Meiosis and Other Factors Affecting Genetic Variability

Mitosis occurs in somatic tissue while meiosis occurs in gametocytes (germ cells). Mitosis gives rise to two identical daughter cells while meiosis results in up to four non-identical sex cells or gametes. Meiosis introduces genetic variability by genetic recombination. Genetic variability occurs through the action of genetic recombination which itself is as a result of independent assortment and crossing-over. Meiosis differs from mitosis in several key ways.2 Meiosis involves two nuclear divisions with no DNA replication between them. It thus produces four daughter cells, each with half the original number of chromosomes. Also, crossing over occurs in prophase I of meiosis. Mitosis involves the nucleus only dividing once after DNA replication. As a result, it produces two daughter cells, each containing the original number of chromosomes, which are genetically identical to those in the parent cell.

meiosis

 

binary-mitosis-meiosis-comparision

 

Table 1: Differences between mitosis and meiosis

Mitosis Meiosis
no tetrad tetrad formation (pairing of homologous chromosomes) and cross over
daughter cells identical to parent cell daughter cells different from parent cell
diploid (2n) daughter cells haploid (n) daughter cells
1 division involved 2 divisions involved
2 daughter cells 4 sperm cells or 1 egg (with polar bodies

 

Independent assortment generates genetic variation and it occurs during Meiosis I when homologous chromosomes randomly align on the metaphase plate. A cell has one maternal and one paternal copy of each somatic chromosome. Independent assortment shuffles these chromosomes, and then places only one copy of each into the gamete. This way, the gamete may have chromosome 1 from the mother, chromosome 2 from the father, chromosome 3 from the father and so on. The mechanism of independent assortment occurs initially during metaphase I. During metaphase I of meiosis, homologous chromosome pair up along the metaphase line in random orientation. During anaphase I of meiosis, the homologous chromosomes are pulled apart. Those on the left will be put into one daughter cell, those on the right will be put into another. Linkage is the cellular event where the genes coding for different characteristics are passed on together at a higher frequency than would be predicted by probability. Linkage can be explained by examining chromosomes. Each chromosome has many genes located along its length. Homologous chromosomes separate from each other through segregation while non-homologous chromosomes separate from each other independently. Each chromosome has many genes located on it, so these genes are typically inherited as a group. A linkage group is a set of genes located on the same chromosome. This means that they tend to be inherited together. The process of crossing-over, which occurs during prophase I of meiosis I, may split up these linkage groups. Crossing-over happens between homologous chromosomes donated by the mother and the father and results in a mixing of the allele combinations in gametes. This means that the child can have gene combinations not found in either parent alone. Two genes are very close to each other on a chromosome will less likely experience crossing-over between them and wont tend to separate.

Genetic recombination, is the process that introduces genetic diversity into the gametes during meiosis. There are 2 processes that make up recombination: by independent assortment that was previously mentioned and crossing over. Crossing over occurs during prophase I, with the actual site of cross over being the chiasma. The tetrad is a pair of homologous chromosomes, which is formed by a process called synapsis. Note that crossing over occurs between homologous chromosomes and not between sister chromatids of the same chromosome, as sister chromatids are identical and so crossing over would not produce any change.  Single crossovers results in genetic recombination. The chromatids involved in this single crossover exchange alleles at a given locus and it results in 2/4 recombinants.

double-crossover

In double crossovers, chromatids from two similar chromosomes touch each other at two points to exchange material.

double-crossover-2

Double crossover can occur through three scenarios. In the first scenario, the chromatids involved in this double crossover do exchange alleles in the beginning, but then it transfers them back, resulting in no net recombination. This is called the 2-strand double crossover and results in 0/4 recombinants. In the second scenario, alleles are exchanged during a crossover but then one of the crossover chromatid exchanges with a different chromatid than the one in the first exchange. It results in 2/4 recombinants and is termed a 3-strand double crossover. In the third scenario 3 the chromatids exchange, then 2 totally different chromatids on the same chromosome exchange. This is called the 4-strand double crossover and results in 4/4 recombinants. Since precision is crucial in chromosome swapping, formation of the tetrad is highly regulated. Synapsis is mediated by a protein structure called the synaptonemal complex. This structure starts to form early in meiotic prophase I. First, proteins named SYCP2 and SYCP3 attach to each of the two homologous chromatin structures that are to be paired. This makes up the lateral elements of the SC. The lateral regions then align and attach via a central region (made of SYCP1 and many other proteins). Both the lateral and central regions together form the synaptonemal complex, and essentially work like a zipper to connect homologous chromosomes.

Sex linkage occurs when genes are located on the chromosomes that determine the sex of an individual.2 The Y chromosome is much shorter than the X chromosome and has fewer genes for traits than found on the X chromosome. Therefore, the X chromosome has many genes for which there is no matching gene on the Y chromosome. Some genes appear on both the X chromosome and Y chromosome. Other genes, however, are found only on the X chromosome or only on the Y chromosome. Females have two copies of the genes that are found only on the X chromosomes. Because males have both a Y chromosome with few genes on it and the X chromosome, many of the recessive characteristics present on the X chromosome appear more frequently in males than in females, who have two X chromosomes. Unusual sex-linked inheritance patterns occur because certain genes are found on only one of the two sex chromosomes. Genes found only on the X chromosome are said to be X-linked genes. Genes found only on the Y chromosome are said to be Y-linked genes. All the sex-linked alleles are carried on the X chromosome. For sex determination in humans, a chromosome match of XX will give a female while XY will result in a male. Cytoplasmic inheritance is the inheritance of material other than genomic DNA. All cellular organelles, such as mitochondria, are inherited from the mother.

A mutation is any change in the DNA sequence of an organism. They can occur for many reasons, including errors during DNA replication. Mutations can also be caused by external factors, such as radiation, carcinogens, drugs, or even some viruses. It is important to understand that not all mutations cause a change in an organism. If a mutation occurs away from the protein-coding sequence and the DNA sequences that regulate its expression, it is unlikely that the change will be harmful to the organism. On occasion, the changes that occur because of mutations can be helpful and will provide an advantage to the offspring that inherit that change. There are several types of mutation:

i) Random mutation – This is the random changes in DNA sequence. It can be due to radiation, chemicals, replication error or other external forces.

ii) Translation error– even if the DNA for a gene is perfect, errors during translation can cause expression of a mutant phenotype.

iii) Transcription error – Even if the DNA of a gene is perfect, errors during transcription can cause expression of a mutant phenotype.

iv) Base substitution – This mutation involves a base (ATGC) changing to a different base.

v) Inversion – This is when a stretch of DNA (a segment of a chromosome) breaks off, then reattaches in the opposite orientation.

vi) Addition– It is also called insertion. An insertion mutation adds one or more nucleotides to the normal DNA sequence. This type of mutation can potentially add amino acids to the protein and change its function.

vii) Deletion – A deletion mutation removes one or more nucleotides and can potentially remove amino acids from the protein and change its function.

viii) Frameshift mutation – It results from single addition/insertion and deletion mutations. A frameshift mutation occurs when insertions or deletions cause the ribosome to read the wrong sets of three nucleotides. A frameshift results in the ribosome reading the wrong set of three nucleotides on the mRNA. Proteins produced by this type of mutation usually bear little resemblance to the normal protein that is usually produced.

ix) Translocation – This refers to when a stretch of DNA (a segment of a chromosome) breaks off, then reattaches somewhere else.

x) Mispairing – This type of mutation occurs when bases are not paired correctly, such as adenosine not pairing with thymine, or guanine not pairing with cytokine.

An advantageous mutation is one that results in a benefit to the fitness of the organism. For example, the mutation that causes flies to become wingless is advantageous in an environment that is very windy. A deleterious mutation results in a harmful effect to the fitness of the organism. For example, a mutation that causes an organism to be sterile. Inborn errors of metabolism are genetic diseases resulting in faulty metabolism. For example PKU (Phenylketonuria) is an inborn error of metabolism where people can’t metabolize phenylalanine. There is no cure, but the treatment involves avoiding things containing the amino acid phenylalanine. A mutagen is a substance that causes mutation while a carcinogen is a substance that causes a mutation that causes cancer. Carcinogens are almost always mutagens, although some are direct mitogens (increase mitosis). However, not all mutagens are carcinogens.

Genetic drift refers to changes in the composition of the gene pool due to chance. Genetic drift tends to be more pronounced in small populations. The founder effect is a more extreme case of genetic drift in which a small population of a species finds itself in reproductive isolation from other populations as a result of natural barriers, catastrophic events, or other bottlenecks that drastically and suddenly reduce the size of the population available for breeding. Bottleneck refers to a disaster that causes a huge decrease in the population and a depletion of the gene pool. Due to the small size of the breeding group, inbreeding, or mating between two genetically related individuals, may occur in later generations. Inbreeding encourages homozygosity, which increases the prevalence of both homozygous dominant and recessive genotypes. A small population may have increased occurrence of certain traits and diseases as there is a reduction in genetic diversity due to genetic drift, the founder effect, and inbreeding.

 

References

1) Churchill, F. (1974). William Johannsen and the genotype concept. Journal of the History of Biology, 5 -30.

2) Freeman, S. (2011.). Biological Science (6th ed.). Hoboken, NY: Pearson.

3) Hartl DL, C. A. (2007). Principles of population genetics. Sunderland, MA: Sinauer.

4) Baker, J. M. (2005). Adaptive speciation: The role of natural selection in mechanisms of geographic and non-geographic speciation. Studies in History and Philosophy of Science Part C: Studies in History and Philosophy of Biological and Biomedical Sciences, 303 – 326.

5) S. Blair Hedges, S. K. (2003, April). Genomic clocks and evolutionary timescales. Retrieved from Hedges Lab: http://www.hedgeslab.org/pubs/146.pdf

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