3.1. Chromosomes & Genome

WHAT IS A CHROMOSOME?

In the nucleus of each cell, the DNA molecule is packaged into thread-like structures called chromosomes. Each chromosome is made up of DNA tightly coiled many times around proteins called histones that support its structure.

Chromosomes are not visible in the cell’s nucleus—not even under a microscope—when the cell is not dividing. However, the DNA that makes up chromosomes becomes more tightly packed during cell division and is then visible under a microscope. Most of what researchers know about chromosomes was learned by observing chromosomes during cell division.

Each chromosome has a constriction point called the centromere, which divides the chromosome into two sections, or “arms.” The short arm of the chromosome is labeled the “p arm.” The long arm of the chromosome is labeled the “q arm.” The location of the centromere on each chromosome gives the chromosome its characteristic shape, and can be used to help describe the location of specific genes.

CLASSIFICATION OF CHROMOSOMES

The position of the centromere in the chromosome (which is constant to a given chromosome) varies i.e., it may occupy different positions. Based on this, four morphological shapes have been identified in chromosomes. These are:

1. Metacentric:

The Centromere occupies a middle position with reference to the length of the chromosome. The two arms thus resulted are almost equal in length. During anaphasic movement in cell division, metacentric chromosomes ap­pear ‘V’ shaped. Eg: Trillium. Tradescantia etc.,

2. Sub metacentric:

When the centromere is located some distance away from the middle region of the chromosome, the position is said to be median and the chromosome will be shorter than the other. During anaphasic movement, sub metacentric chromosomes appear ‘L’ shaped eg: Human beings.

3. Acrocentric:

In this case, the centromere is situated almost near one end of the chromo­some. As a result, one arm of the chromosome will be extremely short and the other very long. The centromere is said to occupy a subterminal postion. eg: Grass hoppers.

4. Telocentric:

When the centromere is situated exactly at one end, the chromosome will be having only one long arm. Telocentric chromosomes are very rare. Truly telocentric chromosomes have been identified by Marks (1957) in certain plants, protozoa and certain mammals.

Cleveland (1949) has also reported in certain protozoa inhabiting the digestive tract of wood termites, the occurrence of telocentric chromosomes. The earlier report regarding chro­mosome IV of Drosophila melangoaster, being telocentric has been disproved as a second arm has been seen.

WHAT IS A GENE?

A gene is the basic physical and functional unit of heredity. Genes, which are made up of DNA, act as instructions to make molecules called proteins. In humans, genes vary in size from a few hundred DNA bases to more than 2 million bases. The Human Genome Project has estimated that humans have between 20,000 and 25,000 genes.

Every person has two copies of each gene, one inherited from each parent. Most genes are the same in all people, but a small number of genes (less than 1 percent of the total) are slightly different between people. Alleles are forms of the same gene with small differences in their sequence of DNA bases. These small differences contribute to each person’s unique physical features.

WHAT IS THE GENOME?

A genome is all of a living thing’s genetic material. It is the entire set of hereditary instructions for building, running, and maintaining an organism, and passing life on to the next generation.

In short, the genome is divided into chromosomes, chromosomes contain genes, and genes are made of DNA.

The human genome of Homo sapiens is stored on 23 chromosome pairs. 22 of these are autosomal chromosome pairs, while the remaining pair is sex-determining. The haploid human genome occupies a total of just over 3 billion DNA base pairs that means 6 billion base pairs per diploid cell.

How many genes does the Human Genome contain?

The Human Genome Project has revealed that there are probably about 20,000-25,000 ‘haploid’ protein coding genes. The completed human sequence can now identify their locations. But only about 1.5% of the genome codes for proteins, while the rest consists of non-coding RNA genes, regulatory sequences, introns, and noncoding DNA (once known as “junk DNA”).

Surprisingly, the number of human genes seems to be less than a factor of two greater than that of many much simpler organisms, such as the roundworm and the fruit fly — see table showing number of genes for different organisms.

How many genes do other organisms have?

 Organism chromosomes –diploid base pairs genome size (#genes) Reference
fruit fly 8 1.65×108 13,600
Budding yeast 16 12,462,637 6,275
human 46 3.3 x 109 ~21,000
human mitochondria 16,569 13
rice 24 4.66 x 108 46,022 -55,615
dog 78 2.4 x 109 ~25,000
mouse 40 3.4 x 109 ~23,000

Humans though have on average three times as many kinds of proteins as the fly or worm because of mRNA transcript “alternative splicing” and chemical modifications to the proteins. This process can yield different protein products from the same gene. ——–Fruit flies share nearly 60% of human genes and are studied by

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