Lecture 4 Topic: Molecular bases of heredity. Allelic and non-allelic interaction of genes

Before going into the expression, regulation and manipulation of gene, let us review what а gene is. The concept of gene has been changing with progress ш research in genetics and molecular biology.

Classical Definition of а Gene. Mendel was the first to visualize а gene as а unit of inheritance ш 1866. He called it an element, which was termed а factor by Correns in 1900. Mendel postulated that the gametes brought from the parents distinct particulate elements (factors) which made their respective characters appear in the offspring. The word gene was introduced by Johanson in 1909 for а single unit of heredity occupying а specific position (locus) in а chromosome. It was later found that the genes lay in а linear order in the chromosomes and were carried along with them to daughter cell in cell division. Behavior of chromosomes and genes was found to be parallel. This and some more genetic information led to the following classical definition of а gene.

A gene is а unit of inheritance which is carried- from а parent by а gamete in а chromosome and controls the expression of а specific character in the young one ш cooperation with its allele, other genes and environment.

Modern Definition of the Gene. The above definition of а gene has become inadequate in view of the new ideas about the gene and its role revealed by intensive research in the last half а century. The following definitions emerged from the outcome of (his research.

А unique sequence of Deoxyribonucleotid pairs. It has been found that DNA is the hereditary material. It is composed of а linear series of deoxyribonucleotide pairs which form а chromosome. Therefore, а gene is regarded а segment of DNA having а limited number of nucleotide pairs in а unique sequence. Different genes have different sequences of nucleotide pairs. The specific sequence of bases in а gene forms the code that directs the cell to manufacture а particular protein.

А unit of recombination. From the study of crossing over, Morgan postulated that а gene was the shortest segment of а chromosome which could be separated from adjacent segments by crossing over. This definition regards the gene а large unit, а specific region of the chromosome determining а distinct trait in the organism.

А unit of mutation. Study of mutation shows that а change in a trait may result from an alteration in the structure of а chromosome, а sequence of bases, or even а single base. Thus, а gene may be defined as the shortest segment of а chromosome which can undergo mutation.

А code for а single protein (enzyme). In 1948, Beadle and Tatum proposed their famous one gene-one enzyme (protein) hypothesis, and considered gene as а unit of hereditary material which codes for the formation of а single protein (enzyme).

А code far one polypeptide. Later, it was found that all proteins do not act as enzymes, some proteins have structural role in cells and some function as membrane receptors. Moreover, certain proteins are composed of more than one polypeptide chains. As a. result of these findings, the Beadle and Tatum hypothesis has been replaced by one gene-one polypeptide chain principle. It visualizes gene as а unit of hereditary material that codes for the synthesis of a single polypeptide.

А unit of Function. 1п 1960s, genes were found to code for rRNA and tRNA besides polypeptides. This discovery made the above concept too narrow to define genes adequately. Regulatory genes or regions, such as the operators of prokaryotic DNA, were also discovered. These genes are never transcribed. Besides these, some viruses have overlapping genes which code for more than one polypeptide. In addition, certain viruses and higher organisms have genes which code for long polyproteins. The latter are then cleaved by proteolytic enzymes into many separate proteins. This means а part of gene can code for а protein. Apart from this, а eukaryote gene may have noncoding introns between coding exons. These researches have led (о the concept that а gene is а segment of DNA molecule that codes for а unit of function. The function nay be to code for а polypeptide, а polyprotein, ribosomal or transfer RNA, or to regulate the activity of other functional units within the DNA.

А segment of nucleic acid. Some RNA viruses can form genomic RNA and mRNA directly without requiring а DNA intermediate. To cover these viruses, а gene may be defined as а segment of nucleic acid molecule responsible for the synthesis of а specific product.

So our concept of а gene has greatly changed from the "factor" of Mendel to the "unique sequence of nucleotide base pairs" of the modern geneticists.

Synthetic Definition. According (о the modern concept, а gene

a) is а segment of nucleic acid, usually DNA, rarely RNA;

b) has а unique sequence of nucleotide base pairs;

c) codes for а specific polypeptide, or one rRNA, or one tRNA, or а

polyprotein or has а regulatory role;

d) can undergo crossing over and mutation at times;

e) may have continuous or split information.

Even this elaborate definition lacks percision. It does not cover the cases in which one gene influences traits, or many genes {polygenes) determine one trait. Overlapping genes of certain viruses are also not covered by it.

Benzer ш 1955 introduced the concept of а cistron as a unit of function. A cistron is a segment of DNA carrying information for the production of; а polypeptide chain.

While the concept of а gene changes with new findings, its definition as а unit of heredity is still valid.