Meiosis Essay, Research Paper

State that meiosis is a reduction division in terms of diploid and haploid

numbers of chromosomes Outline the process of meiosis including pairing of chromosomes followed by

two divisions which result in four haploid cells In meiosis, the number of chromosomes is halved, the daughter cells receiving

only one of each type of chromosome instead of two. It occurs in the formation

of gametes (sex cells). In the diploid parent cell, chromosomes occur in

homologous pairs (the same size). The parent splits in two

(1stmeiotic division), and the homologous chromosomes of each pair

line up together and then get separated from one another and go into different

cells. The products then divide again (2ndmeiotic division, and the

chromatids are separated and a total of 4 daughter cells result Explain how the movement of chromosomes during meiosis can give rise to

genetic variety in the resulting haploid cells Chromosones are inherited as a group; that is, during cell division they act

and move as a unit rather than independently. The existence of linkage groups is

the reason some traits do not comply with Mendel’s law of independent assortment

(recombination of genes and the traits they control); i.e., the principle

applies only if genes are located on different chromosomes. Variation in the

gene composition of a chromosome can occur when a chromosome breaks, and the

sections join with the partner chromosome if it has broken in the same places.

This exchange of genes between chromosomes, called crossing over, usually occurs

during meiosis, when the total number of chromosomes is halved. Explain that non-disjunction can lead to changes in chromosone number,

illustrated by reference to down?s syndrome Congenital disorder caused by an extra chromosome on the chromosome 21 pair,

thus giving the person a total of 47 chromosomes rather than the normal 46.

Persons born with Down syndrome are characterized by several of the

following: broad, flat face; short neck; up-slanted eyes, sometimes with an

inner epicanthal fold; low-set ears; small nose and enlarged tongue and lips;

sloping underchin; poor muscle tone; mental retardation; heart or kidney

malformations or both; and abnormal dermal ridge patterns on fingers, palms, and

soles. The mental retardation seen in persons with Down syndrome is

usually moderate, though in some it may be mild or severe. Congenital heart

disease is found in about 40 percent of people with Down syndrome.

Most persons with Down syndrome have an extra (third) chromosome–a

condition known as trisomy–associated with the chromosome 21 pair. Almost all

individuals with Down syndrome have this trisomy, but a small number

(perhaps 4 percent) have an abnormality called translocation, in which the extra

chromosome in the 21 pair breaks off and attaches itself to another chromosome.

The cause of the chromosomal abnormalities in Down syndrome remains


State Mendel?s law of segregation The characteristics of an organism are determined by internal factors that

occur in pairs. Only one of a pair of such factors can be represented in a

single gamete. Explain the relationship between mendel?s law of segregation and

meiosis Meiosis is the process that separates allele pairs to create the gametes (sex

cells; sperm, egg) that later fuse together during fertilisation.

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