Genetics Basics - Lesson 5: Meiosis
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5.1 Meiosis Defined
Meiosis creates gametes (egg and sperm cells).
During meiosis, genetic information is exchanged between the maternally and paternally inherited copies of a pair of chromosomes in order to create new combinations of genes. This process of genetic recombination helps to increase genetic variability within a species. It allows for the transmission of virtually limitless combinations of genes from parent to offspring.
The resulting gametes have 23 new chromosomes, one member of each of the 23 pairs, representing unique combinations of the original maternal and paternal copies.
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5.2 Meiosis Terminology—Homolog
Homologous chromosomes, or homologs, consist of the members of any given chromosome pair. One member of the homolog is the chromosome from the mother and the other member is from the father.
During meiosis, homologs pair together.
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5.3 Meiosis Terminology—Chromatid
A chromatid is formed during meiosis and is one of two identical DNA strands formed after DNA synthesis.
Sister chromatids are from the same chromosome, and they are connected together at the centromere.
The classic drawing of a chromosome shaped like the letter X actually depicts sister chromatids.
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5.4 The Two Parts of Meiosis—Meiosis I
Meiosis is divided into two parts: meiosis I and meiosis II.
In meiosis I, each chromosome in a cell is replicated to yield duplicated sister chromatides for each member of the homologous chromosomes.
Physical contact between chromatids may occur, resulting in the formation of chiasmata (from the Greek khiasma: to cross).
Chiasmata are thought to represent the process of crossing over, or recombination, in which an exchange of DNA between two of the four chromatids occurs.
A chiasma occurs at least once per chromosome pair. Thus, a parental haplotype (the arrangement of many alleles along a chromosome) will not remain intact upon transmission to an offspring but, instead, will become a new combination of the original maternal and paternal haplotypes.
Following the process of crossing over, at least two of the four chromatids become unique, or unlike those of the parent.
The cellular division in meiosis I is called the reduction division because it results in the reduction of the chromosome number from diploid (cell has two copies of each chromosome, one maternal and one paternal chromosome) to haploid (cell has only one copy of each chromosome, either maternally-derived or paternally-derived chromosome). At this stage it is important to remember the remaining chromosomes are comprised of the sister chromatids, which will separate in meiosis II.
5.4.1 Introduction
Most cells reproduce to form identical copies, or daughter cells, for the purpose of growth or repair. This process is called mitosis. However, parental gametes – the sperm and egg cells – are unique from all other cells in that they reproduce through a process called meiosis. The purpose of meiosis is to shuffle genetic information and cut the cellular chromosome number in half, from 46 chromosomes to 23 chromosomes. In this way, when an egg and sperm cell combine during fertilization, the resulting embryo will inherit the appropriate amount of unique genetic information from each parent.
If sperm and egg cells were to divide and keep the same number of chromosomes that other cells do, then the resulting embryo would have twice the normal number of chromosomes. For example, if a human egg and sperm each contained 46 chromosomes, then the resulting embryo would contain 92 chromosomes– a chromosome number not compatible with normal development or survival.
The process of meiosis involves two divisions of the genetic material. The first division is called the reduction division – or meiosis I – because it reduces the number of chromosomes from 46 chromosomes or 2n to 23 chromosomes or n (n describes a single chromosome set). In humans, the reduction division results in two sperm cells each with 23 chromosomes or an egg cell with 23 chromosomes and one discarded polar body. The second division – or meiosis II – is called equational division. The two sperm cells split again producing four sperm cells, each with 23 or n chromosomes. In oogenesis, the egg cell and polar body divide to form the ovum and two additional polar bodies. In this way, the offspring are assured to get one half of the chromosomes from the mother and one half from the father.
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| 1. Metabolically active cell with a diploid nucleus (2n=6). 2. DNA replication occurs. |
5.4.2 Interphase I
A cell that is not dividing is said to be in interphase. It contains 23 pairs of homologous or matching chromosomes – one that was inherited from the person’s mother and one that was inherited from the person’s father. When referring to one of a pair of homologous chromosomes it is common to refer to this as a homolog, for short. During Interphase I, the cell duplicates its nuclear material. Each homolog replicates, forming two identical sister chromatids. At this point in time, there is the equivalent of 4 copies of each chromosome in the cell. In reality, chromosomes are not yet condensed enough to be visible until the end of prophase 1.
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| 1. Chromosomes become shorter and thicker. 2. The nuclear envelope begins to disintegrate. |
5.4.3 Early Prophase I
In prophase I, the chromosomes become shorter and thicker and more easily stained. They can now be viewed underneath a light microscope. The sister chromatids are joined at the centromere(see diagram). The nuclear envelope begins to disappear.
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| 1. Chromosomes have become fully shortened and thickened. 2. Homologous chromosomes lie side by side (in a process called synapsis). 3. The nuclear envelope disappears. 4. Spindle fibers begin to form. |
5.4.4 Late Prophase I
Later in prophase I, homologous chromosomes – for example, the maternal chromosome #1 and the paternal chromosome #1 – line up next to one another in a process called synapsis. An aggregate of RNA and protein holds the paired chromosomes together. Homologous chromosomes switch material in a process known as recombination or crossing over, which is mediated by the formation of chiasmata. The two chromatids attached at the chiasma exchange parts. In this way some of the genes originally on the father’s chromosome are now on the mother’s chromosome and vice versa. This shuffling process helps to create and maintain variation in a species. The nuclear envelope begins to disappear and spindle fibers begin to form at the poles of the cell.
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| 1. Recombination occurs. |
5.4.5 Recombination
Homologous chromosomes switch materials in a process known as recombination, or ‘crossing over’, which is mediated by the formation of chiasmata.
The two chromatids attached to the chiasmata exchange parts. In this way, some of the genes originally on the father’s chromosomes are now on the mother’s chromosomes, and vice versa. This shuffling process helps to create and maintain variation in a species.
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| 1. Pairs of homologous chromosomes migrate to the equatorial plane of the cell. |
5.4.6 Metaphase I
In metaphase I, the pairs migrate to the equatorial plane of the cell. The spindle fully forms and attaches to the centromeres.
“The pattern in which homologs align during metaphase I determine the combination of maternally and paternally derived chromosomes in the progeny cells.
Each chromosome moves independently of all the others – a phenomenon called independent assortment. For example, the copy of chromosome 1 that a gamete receives in no way influences if the same gamete will receive the maternally-derived or paternally-derived chromosome 4.
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| 1. Homologous chromosomes (each consisting of two chromatids held together by a centromere) move to opposite poles. |
5.4.7 Anaphase I
During anaphase I, the homologous chromosomes - each consisting of two sister chromatids held together by a centromere – separate and begin to move to the opposite poles of the cell, pulled by the shrinking spindle fibers.
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| 1. Movement of chromosomes to the poles is completed. (Note: Although there is one of each type of chromosome at each pole, there is a mixture of maternal and paternal chromosomes.) 2. The spindle disappears. 3. Cell division starts (a process called cytokenesis). |
5.4.8 Telophase I
In telophase I, the movement of the homologous chromosomes to the poles is completed. Remember that although there is one of each type of chromosome at each pole, there is a mixture of maternal and paternal chromosomes due to the recombination that occurred during prophase I. The spindle now disappears. Cytokinesis, or cell division occurs, producing two haploid (n) daughter cells, each with only one set of chromosomes (1n). However, the chromosomes are still in their replicated state and consist of sister chromatids. The cells move back into interphase, marking the end of meiosis I.
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5.5 The Two Parts of Meiosis-Meiosis II
In meiosis II, the second phase of meiosis, the process is identical to a mitotic (somatic cell) division, in which genetic material is transmitted equally, identically, and without recombination to daughter cells.
In contrast to a mitotic division, which yields two identical diploid daughter cells, the end result of meiosis is haploid daughter cells with chromosomal combinations different from those originally present in the parent.
In sperm cells, four haploid gametes are produced.
In egg cells, meiosis results in a single haploid gamete, with the remainder of the genetic material lost in the formation of three nonviable polar bodies.
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| 1. The chromosomes uncoil. 2. The nuclear envelope reappears, resulting in two haploid cells (n=3). 3. In contrast to regular cell division (mitosis), this interphase is briefer. There is no DNA synthesis between the first and second meiotic divisions. |
5.5.1 Interphase
In contrast to regular cell division, or mitosis, this second interphase takes less time. The chromosomes uncoil into very thin threads and the nuclear envelope reappears. Most importantly, although proteins are manufactured during this interphase period, the genetic material is NOT replicated a second time.
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| 1. Chromosomes again become thicker and shorter. 2. The nuclear envelope begins to disintegrate. |
5.5.2 Prophase II
Prophase II marks the beginning of the second meiotic division. Meiosis II is similar to mitosis in that each chromosome consists of two sister chromatids attached at the centromere. The goal of meiosis II is to separate the sister chromatids.
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| 3. The chromosomes (consisting of two chromatids -- remember, the homologous chromosomes separated during Meiosis I) begin to move to the equatorial plane. 4. Spindle fibers begin to form. |
5.5.3 Prophase II
In prophase II, the chromosomes again become condensed. The nuclear envelope begins to disintegrate. The chromosomes, consisting of the two sister chromatids only, begin to move to the equatorial plane of the cell. Spindle fibers once again begin to form at the poles.
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| 1. The chromosomes become aligned on the equatorial plane. 2. Fibers of the spindle are reformed. |
5.5.4 Metaphase II
During metaphase II the chromosomes become aligned on the equatorial plane and spindle fibers attach to the centromeres.
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| 1. Centromeres divide, separating the chromatids, which move to the opposite poles as chromosomes. |
5.5.5 Anaphase II
In anaphase II, the centromeres divide, effectively separating the sister chromatids, which move to the opposite poles as they are pulled by the spindle fibers.
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| 1. The movement of the chromosomes to the poles is complete. |
5.5.6 Telophase II
In telophase II, the movement of the chromosomes to the poles is completed and the spindle disappears. In spermatogenesis, nuclear envelopes form around the four nuclei, which then separate into four individual spermatids. In oogenesis, the ultimate result is one ovum and 3 polar bodies, which are discarded.
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| 1. The movement of the chromosomes to the poles is complete. 2. The spindle disappears. 3. Cell division starts. |
5.5.7 Telophase II
The result of meiosis II is the formation of four unique cells, each carrying a new assortment of genes and chromosomes, and each with half the number of original chromosomes.
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| 1. The chromosomes uncoil and resume interphase appearance. 2. The nuclear envelope reappears, resulting in four haploid nuclei (n=3). 3. The second cell division completes, resulting in four haploid cells or gametes. |
5.5.8 Summary
In summary, meiosis is a form of cell division that results in half the normal number of chromosomes in sex cells, or gametes, thus maintaining the chromosome number of a species from generation to generation. Additionally, genetic variability results from the recombination and independent assortment of the genetic material.
5.6 Review of the Genetic Consequences of Meiosis
In meiosis I there is a reduction of the chromosome number from diploid (two separate copies of each chromosome) to haploid (one copy of each chromosome) during the formation of gametes.
There is a random assortment of the homologs.
Recombination (or crossing over) results in additional variety in the genetic material passed on to each cell.
