Biology Test - 61

The genome of an organism is inscribed in DNA, or in some viruses RNA. The portion of the genome that codes for a protein or an RNA is referred to as a gene. Those genes that code for proteins are composed of tri-nucleotide units called codons, each coding for a single amino acid.

In genetics, dominance is the phenomenon of one variant of a gene on a chromosome masking or overriding the effect of a different variant of the same gene on the other copy of the chromosome. The first variant is termed dominant and the second recessive

Tjio and Levan's contribution is very significant because they gave the number of human chromosomes.

Polytene chromosomes were first discovered in the 1880s in insects (Balbiani 1881). One of the earliest published examples of polytene chromosomes is of Chironimus salivary gland chromosomes found in Walther Flemming's 1882 book, Zellsubstanz, Kern, und Zelltheilung (Flemming 1882)

In humans, each cell normally contains 23 pairs of chromosomes, for a total of 46. Twenty-two of these pairs, called autosomes, look the same in both males and females. The 23rd pair, the sex chromosomes, differ between males and females.

Strasburger - He observed thread like structures during cell division. (ii) Balbiani - Described rod like structures in nucleus before cell division. (iii) Waldeyer - Coined the term 'chromosomes'.

The Y chromosome is also acrocentric. In an acrocentric chromosome the p arm contains genetic material including repeated sequences such as nucleolar organizing regions, and can be translocated without significant harm, as in a balanced Robertsonian translocation.

The observable traits expressed by an organism are referred to as its phenotype. An organism's underlying genetic makeup, consisting of both physically visible and non-expressed alleles, is called its genotype.

DNA (deoxyribonucleic acid) is the cell's genetic material, contained in chromosomes within the cell nucleus and mitochondria. Except for certain cells (for example, sperm and egg cells and red blood cells), the cell nucleus contains 23 pairs of chromosomes. A chromosome contains many genes.