This passing of genes from one generation to the next is called heredity. Simple organisms pass on genes by duplicating their genetic information and then splitting to form an identical organism. More complex organisms, including humans, produce specialised sex cells (gametes) that carry half of the genetic information, then combine these to form new organisms. The process that produces gametes is called meiosis.
You may have read our article on gene mutations and discovered how variations in the DNA base sequence result in genetic variety. You will study about the role of meiosis and random fertilization in the generation of genetic diversity in this section.
In humans, one diploid cell (with 46 chromosomes or 23 pairs) goes through two cycles of cell division but only one round of DNA replication. As a consequence, four haploid daughter cells known as gametes or egg and sperm cells (each with 23 chromosomes – one from each pair in the diploid cell) are produced.
A zygote is formed when an egg cell and a sperm cell unite during conception (46 chromosomes or 23 pairs). This is a new individual’s first cell. The halving of chromosome numbers in gametes guarantees that zygotes have the same amount of chromosomes from generation to generation. This is crucial for sustained sexual reproduction across many generations.
Interphase: Replication of DNA in preparation for meiosis. After replication, each chromosome becomes a structure comprising 2 identical chromatids.
Prophase I: The chromosomes condense into visible X shaped structures that can be easily seen under a microscope, and homologous chromosomes pair up. Recombination occurs as homologous chromosomes exchange DNA. At the end of this phase, the nuclear membrane dissolves.
Metaphase I: Paired chromosomes line up along the middle of the cell.
Anaphase I: The pairs of chromosomes separate and move to opposing poles. Either one of each pair can go to either pole.
Telophase I: Nuclear membranes reform. Cell divides and 2 daughter cells are formed, each with 23 chromosomes.
Prophase II: There are now 2 cells. DNA does not replicate again.
Metaphase II: Individual chromosomes line up along the middle of the cell.
Anaphase II: The chromosome copies (chromatids) separate and move to opposing poles.
Telophase II: Nuclear membranes reform. There are 4 new haploid daughter cells. In males, 4 sperm cells are produced. In females, 1 egg cell and 3 polar bodies are produced. Polar bodies do not function as sex cells.
During fertilization, one gamete from each parent joins together to produce a zygote. Each gamete in meiosis has a unique set of DNA due to recombination and independent assortment. The resultant zygote has a one-of-a-kind mix of genes.
During prophase I, recombination or crossing over occurs. Homologous chromosomes – one inherited from each parent – pair gene by gene along their lengths. Breaks occur throughout the chromosomes, and they reunite, exchanging genes. The chromosomes now contain a one-of-a-kind mix of genes.
Meiosis involves two rounds of cell division but only one round of DNA replication. As a consequence, four haploid daughter cells known as gametes are produced.
During meiosis, the chromosomes travel randomly to distinct poles, a process known as independent assortment. After meiosis, a gamete will have 23 chromosomes, but independent assortment implies that each gamete will have one of many distinct chromosomal combinations.
This reshuffling of genes into unique combinations enhances genetic variety in a population and explains why siblings with the same parents differ.
Crossing over and independent segregation produce genetic variation during meiosis. Random fertilization adds to genetic diversity after meiosis is complete. We’ll go through each of these events in depth below.
Crossing over is a process that happens solely during prophase I of meiosis I and includes the exchange of portions of DNA between homologous chromosomes. A chromatid wraps around the homologous chromatid of the opposite chromosome, allowing these portions of DNA to ‘break’ off and switch between the pair to form recombinant chromatids. As novel gene combinations are produced, alleles are switched or new alleles are created!
In meiosis I and meiosis II, independent segregation occurs (metaphase I and metaphase II). This shows how chromosomes might assemble along the metaphase plate, resulting in massive genetic variety. This process is completely random, and we utilize math to show how much genetic variety is introduced.
A pair of homologous chromosomes is made up of two separate chromosomes. As a result, the number of potential alignments along the metaphase plate is 2n, where n represents the number of homologous chromosomal pairs in a cell. This gives us 223 potential possibilities in a human cell, which is more than 8 million.
Random fertilization produces genetic variety in the same way that sexual reproduction requires the random fusing of two gametes, all of which are genetically distinct owing to crossing over and individual segregation. This results in enormously diverse genetic uniqueness combinations in creatures that reproduce sexually. We utilize algebra once again to compute the number of distinct chromosomal combinations that may result from random fertilization.
We computed almost 8 million potential chromosomal combinations after crossing over and independent segregation. Because sexual reproduction requires the fusing of two gametes, we have (223) 2 combinations, which equals 70 trillion!
Chromosomal mutations are alterations in the structure or number of chromosomes. Non-disjunction is a frequent chromosomal mutation that occurs during meiosis. The inability of chromosomes to divide evenly during the anaphase stage of nuclear division is referred to as non-disjunction. Because this is a natural occurrence, the ensuing gametes will not contain the anticipated number of chromosomes.
Polyploidy and aneuploidy are the two primary consequences.
Polyploidy occurs when homologous chromosomes fail to split during meiosis. This results in gametes with more than two sets of chromosomes, such as triploid (three sets of chromosomes) or even tetraploid cells (four sets of chromosomes). Polyploidy is a typical process in plants that causes an increase in gene expression as well as physical alterations such as cell expansion. Polyploidy is highly uncommon and fatal in humans, however polyploid cells may arise in certain instances.
Aneuploidy occurs when sister chromatids fail to split during meiosis, resulting in gametes with one extra or one less chromosome. This often results in genetic abnormalities, such as Down syndrome. When a gamete with one additional chromosome at location 21 fuses with a normal gamete, a zygote with three copies of chromosome 21 is formed.
During meiosis I prophase, double-chromatid homologous pairs of chromosomes cross across and often swap chromosomal segments. This recombination produces genetic variety by enabling genes from both parents to intermix, resulting in chromosomes with distinct genetic complements.
The chromatids do not split during meiosis I, therefore each daughter cell obtains just one copy of each chromosome, the haploid number, and each copy includes two chromatids. The chromatids separate and are dispersed to each resultant gamete during meiosis II.
1 response Meiosis increases genetic variation. This is due to the fact that it generates four daughter cells, none of which are genetically identical, while mitosis generates two identical daughter cells (which are identical to the parent cell).
Fertilization of gametes from parents occurs when gametes from two parents combine and create an embryo. This embryo then develops into a new person. Because the genetic material originates from two separate people, this method increases genetic variety in the child.
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