Do you have ALL your parents' chromosomes?
No, you only received half of your mother's chromosomes and half of your father's chromosomes. If you inherited them all, you would have twice the number of chromosomes that you're supposed to have. Humans typically have 23 pairs of chromosomes. If you received all your parents' chromosomes, you would have 46 pairs!
Introduction to Meiosis
Sexual reproduction combines gametes from two parents. Gametes are reproductive cells, such as sperm and egg. As gametes are produced, the number of chromosomes must be reduced by half. Why? The zygote must contain genetic information from the mother and from the father, so the gametes must contain half of the chromosomes found in normal body cells. When two gametes come together at fertilization, the normal amount of chromosomes results. Gametes are produced by a special type of cell division known as meiosis. Meiosis contains two rounds of cell division without DNA replication in between. This process reduces the number of chromosomes by half.
Human cells have 23 pairs of chromosomes, and each chromosome within a pair is called a homologous chromosome. For each of the 23 chromosome pairs, you received one chromosome from your father and one chromosome from your mother. Alleles are alternate forms of genes found on chromosomes. Homologous chromosomes have the same genes, though they may have different alleles. So, though homologous chromosomes are very similar, they are not identical. The homologous chromosomes are separated when gametes are formed. Therefore, gametes have only 23 chromosomes, not 23 pairs.
Haploid vs. Diploid
A cell with two sets of chromosomes is diploid, referred to as 2n, where n is the number of sets of chromosomes. Most of the cells in a human body are diploid. A cell with one set of chromosomes, such as a gamete, is haploid, referred to as n. Sex cells are haploid. When a haploid sperm (n) and a haploid egg (n) combine, a diploid zygote will be formed (2n). In short, when a diploid zygote is formed, half of the DNA comes from each parent.
Overview of Meiosis
Before meiosis begins, DNA replication occurs, so each chromosome contains two sister chromatids that are identical to the original chromosome. Meiosis (Figure below) is divided into two divisions: Meiosis I and Meiosis II. Each division can be divided into the same phases: prophase, metaphase, anaphase, and telophase. Cytokinesis follows telophase each time. Between the two cell divisions, DNA replication does not occur. Through this process, one diploid cell will divide into four haploid cells.
Overview of Meiosis. During meiosis, four haploid cells are created from one diploid parent cell.
During meiosis I, the pairs of homologous chromosomes are separated from each other. This requires that they line up in their homologous paris during metaphase I. The steps are outlined below:
- Prophase I: The homologous chromosomes line up together. During this time, a process that only happens in meiosis can occur. This process is called crossing-over (Figure below), which is the exchange of DNA between homologous chromosomes. Crossing-over forms new combinations of alleles on the resulting chromosome. Without crossing-over, the offspring would always inherit all of the alleles on one of the homologous chromosomes. Also during prophase I, the spindle forms, the chromosomes condense as they coil up tightly, and the nuclear envelope disappears.
- Metaphase I: The homologous chromosomes line up in their pairs in the middle of the cell. Chromosomes from the mother or from the father can each attach to either side of the spindle. Their attachment is random, so all of the chromosomes from the mother or father do not end up in the same gamete. The gamete will contain some chromosomes from the mother and some chromosomes from the father.
- Anaphase I: The homologous chromosomes are separated as the spindle shortens, and begin to move to opposite sides of the cell.
- Telophase I: The spindle fibers dissolves, but a new nuclear envelope does not need to form. This is because, after cytokinesis, the nucleus will immediately begin to divide again. No DNA replication occurs between meiosis I and meiosis II because the chromosomes are already duplicated. After cytokinesis, two haploid cells result, each with chromosomes made of sister chromatids.
Since the separation of chromosomes into gametes is random during meiosis I, this process results in different combinations of chromosomes (and alleles) in each gamete. With 23 pairs of chromosomes, there is a possibility of over 8 million different combinations of chromosomes (223) in a human gamete.
During crossing-over, segments of DNA are exchanged between sister chromatids. Notice how this can result in an allele (A) on one sister chromatid being moved onto the other sister chromatid.
During meiosis II, the sister chromatids are separated and the gametes are generated. This cell division is similar to that of mitosis, but results in four genetically unique haploid cells. The steps are outlined below:
- Prophase II: The chromosomes condense.
- Metaphase II: The chromosomes line up one on top of each other along the middle of the cell, similar to how they line up in mitosis. The spindle is attached to the centromere of each chromosome.
- Anaphase II: The sister chromatids separate as the spindle shortens and move to opposite ends of the cell.
- Telophase II: A nuclear envelope forms around the chromosomes in all four cells. This is followed by cytokinesis.
After cytokinesis, each cell has divided again. Therefore, meiosis results in four haploid genetically unique daughter cells, each with half the DNA of the parent cell (Figure below). In human cells, the parent cell has 46 chromosomes, so the cells produced by meiosis have 23 chromosomes. These cells will become gametes.
An overview of meiosis.
allele: Alternate form of a gene.
crossing-over: Exchange of DNA between homologous chromosomes that occurs during prophase I of meiosis.
diploid: Having two sets of chromosomes; 2n.
gamete: Reproductive cell, such as sperm or egg.
haploid: Having one set of chromosomes, as in sperm and egg; n.
homologous chromosomes: Pair of chromosomes that have the same size and shape and contain the same genes, but different alleles.
meiosis: Process of cell division during which the chromosome number is halved in order to produce gametes.
mitosis: Division of the nucleus.
sexual reproduction: Process of forming a new individual from two parents.
spindle: Structure that helps separate the sister chromatids during mitosis; also separates homologous chromosomes during meiosis.
zygote: Cell that forms when a sperm and egg unite; the first cell of a new organism.
- Meiosis is a process of cell division that reduces the chromosome number by half and produces sex cells, or gametes.
- Meiosis is divided into two parts: Meiosis I and Meiosis II. Each part is similar to mitosis and can be divided into the same phases: prophase, metaphase, anaphase, and telophase.
- Crossing-over occurs only during prophase I.
- Four genetically unique haploid cells result from meiosis.
Use the resource below to answer the questions that follow.
- How does a diploid organism differ from a haploid organism?
- When does recombination of chromatids occur? What effect does recombination of chromatids have on the diversity of offspring?
- What happens during metaphase I of meiosis? How does this compare to the metaphase of mitosis?
- What is the product of meiosis?
- What is the difference between a haploid cell and a diploid cell?
- Describe the steps of Meiosis I and Meiosis II.
- Describe crossing-over.
- What is the outcome of meiosis?