parent to produce offspring. Fertilization is random and creates many new combinations of genes in offspring
Nondisjunction
Chromosomes that fail to separate correctly undergo nondisjunction. Nondisjunction can occur during meiosis 1 or 2, leading to the creation of gametes that have the wrong number of chromosomes
when a gamete with an abnormal number of chromosomes undergoes fertilization, the result is an aneuploid indivdual