500 word essay

Alleles and chromosmes

Alleles are different forms of same gene.

Genes are linearly arranged on chromosomes.

Chromosomes contain genetic material of cell i.e. DNA. So chemically alleles, genes,

chromosomes are all DNA !

Explanation:

• Let us start the story at the level of an eukaryotic diploid cell. There is a nucleus, inside

which you see chromosomes in pairs. These are called homologous pairs: they are in

duplicate.

• Each chromosome contains a linear DNA molecule, closely associated with histone

proteins. DNA is genetic material, made of only four different nucleotides arranged in a

double helix. It contains chemical codes which guide life processes, written by using

those four letters.

• Along the length of DNA molecule, only some stretches are 'meaningful' and are called

genes. Hence, several genes are located on same chromosome. Homologous

chromosomes carry same genes, generally in same order.

• Now if we consider one gene, the sequence of genetic letters in one chromosome may

slightly differ from the sequence in another. This is why one gene may have different

alleles. For example, at the genetic locus where gene for our eye colour is located, so

much variations are present that you see various different colours of iris in different

people.

Chromosomes Chromosome proteins, called histones, tightly bind to the DNA double helix. This binding

compresses the long DNA molecules so that they fit within a cell. Humans contain 23 pairs of

chromosomes, and if you unwound all the DNA from a human cell and placed it end to end, it

would exceed six feet in length.

Read more about what a chromosome is?

A single, or haploid, set of chromosomes is stored in each parent’s sex cells. At fertilization, the

new embryo’s cells have a double, or diploid, sets of chromosomes. During cell division, a cell

replicates its complement of chromosomes so that each daughter sell gets the full diploid set.

Genes and DNA Allele Genes appear throughout the length of each chromosome, and each chromosome pair has a

unique set of genes. You can only recognize genes from their information content -- the

sequence of nucleotide bases. Otherwise, genes are indistinguishable from the rest of the

chromosome.

The site of a gene on a chromosome is its locus. You can designate a locus by counting the

number of bases from the beginning of the chromosome to the start of the gene.

Let's look at the allele definition. In a diploid organism, the two corresponding genes in a

chromosome pair, or alleles, might be identical or might have different base sequences. Each

parent contributes one allele in each pair. Some phenotypes -- the physical expression of

genetic information -- require the interaction of several different genes, making the

relationships among alleles more complex.

Dominant and Recessive Alleles In a diploid individual, two identical, or homozygous, alleles express the same trait -- that is, the

same structural protein or enzyme. Heterozygous alleles encode different information for the

same trait. Often, one DNA allele dominates over the other, meaning that its coding determines

the phenotype of the gene.

The cell can only express a recessive trait if both alleles are homozygous for that trait. For

example, the color of a flower might depend on the information stored in the plant’s flower-

color alleles. If red is dominant, the flower can only be some other color if the red DNA allele is

absent. Mutations, which alter the base sequences of alleles, can create evolutionary changes

in a species or even the development of new species, but can also lead to defective offspring.