Recognizing the significance of meiosis to sexual reproduction

Meiosis - The Genetics of Reproduction Genetics and Evolution The genetic information found in DNA is essential in creating all the characteristics of an organism. This remains the case when passing genetic information to offspring that can occur via a process called meiosis where four haploid cells are created from their diploid parent cell. For a species to survive, and genetic information to be preserved and passed on, reproduction must occur. This can be done by passing on the information found in the chromosomes via the gametes that are created in meiosis.

Chromosome Complement Humans are diploid creatures, meaning that each of the chromosomes in our body are paired up with another. Haploid cells possess only one set of a chromosome. For example, a diploid human cell possesses 46 chromosomes and a gamete (Sex Cell) created by a human is haploid, it possesses 23 chromosomes.

Reproduction Reproduction occurs in humans with the fusion of two haploid cells (gametes) that create a zygote. The nuclei of both these cells fuse, bringing together half the genetic information from the parents into one new cell that is now genetically different from both its parents. This increases genetic diversity, as half of the genetic content from each of the parents brings about unique offspring, which possesses a unique genome presenting unique characteristics. Meiosis as a process can increase genetic variation in many ways.

Chromosome Structure A chromosome is an organized structure of DNA and protein found in cells. It is a single piece of coiled DNA containing many genes, regulatory elements and other nucleotide sequences. Chromosomes also contain DNA-bound proteins, which serve to package the DNA and control its functions.

Homologous Chromosomes Each chromosome has a match, called a homolog. This is why normal organismsalways have an even number of chromosomes. One homolog you received from your mother, the other you received from your father. They are not exactly alike, but they are the same size, shape, and have the same banding pattern.

Sex Chromosomes In humans, there are two chromosomes that determine sex. The X and the Y chromosome. Reproduction occurs in humans with the fusion of two haploid cells (gametes) that create a zygote. The nuclei of both these cells fuse, bringing together half the genetic information from the parents into one new cell that is now genetically different from both its parents. This increases genetic diversity, as half of the genetic content from each of the parents brings about unique offspring, which possesses a unique genome presenting unique characteristics. Meiosis as a process can increase genetic variation in many ways. If you have an XX - you are female If you have an XY - you are male Diploid vs Haploid Body cells have the full set of chromosomes – they are DIPLOID (In humans, 46) Sex cells (sperm and eggs) have half a set – they are HAPLOID (In humans, 23) GAMETOGENESIS - process of creating gametes (Spermatogenesis & Oogenesis)

The Process of Meiosis The process of meiosis essentially involves two cycles of division, involving a gamete mother cell (diploid cell) dividing and then dividing again to form 4 haploid cells. These can be subdivided into four distinct phases which are a continuous process. 1st Division • Prophase - Homologous chromosomes in the nucleus begin to pair up with one another and then split into chromatids (one half of a chromosome) where crossing over can occur. Crossing offer can increase genetic variation. • Metaphase - Chromosomes line up at the equator of the cell, where the sequence of the chromosomes lined up is at random, through chance, increasing genetic variation via independent assortment. • Anaphase - The homologous chromosomes move to opposing poles from the equator • Telophase - A new nuclei forms near each pole alongside its new chromosome compliment. At this stage two haploid cells have been created from the original diploid cell of the parent.

2nd Division • Prophase II - The nuclear membrane disappears and the second meiotic division is initiated. • Metaphase II - Pairs of chromatids line up at the equator • Anaphase II - Each of these chromatid pairs move away from the equator to the poles via spindle fibres • Telophase II - Four new haploid gametes are created that will fuse with the gametes of the opposite sex to create a zygote. Overall, this process of meiosis creates gametes to pass genetic information from parents to offspring, continuing the family tree and the species as a whole. Each of these gametes possesses unique genetic information due to situations in meiosis where genetic diversity is increased.

Key points of Meiosis The process results in 4 daughter cells. Daughter cells are haploid (N). Daughter cells have unique combinations of chromosomes. Daughter cells do not have homologous pairs. Meiosis creates gametes (sperm and eggs). Meiosis ensures variability in offspring. Gametes combine to create a zygote which is diploid (2N) - process of sexual reproduction.

Independent Assortment and Crossing Over - Genetics and Evolution The process of meiosis results in 4 haploid gametes created from the parent cell. Half the genetic information from a parent is present in these haploids, which fuse with gametes of the opposite sex to create a zygote, with a complete chromosome compliment that will create offspring after prolonged growth. The process of meiosis increases genetic diversity in a species. The sex organs which produce the haploid gametes are the site of many occurrences where genetic information is exchanged or manipulated.

Independent Assortment of Chromosomes Alleles for a particular phenotype determine what characteristic an organism will express, as with the following example Mendel’s law of independent assortment applies only to traits carried on different chromosomes, i.e. unlinked genes Independent assortment occurs as a result of the alignment of homologs during metaphase I, determining which maternal and paternal chromosomes assort to each daughter cell Each pair of alleles separates independently of every other pair of unlinked alleles.

The previous diagram indicates that even though the two homologous chromosomes contain the same genetic information, the assortment of the chromosomes (the order they lie in) can determine what genetic information is present in each of the 4 gametes produced. With 23 chromosomes in a human gamete, their are 223 combinations (8388608 combinations).

Crossing Over During meiosis, when homologous chromosomes are paired together, there are points along thechromosomes that make contact with the other pair. This point of contact is deemed the chiasmata, andcan allow the exchange of genetic information between chromosomes. This further increases geneticvariation. There are also many other ways in which genetic variation is increased in a species gene pool, all of which are described in the following pages.