Gene concept

Gene concept AGB 121 Dept. Of AGB Veterinary College, Hebbal, Bangalore

Classical Genetics – Mendelian genetics • Fundamental principles underlying transmission of genetic traits

– Forward genetics • Identify or generate phenotypically mutant organisms • Determine mode of inheritance and interactions with other mutants • Identify mutated gene(s) • Correlate observed phenotype with biochemical mechanisms Dr R Jayashree, Asst. Prof(AGB) Veterinary College, Bangalore

• In 1909, the Botanist Wilhelm Johannsen coined the term "gene" to describe the heritable characters discussed by Mendel.

• 1907 -1919, the geneticist Thomas Hunt Morgan • and his colleagues conducted experiments using the fruit fly Drosophila melanogaster.

• Fruit flies have marked advantages for use in transmission genetics research: they are abundant in nature, and they are very easily fed and accommodated.

• Chromosome theory of heredity Dr R Jayashree, Asst. Prof(AGB) Veterinary College, Bangalore

• In 1933 Jean Brachet was able to show that DNA is found in chromosomes and that RNA is present in the cytoplasm of all cells.

• In 1941 Edward Lawrie Tatum and George Wells Beadle show that genes code for proteins.

• "Gene" is a theoretical term. Like all theoretical terms, its meaning has dramatically changed over and over in time, and it has been defined in so many different operational ways.

• Johansen (1909) coined the word ‘Gene’ Dr R Jayashree, Asst. Prof(AGB) Veterinary College, Bangalore

Neo Classical concepts „ R Garrod (1908) proposed One gene –one E product hypothesis „ G W Beadle and E L Tatum (1948) proposed One gene One gene – one enzyme one ‰ enzyme hypothesis „ Yanofsky (1965) proposed One gene (cistron) – one polypeptide hypothesis polypeptide hypothesis Dr R Jayashree, Asst. Prof(AGB) Veterinary College, Bangalore

Neo Classical concept • The nature of the genetic material became more accurate when Avery, Macleod and McCarty (1944) demonstrated that the substance causing transformation in bacteria was DNA.

• Support for the DNA theory of inheritance was gained when Alfred D. Hershey and Martha Chase demonstrated that DNA alone was responsible for the multiplication of bacteriophages.

Dr R Jayashree, Asst. Prof(AGB) Veterinary College, Bangalore

One gene One enzyme concept- Beadle & Tatum

Dr R Jayashree, Asst. Prof(AGB) Veterinary College, Bangalore

Griffith experiment- Pneumococcus sp.

Rough colony-Carbhohydrate capsule –Prevent the occurrence of Pneumonia-Type II R Organism when grown in solid agar medium produces smooth colony which are virulent-Type III S

Injected LiveType III S

Injected Dead Type III S

Injected Live Type II R

Injected Live +Dead Type III S Dr R Jayashree, Asst. Prof(AGB) Veterinary College, Bangalore

Avery, Macleod & McCarty Experiment • The DNA extracted from one strain of Pneumococcus – Highly purified and allowed to penetrate the cells of another strainRecipient strain developed certain characters of donor strain • Type II R – Rough colonies • DNA extract of heat killed Type III S – No colonies Avery's group (Avery et al., 1944) was able to isolate from this extract the substance which was responsible for transformation. That substance was DNA. Dr R Jayashree, Asst. Prof(AGB) Veterinary College, Bangalore

Identifying DNA as genetic material • Type II R + DNA extract of Type III S colony +Protease enzyme= IIIS smooth colony • Type II R + Heat killed Type III S + RNAse = IIIS smooth colony • Type II R+ DNA extract of heat killed Type IIIS+DNAse- No colony

Dr R Jayashree, Asst. Prof(AGB) Veterinary College, Bangalore

Hershey Chase Experiment

Only viral DNA enters the bacteria the protein coat of the virus remains outisde The phosporous label was associated with Bacterial DNA and the sulphur label was in the protein coat left in the medium Dr R Jayashree, Asst. Prof(AGB) Veterinary College, Bangalore

Hershey and Chase (1952) DNA is the only component of bacterial virus that enters its bacterial host, and therefore it must be presumed to be the sole bearer of viral genetic information.

Benzer (1955, 1957, 1959, 1961) The recon is the smallest unit of genetic material which can be separated from other such units by means of genetic recombination but which cannot be divided further.

A muton is the smallest unit of genetic material a change in which is suffcient to cause a mutant phenotype.

Dr R Jayashree, Asst. Prof(AGB) Veterinary College, Bangalore

Principle of Cis-trans test

In the cis-heterozygote the mutations are in the same chromosome, but in the trans-heterozygote they are in homologous chromosomes.

Genotype of the cis-heterozygote is designated as a b/.., Genotype of the trans-heterozygote as a./.b.

Dr R Jayashree, Asst. Prof(AGB) Veterinary College, Bangalore

• If the cis-heterozygote is of a wild type phenotype and the trans-heterozygote is mutant, a and b are mutations of the same cistron. • If, however, both cis- and trans-heterozygotes are phenotypically of a wild type, a and b are mutations of different cistrons.

Dr R Jayashree, Asst. Prof(AGB) Veterinary College, Bangalore

Gene as a Unit of • Transmission or Cistron-100 to 30000 nucleotide pairs – transmit character from one generation to another • Recombination or Recon-not more than two pairs of nucleotide capable of separation from one chromosome and exchange with other • Mutation or Muton-delimited to a single nucleotide which when changed produce a phenotypic trait Dr R Jayashree, Asst. Prof(AGB) Veterinary College, Bangalore

Break through in Gene concept • James D.Watson and Francis H. C. Crick (1953) demonstrated that deoxyribonucleic acid (DNA) is a double-stranded helix of nucleotides. • Alexander Dounce and George Gamow independently presented the so-called colinearity hypothesis, according to which the linear structure of DNA determines the linear primary structure of a polypeptide.

Dr R Jayashree, Asst. Prof(AGB) Veterinary College, Bangalore

Break down of Neo classical concept • Linn and Arber (1968) and Meselson and Yuan (1968) found specific restriction endonucleases, i.e., enzymes that cut DNA in bacteria, Cloned DNA molecules can be physically mapped, using the cutting points of the restriction endonucleases as markers, (Southern, 1975) and sequenced by means of sophisticated biochemical methods (Maxam & Gilbert, 1977; Sanger et al., 1977).

Dr R Jayashree, Asst. Prof(AGB) Veterinary College, Bangalore

Genetic Information • Gene – basic unit of genetic information.

Genes determine the inherited characters.

• Genome – the collection of genetic information.

• Chromosomes – storage units of genes.

• DNA - is a nucleic acid that contains the genetic instructions specifying the biological development of all cellular forms of life Dr R Jayashree, Asst. Prof(AGB) Veterinary College, Bangalore

Chromosome Logical Structure • Locus – location of a gene/marker on the chromosome.

• Allele – one variant form of a gene/marker at a particular locus.

Locus1 Possible Alleles: A1,A2

Locus2 Possible Alleles: B1,B2,B3 Dr R Jayashree, Asst. Prof(AGB) Veterinary College, Bangalore

Genetic Concepts • Homologous Chromosomes – Share centromere position – Share overall size – Contain identical gene sets at matching positions (loci)

gene for color

gene for shape

Dr R Jayashree, Asst. Prof(AGB) Veterinary College, Bangalore

Genetic Concepts • Gene – sequence of DNA which is transcribed into RNA (rRNA, tRNA or mRNA) • Locus – the position on a chromosome of a particular DNA sequence (gene)

G Locus – gene for color

W Locus – gene for shape

Dr R Jayashree, Asst. Prof(AGB) Veterinary College, Bangalore

Genetic Concepts • Heredity describes how some traits are passed from one geneartion to another.

• The traits are expressed by genes, which are small sections of DNA that are coded for specific traits.

• Genes are found on chromosomes.

• Genes replicate during cell division • Genes undergo sudden changes called mutation • Genes are capable of self duplication producing their own exact copies Dr R Jayashree, Asst. Prof(AGB) Veterinary College, Bangalore

Modern Genetic Concepts • Genome – Collection of all genetic material of organism

• Genotype – Set of alleles present in the genome of an organism

• Phenotype – Result of Gene Expression – Genes (DNA) are transcribed into RNA – mRNA is translated into protein, tRNA & rRNA work in translation process – Biochemical properties of proteins, tRNAs & rRNAs determine physical characteristics of organism – Mahner and Kary (1997) analysed in detail the genotype and phenotype. Dr R Jayashree, Asst. Prof(AGB) Veterinary College, Bangalore

Genetic Concepts • DNA is mutable • A variation in DNA sequence at a locus is called an allele – Diploid organisms contain 2 alleles of each locus (gene) • Alleles can be identical – homozygous • Alleles can be different – heterozygous • If only one allele is present – hemizygous – Case in males for genes on X and Y chromosomes

Dr R Jayashree, Asst. Prof(AGB) Veterinary College, Bangalore

Gene types on the basis of behavior of genes • Basic genes-bring expression of a Character • Lethal genes • Multiple genes-two or more genes for a single trait • Cummulative genes- additive effect on other genes • Pleiotropic- effect more than one character • Modifying genes-interacts with other genes to produce

modified effect • Inhibitory gene-Inhibits the expression of another gene Dr R Jayashree, Asst. Prof(AGB) Veterinary College, Bangalore

Concept of gene action- Operon Model • Structural gene – It regulates to produce a specific mRNA and determines the kind of protein to be synthesized • Operator genes -These genes act as turn on or turn off the activities of the structural genes • Regulator genes -This produces certain proteins called repressors which prevent the operator genes Dr R Jayashree, Asst. Prof(AGB) Veterinary College, Bangalore

Gene splicing

Dr R Jayashree, Asst. Prof(AGB) Veterinary College, Bangalore

Gene Expression DNA Gene Transcription RNA (messenger RNA) Translation Protein (sequence of amino acids)

Functioning of proteins within living cells influences an organism’s traits.

Dr R Jayashree, Asst. Prof(AGB) Veterinary College, Bangalore

Mutation & Phenotypic Variation Pigmentation gene, dark allele

(a) Molecular level

Pigmentation gene, light allele

Transcription and translation

Highly functional pigmentation enzyme

Poorly functional pigmentation enzyme

(b) Cellular level Pigment molecule Wing cells Dr R Jayashree, Asst. Prof(AGB)

Lots of pigment Veterinary made College, Bangalore Little pigment made

Mutation & Phenotypic Variation

Dark butterfly Organismal level

Light butterfly

Dark butterflies are usually in forested regions.

Light butterflies are usually in unforested regions.

Dr R Jayashree, Asst. Prof(AGB) Veterinary College, Bangalore

Molecular Genetics Chromosome

Cell

DNA Nucleus

Nucleotides

Dr R Jayashree, Asst. Prof(AGB) Veterinary College, Bangalore

DNA (deoxyribonucleic acid)

mRNA

Protein

Dr R Jayashree, Asst. Prof(AGB) Veterinary College, Bangalore

Dr R Jayashree, Asst. Prof(AGB) Veterinary College, Bangalore

Translation

Dr R Jayashree, Asst. Prof(AGB) Veterinary College, Bangalore

Translation

Dr R Jayashree, Asst. Prof(AGB) Veterinary College, Bangalore

Translation

Dr R Jayashree, Asst. Prof(AGB) Veterinary College, Bangalore

Human Genome Project • Sequenced almost all 3 billion DNA base pairs (2003) • Current work includes: – ENCODE Project (ENCyclopedia Of DNA Elements) to characterise functional elements in genome • 20,000-25,000 genes (1.5% of genome) • The bits in between (98.5% of genome)

– Characterise human DNA sequence variation • Find and describe DNA sequence variation (International HapMap Project) • Find significance of sequence variation (e.g. contribution to complex diseases) Dr R Jayashree, Asst. Prof(AGB) Veterinary College, Bangalore

HapMap project

http://www.ncbi.nlm.nih.gov/ National Center for Biotechnology Information advances science and health by providing access to biomedical and genomic information.

Dr R Jayashree, Asst. Prof(AGB) Veterinary College, Bangalore

Frequency

Case

0.200

Control 0.165

Odds ratio: 1.26

Dr R Jayashree, Asst. Prof(AGB) Veterinary College, Bangalore

1. Eye-catching headline of the form “Gene for…”

2. Highly qualified factual paragraph

Dr R Jayashree, Asst. Prof(AGB) Veterinary College, Bangalore