Cell Division and Genetics: Meiosis, Mitosis, and Alterations
Meiosis, Mitosis, and Early Genetics
4 – Meiosis, mitosis, and early genetics and gene expression mechanisms, genetic alterations: enzymopathies and metabolic diseases, chromosomal anomalies: Down syndrome, Turner, Klinefelter, the “cat’s meow,” Edwards, and Patau.
Of the various forms of reproduction (division to multiply) in living cells, in humans, mitosis and meiosis both occur in interphase, prophase, metaphase, anaphase, and telophase (practice with plasticine models). Chromosomes are in the nucleus and are visible only when dividing the cell, appearing as an “X” or “V” with two chromatids (two DNA chains) joined by the centromere and perceived as “arms.”
The karyotype, or idiogram, shows the chromosomes in each gamete (egg and sperm) are 23 (n), composing a haploid. From the zygote onwards, there are 46 pairs of diploid counterparts (2×23, 2n) equivalent to the parental mother. There is one pair (the 23rd) in women (XX) and males (XY) which are the gonosomal or sex chromosomes; the rest are called autosomes.
Meiosis
Involves sex and is a source of genetic variability, occurring through the formation of gametes (gametogenesis) in the ovaries and testes. From a cell with 23 pairs of chromosomes, four cells are produced, each with 23. In women, one larger gamete (egg) is formed, while three cells degenerate (polar bodies). Meiosis is initiated before birth.
In essence, meiosis involves:
- a) DNA replication in the initial stem cell (zipper model), resulting in chromosomes with two sister chromatids together.
- b) Reductional phase involving the distribution of chromosomes of each pair of homologues (each chromosome with two sister chromatids) into two daughter cells.
- c) Equational stage consisting of the separation of sister chromatids at the centromere and the allocation to a total of four daughter cells, with half the total number of chromosomes compared to the initial stem cell. A special phenomenon called “crossover” involves the exchange of non-sister chromatid segments in the pair of homologues, introducing genetic variation in the daughter cells, called recombination.
Mitosis
Except for mutations (copying errors during DNA replication), mitosis leads to identical cells. It occurs when the zygote divides, during development, and throughout life, replenishing cells and tissues. From a cell with 46 chromosomes, two cells with 46 chromosomes are produced.
In essence, mitosis involves:
- a) DNA replication (zipper model) giving rise to chromosomes with sister chromatids together.
- b) Separation of sister chromatids at the centromere and sharing of new chromosomes, giving two daughter cells with 46 chromosomes.
Genetics
Classic genetics, initiated by Mendel, is based on observing parental traits and analyzing the method and proportions in which they appear in offspring (Mendelian inheritance).
Molecular genetics deals with gene composition, heredity mechanisms (DNA duplication, based on the rack model and base complementarity), and how the genotype (genes) produces observable traits (phenotype) through gene expression stages, including:
- a) Transcription: The message from DNA to mRNA by complementary copy from DNA.
- b) Translation: In the cytoplasm, ribosomes and tRNA synthesize a protein chain when ribosomes “read” the mRNA strand. This can be a structural protein or a metabolic enzyme (leading to the phenotype). There is a correspondence (see the genetic code chart, Dr. Ochoa) between base triplets and amino acids from nutrition that come together to form proteins.
Definitions
- Gene: Localized information (locus) on a DNA segment on a chromosome, resulting in a phenotypic trait.
- Allele: Variants of a gene. The gene for blood group has three alleles: Ia, Ib, i. A diploid has two alleles, one on each chromosome at the same locus. If they are equal, it is homozygous; if different, it is heterozygous.
- Dominant allele: Expressed or imposed allele, written in capital letters.
- Recessive allele: “Dominated” allele, written with the same letter but in lowercase.
- Codominant alleles: Alleles affecting the same trait, resulting in intermediate inheritance, symbolized by the same uppercase letter with distinct subscripts (e.g., AB blood group, genotype Ia Ib).
Two different genes can be on different loci within the same chromosome (linked genes) or on different chromosomes (independent genes). Practical examples and exercises on inheritance can illustrate these concepts.
Genetic Alterations
Diseases caused by abnormal gene information result in dysfunctional enzymes or proteins. These can occur in genes on autosomes and may be due to dominant or recessive alleles (e.g., albinism (aa), phenylketonuria (ff), neurofibromatosis (N-)). They can also occur in gonosomes (sex-linked inheritance, e.g., hemophilia).
Phenylketonuria
A defective enzyme prevents the metabolism of phenylalanine, leading to mental retardation and a urine odor.
Neurofibromatosis
A protein causes multiple abnormalities, including nerve tumors and skin spots.
Albinism
Enzymatic deficiency causing the inability to form melanin.
Chromosomal Abnormalities
Occur due to errors during chromosome division or disjunction in gametogenesis or mitosis (mosaicism). These cause syndromes affecting multiple systems (e.g., trisomy: Down syndrome (21), Edwards syndrome (18), Patau’s syndrome (13); deletions: “cat’s meow” syndrome (deletion on chromosome 5); gonosomal: Klinefelter syndrome (XXY), Turner syndrome (monosomy X)).