CONCEPT OF GENE

DURGESH SIROHI

M.Sc. IInd sem.

CONTENTS

CLASSICAL CONCEPT OF GENEMODERN CONCEPT OF GENECHEMICAL NATURE OF GENECONCEPT OF GENECOMPLEMENTATION TESTFINE STRUCTURE OF GENEREFERENCES

Classical concept of gene Classical concept of gene was introduced by Sutton (1902)

and was elaborated by Morgan (1913). Bidge (1923), Muller (1927) and others which outlined as follows.

i) Genes are discrete particles inherited in mendelian fashion that occupies a definite locus in the chromosome and responsible for expression of specific phenotypic character.

ii) Number of genes in each organism is more than the number of chromosomes; hence several genes are located on each chromosome.

iii) The genes are arranged in a single linear order like beads on a string.

iv) Each gene occupies specific position called locus. v) If the position of gene changes, character changes. vi) Genes can be transmitted from parent to off springs. vii) Genes may exist in several alternate forms called alleles. viii) Genes are capable of combined together or can be

replicated once during a cell division. ix) Genes may undergo for sudden changes in position and

composition called mutation. x) Genes are capable of self duplication producing their own

exact copies.

Modern concept of gene

i) Genes as unit of transmission or cistron :The part of DNA specifying a single polypeptide chain

is termed as cistron. A cistron can have 100 nucleotide pairs in length to 30,000 nucleotide pairs. It transmits characters from one generation to other as unit of transmission.

ii) Genes as unit of recombination or recon :The smallest segment of DNA capable of being

separated and exchange with other chromosome is called recon. A recon consists of not more than two pairs of nucleotides.

iii) Gene as unit of mutation or muton :Muton is the smallest unit of genetic material which

when changed or mutated produce a phenotypic trait. Thus muton is delimited to a single nucleotide.

The current definition of a gene used by scientific organizations that annotate genomes still relies on the sequence view. Thus, a gene was defined by the Human Genome Nomenclature Organization as “a DNA segment that contributes to phenotype/function. In the absence of demonstrated function a gene may be characterized by sequence, transcription or homology” (Wain et al. 2002). The Sequence Ontology Consortium reportedly called the gene a “locatable region of genomic sequence, corresponding to a unit of inheritance, which is associated with regulatory regions, transcribed regions and/or other functional sequence regions” (Pearson 2006).

THE CHEMICAL NATURE OF THE GENE

DNA

x • Deoxyribonucleic Acid• Double helix• Carries genetic

information• Located in the nucleus• The monomer is a

nucleotide– A phosphate– A ribose sugar– A nitrogenous base

Bases in DNA

• A – adenine• T – thymine• C – cytosine• G – guanine

Pyrimidines

Purines

Nucleotide

It is the units that

make up DNA. Nucleotides are composed of a 5-carbon sugar (deoxyribose), a phosphate group, and a nitrogen base (each nucleotide has only one of the four; A, C, T, G)

What is a polymer?

A polymer is made up of many nucleotides covalently bonded together (polynucleotide). They are chains of macro-molecules linked via covalent bonds.

CONCEPT OF GENE

The existence of genes was first suggested by Gregor Mendel (1822–1884), who, in the 1860s, studied inheritance in pea plants (Pisum sativum) and hypothesized a factor that conveys traits from parent to offspring.

He spent over 10 years of his life on one experiment. Although he did not use the term gene, he explained his results in terms of inherited characteristics.

Beadle and Tatum (1942)--One Gene, One Enzyme

Bread mold Neurospora can normally grow on minimal media, because it can synthesize most essential metabolites.

If this biosynthesis is under genetic control, then mutants in those genes would require additional metabolites in their media.

This was tested by irradiating Neurospora spores and screening the cells they produced for additional nutritional requirements (auxotrophs).

ONE-GENE-ONE POLYPEPTIDEHYPOTHESIS

This hypothesis proposed by Ingram accounts for monomeric, dimeric enzymes & for non enzyme proteins.

According to this hypothesis, a gene specifies a single polypeptide chain.

Complementation testThe complementation test was developed

by American geneticist Edward B. Lewis.A complementation test (sometimes

called a "cis-trans" test) can be used to test whether the mutations in two strains are in different genes. Complementation will not occur if the mutations are in the same gene. The convenience and essence of this test is that the mutations that produce a phenotype can be assigned to different genes.

THE FINE STRUCTURE OF GENE Seymour Benzer (1950-60)

Benzer studied of the rII region of T4:(rII mutant causing rapid lysis of E. coli than wild type)

Two types of traits:

plaque morphology

Host range property

1. Permissive host E. coli B; all (rII- & rII+phages grow.

2. Restrictive host E. coli K12; rII+ recombinants grow.

Recombinants of two rII mutants of T4.

Gene order is determined by frequency of recombinants.

If recombination rate is high, genes are far apart.

If recombination rate is low, genes are close together.

Recombination frequency

EUKARYOTIC GENE

Eukaryotic genes are considerably more complicated than their prokaryotic counterparts. First, a gene is no longer a contiguous stretch of bases between the start codon and a stop codon. It is broken or spliced into coding regions called exons with intervening non-coding sections called introns. Splicing mechanisms in the eukaryotic cell stitch the exons together before translation. Alternative splicing mechanisms allow the exons to be put together in a variety of ways, thus a single gene can code for a variety of proteins. The one gene - one protein mapping that is characteristic of prokaryotes is lost. Second, most DNA in eukaryotes is non-coding; only about 3% codes for proteins. In addition, many of the regulatory signals may be quite far from the start codon.

Genes By definition, a gene includes the entire nucleic acid sequence necessary for the expression of its product (peptide or RNA).  Such sequence may be divided into regulatory region and transcriptional region.  The regulatory region could be near or far from the transcriptional region.  The transcriptional region consists of exons and introns.  Exons encode a peptide or functional RNA.  Introns will be removed after transcription. As shown in the following figure, a typical DNA molecule consists of genes, pseudogenes and extragenic region.   Pseudogenes are nonfunctional genes.  They often originate from mutation of duplicated genes.  Because duplicated genes have many copies, the organism can still survive even if a couple of them become nonfunctional

Figure.  General organization of the DNA sequence.  Only the exons encode a functional peptide or RNA.  The coding region accounts for about 3% of the total DNA in a human cell.

CONCLUSION The classical view of a gene as a unit of hereditary information

aligned along a chromosome, each coding for one protein, has changed dramatically over the past century. For Morgan, genes on chromosomes were like beads on a string. The molecular biology revolution changed this idea considerably. To quote Falk (1986), ‘‘ the gene is neither discrete nor continuous, nor does it have a constant location, nor a clear cut function, not even constant sequences nor definite borderlines.’’

What has not changed is that genotype determines phenotype, and at the molecular level, this means that DNA sequences determine the sequences of functional molecules. In the simplest case, one DNA sequence still codes for one protein or RNA.

REFERENCESKarp, G.(2010).Cell Biology: International

student version, 6th edition. John Wiley & sons, Inc., USA.

Reece, J. B, Urry. L.A, Cain, M.L, Wasserman, S.A, Minorsky, P.V. and R. B.Jackson(2011). Campbell Biology,9th edition. Pearson, Boston New York.

Russell, P.J.(2006). I Genetics: A Molecular Approach. Pearson, San Francisco, Boston New York.

Singh, B.D.(1996). Fundamentals of genetics. Kalyani Publishers, New Delhi.