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MCAT TOPIC REVIEWS & RESOURCES

Biological and Biochemical Foundations of Living Systems

BIO
Meiosis and Other Factors Affecting Genetic Variability

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Significance of meiosis

Meiosis is a special form of cell division that produces gametes for sexual reproduction. Combined with its function of producing gamete cells with 1n number of chromosomes (haploid), meiosis provides opportunity for increasing genetic diversity.

Meiosis begins with replicated chromosomes, each having two sister chromatids, and proceeds as two rounds of cell division by the stages of prophase, metaphase, anaphase, and telophase. The first round, Meiosis I, is a reductive division that reduces the number of chromosomes by half, separating and dividing homologous chromosomes between two daughter cells. The second division is similar to mitosis but performed on haploid cells, separating and dividing replicated sister chromatids between its two daughter cells. Together the two stages of meiosis create a total of four haploid cells from one diploid cell.

Segregation of genes

Early Mendelian genetics observed phenotype ratio patterns in progeny that supported a probabilistic distribution of genetic factors passed on to the next generation. This law of segregation held that alleles for a particular gene are divided among the inherited material passed on to progeny, a principle seen in action in the separation of homologous chromosomes during Meiosis I.

Independent assortment

The law of independent assortment holds that the segregation of one gene does not affect the segregation of another, that they assort independently. Using a Mendelian example, the ratio of flower color among progeny would be independent of the ratio of pea shape due to the independent assortment of their respective genes. This principle too is displayed in the independent separations of homologues for each chromosome.

Linkage

Linkage, the physical presence of two given genes on the same chromosome, raises an exception to independent assortment, whereby the combination of alleles for all genes existing on a chromosome will necessarily segregate together into a daughter cell on account of being a part of the same molecule.

Recombination

Recombination, the crossing over or trading of alleles between two homologous chromosomes that takes place during Meiosis I, in turn, raises an exception to linkage, allowing a means to separate genes differently than if the original chromosomes had stayed intact without cross overs.

Single crossovers

A single crossover event on a chromosome will switch out all of the alleles downstream from the point of crossover for those present on the homologous chromosome. Genes that are closer together are less likely to have a single crossover event between them (and more likely if farther apart).

Double crossovers

A double crossover allows for the chance of a portion of the original combination of alleles along a chromosome to again segregate together, as the ends of the homologues are switched again at some point downstream of the first crossover. Genes that are closer together are even less likely to have a double crossover between them than a single crossover (and also more likely if farther apart).

Synaptonemal complex

Recombination and its crossing over events take place during a physical association of two homologous chromosomes (each also carrying a replicated sister chromatid) in what is called a synaptonemal complex. The crossing of one homologue over another creates a visual X named a chiasma.

Tetrad

Because each of the homologous chromosomes coming together during recombination consists of two sister chromatids, the pair of homologues complexes as a total of four chromatids, a tetrad.

Sex-linked characteristics

Traits traced to gene products of genes on the X- or Y-chromosomes are sex-linked due to their inheritance in relation to the chromosomes themselves and ergo sex determination. Recessive traits of X-linked genes will have a greater propensity to be displayed in the phenotype for males (possessing one X and one Y chromosome) because, only having one copy of the gene removes the possibility for masking by a dominant allele as would be the case in a heterozygous female (having two copies of the X chromosome, one with a dominant allele for the gene in question and one with the recessive allele).

Very few genes on Y chromosome

The Y-chromosome is shorter in comparison to its partner chromosome, the X-chromosome, and has less genes. Because the Y-chromosome is normally inherited in one (male) or zero (female) copies, its alleles will therefore be present as the only copy for each of its genes.

Sex determination

Sex determination in humans relies on inheritance from a female's XX genotype of an X-chromosome and inheritance from a male's XY genotype of either an X- or Y-chromosome , thus making the selection of sex chromosome in the male gamete the deciding factor.

Cytoplasmic/extranuclear inheritance

As a germ cell divides its chromosomes, so too is the cytoplasm and extranuclear material divided between the products of each stage of meiosis. The female derived ovum contains and therefore provides much more cytoplasmic material upon fusion with the smaller male derived sperm, including the contribution of mitochondria carrying maternal mitochondrial DNA.

Mutation

Changes in the genome can and do occur, giving rise to mutation, the raw material for genetic variability.

General concept of mutation — error in DNA sequence

A mutation introduces a change in the DNA, as large as kilobase-sized sequences of nucleotides added or removed, or as small as the switching of a single base. Whether a mutation is advantageous, deleterious, or neutral in its effect is determined by the nature of the mutation's impact on gene product or control and the environment in which an altered phenotype is expressed.

Types of mutations:

  • random — usually occurs as an error of replication or damage by external factors
  • translation error — produces a polypeptide with one or more incorrect amino acids or a truncation
  • transcription error — produces an incorrect mRNA transcript which may be overcome by the volume of mRNA produced for a given round of gene expression
  • base substitution — a single change in a nucleotide along the DNA sequence
  • inversion — a portion of the DNA sequence is inverted
  • addition — a sequence of nucleotides is added, including duplications or gene amplifications
  • deletion — a sequence of nucleotides is removed
  • translocation — a sequence of nucleotides is moved from one place in the genome to another
  • mispairing — an incorrect nucleotide is inserted on one strand of the double-stranded DNA

A mutation in a gene can be classified depending on its effect on its polypeptide product:

  • silent mutations — a change in the nucleotide sequence that ends up encoding for the same amino acids, potentially relying on wobble pairing
  • missense mutations — a change in the nucleotide sequence that encodes for the wrong amino acid
  • nonsense mutations — a change in the nucleotide sequence that encodes for early truncation via a Stop codon
  • frameshift mutations — a change in the nucleotide sequence in a multiple other than three that causes the code to be read off its reading frame position, creating dramatic differences in amino acid sequence or early truncation

Advantageous vs. deleterious mutation

Mutations play a role as the raw material for natural selection because of the potential positive or negative impact they can have on the survival of the individual organism and its chances of reproduction. Whether a specific mutation is advantageous or deleterious will depend not only on the effect the mutation has on phenotype but the environment that phenotype is presented in. A point mutation in the gene for hemoglobin that leads to sickle cell disease (when homozygous) also being protective against malaria (when a heterozygous carrier) illustrates the impact of environment on how a mutation will affect overall survivability of an individual.

Inborn errors of metabolism

A specific class of inherited mutations that affect important metabolic enzymes are called inborn errors of metabolism.

Relationship of mutagens to carcinogens

Mutagens cause mutations which, it has been shown, can be advantageous or deleterious. Generally, in a highly evolved organism, in the face of mutation, there are more opportunities for loss of function (deleterious effects) than for gain in fitness (advantageous effects). Carcinogens are a specific subclass of mutagens that produce deleterious cancer-causing effects.

Genetic drift

Genetic drift, the changes in allele frequencies for genes across a population, can be caused by random fluctuations but produce dramatic changes in the genetic makeup of the population over time.

Synapsis or crossing-over mechanism for increasing genetic diversity

While sexual reproduction in itself introduces an element of genetic diversity by combining homologues from two separate individuals into the genetic makeup for a new individual, the events of synapsis and crossing-over which take place in the production of gametes in each parental individual lend a great deal to increasing genetic diversity by recombining combinations of gene alleles on a single chromosome which otherwise would always segregate together.

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