Concept of Gene And Protein Synthesis

PRESENTED BY : SHITAL MAGAR M.PHARM SECOND SEM

Department of Pharmacology R.C Patel Institute of Pharmaceutical Education and

Research, Shirpur.

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The word ‘gene’ has two meanings: (1) the determinant of an observable trait or characteristic of an organism, or (2) the DNA sequence that determines the chemical structure of a specific polypeptide molecule or RNA molecule.

The gene is operationally depened on the basis of four genetic phenomena: genetic transmission, genetic recombination, gene mutation, and gene function.

The word ‘gene’ was coined by W. Johannsen in 1909,but the modern concept of the gene originated with Gregor Mendel, who in the 1860s studied the inheritance of Characteristics on true-breeding varieties of garden peas.

What is Gene ? ( Petter Portin 2002 )

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3Classical concept of Gene?

Neoclassical concept of Gene? Avery et al. (1944) demonstrated that the substance causing transformation in bacteria was DNA. Transformation had been discovered by Griffith (1928), He observed that killed bacterial cells injected into mice were able to transform genetically different living bacteria into their own kind.

The classical view of the gene begins with the work of Mendel (1866), in which he explained definitively the transmission of genes or elements as he called these units of inheritance .

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4The Present Concept of Gene

Singer and Berg (1991) have pointed out that many different definitions of the gene are possible. If we want to adopt a molecular definition, they suggest the following definition: “A eukaryotic gene is a combination of DNA segments that together constitute an expressible unit”.

Each gene includes one or more DNA segments that regulate the transcription of the gene and thus its expression'' Thus the segments of a gene include (1) a transcription unit, which includes the coding sequences, the introns, the flanking sequences the leader and trailer sequences, (2) the regulatory sequences.

5Summary of Conceptions of Gene

YEAR, SCIENTIST GENE CONCEPT

1866 G.J. Mendel A unit factor that controls specific phenotypic trait

1902 Sir A.E.Garrod One gene –one metabolic block theory

1940 Beadle & tatum One gene-one enzyme theory

1957 U.M.Ingram One gene-one polypeptide theory

1999 Griffiths & neumann Molecular and evalutionary gene theory

2006 Griffiths & stotz Nominal gene theory

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6Structure of Gene Merrick,1994

Gene structure is the organisation of specialised sequence elements within a gene. In most organisms, genes are made of DNA, where the particular DNA sequence determines the function of the gene.

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7Chromosome, DNA and Gene

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8DNA and RNA

Components DNA RNA

Sugar Deoxyribose Ribose

Bases A,G,C,T A,G,C,U

Strands Double strands Single strand

Genetic material Most life Some viruses

Enzyme None Many with

Structure Double helix linear or folded

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9Types of RNA

Messenger RNA (mRNA) copies DNA’s code & carries the genetic information to the ribosomes.

Ribosomal RNA (rRNA), along with protein, makes up the ribosomes.

Transfer RNA (tRNA) transfers amino acids to the ribosomes where proteins are synthesized.

10What is Gene Expression ?

The process by which a gene's information is converted into the structures and functions of a cell by a process of producing a biologically functional molecule of either protein or RNA (gene product) is made. Gene expression is assumed to be controlled at various points in the sequence leading to protein synthesis.

TRANSCRIPTION TRANSLATION

PROTEIN

RNADNAm-RNA degradation

control

RNA processing control

RNA transport and location control

Protein activity control

11What is Gene Expression ? Jacob and Monad (1961) studied control of protein synthesis in E. coli and

lactose digesting enzymes Genes can be switched on or off as necessary A gene that is ‘on’ will be transcribed In E.coli, the enzyme lactase will be produced if the gene ‘on’ If the gene is ‘off’ mRNA will not be created and translation can not occur

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12What is Gene Expression ?

The operon model, Proposed by Jacob and Monad. Explains how genes switch on and off. Operon=promoter, operator and structural genes. Lac operon is found in E.coli.

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13How Gene is Express ?

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14How Gene is Express ?

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15Types Of Gene

On the basis of their behaviour the genes may be categorized into the following types:

i) Basic genes: These are the fundamental genes that bring about expression of particular character.

ii) Lethal genes: These bring about the death their possessor.

iii) Multiple gene: When two or more pairs of independent genes act together to produce a single phenotypic trait.

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vi) Cumulative gene: Some genes have additive effects on the action of

other genes. These are called cumulative genes.

v) Pleiotropic genes: The genes which produce changes in more than one

character is called pleiotropic gene.

vii) Inhibitory gene: The gene which suppresses or inhibits the expression of

another gene is called inhibitory gene.

Types Of Gene

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17Protein Synthesis

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18Protein….. Proteins are single, unbranched chains of amino acid monomers There are 20 different amino acids. The amino acid sidechains in a peptide can become modified,

extending the functional repetoire of aminoacids to more than hundred different amino acids.

A protein’s amino acid sequence determines its three-dimensional structure (conformation).

In turn, a protein’s structure determines the function of that protein.

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19Structure Of Protein

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20Protein Synthesis: An Overview

Genetic information is contained within the nucleus of a cell.

DNA in the nucleus directs protein synthesis but protein synthesis occurs in ribosomes located in the cytoplasm.

How does a ribosome synthesize the protein required if it does not have access to DNA.

Information is copied from DNA into mRNA, this is transcription.

mRNA leaves the nucleus and enters the cytoplasm of the cell.

Ribosomes use the mRNA as a blueprint to synthesize proteins composed of this is translation.

21The central dogma of protein Synthesis

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22Protein Synthesis

Nucleus

DNA

TranscriptionIntron

Intron Pre-mRNA

mRNANuclear pore

Export Translation

Cytoplasm

Protein

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23Genetic Code Millard Susman,2001

Proteins are composed of 20 different amino acids.

A sequence of 3 nucleotides is used to code each amino acid.

Each triplet of nucleotides is called a codon.

Start codon AUG codes for amino acid methionine.

3 stop codons

There are 64 codons in the genetic code 43=64

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24Characteristics of the Code

Continuity - The genetic code reads as a long series of three-letter codons that have no spaces or punctuation and never overlap.

Redundancy – Several different codons can code for the same amino acid, but no codon ever has more than one amino acid counterpart.

Universality – the genetic code is the same in almost all living organisms, from bacteria to mammals

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25Transcription

Once the RNA polymerase leaves the promoter region, a new

RNA polymerase can bind there to begin a new mRNA transcript.

Since prokaryotes lack a membrane bound nucleus translation

can begin even before the mRNA dissociates. However the pre-

mRNA from eukaryotic cells needs some modification before it

leave the nucleus.

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26Transcription: Initiation

RNA polymerase binds to a segment of DNA and opens up the double helix RNA polymerase recognizes the promoter region which is a sequence of

DNA rich in A and T bases (TATA box) found only on one strand of the DNA. An RNA polymerase cannot recognize the TATA box and other landmarks of

the promoter region on its own. Another protein, a transcription factor that recognizes the TATA box, binds to the DNA before the RNA polymerase can do so.

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27Transcription: Initiation For transcription to be initiated, both promoter sequences must be present in

their correct locations. The nucleotide sequences in the promoters are slightly different from one another, which means the RNA polymerase will bind in only 1 orientation, thus RNA polymerase can only face 1 way during transcription. This ensures transcription will proceed in only 1 direction.

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28Transcription: Elongation

The RNA polymerase uses only one of the strands of DNA as a template for mRNA synthesis. This is called the template strand or sense strand. The coding strand or anti-sense strand contains the complementary nucleotide sequence to the sense strand. RNA polymerases can add nucleotides only to the 3’ end of a DNA sequence. Thus, an RNA molecule elongates in the 5’ to 3’ direction. Consider the following DNA sequence 3’ TACTTACTCGTCTTG 5’.

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29Transcription: Elongation

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As the RNA polymerase molecule passes, the DNA helix re-forms. Synthesis continues until the end of a gene is reached where RNA polymerase recognizes a terminator sequence.

Transcription: Termination

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31Translation

After transcription mRNA exits the nucleus via nuclear pores and ribosomes bind to mRNA

Ribosomes synthesize different proteins by reading the coding sequence on mRNA

The mRNA is read in triplets of nucleotides each of which encodes an aa.

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Consider the following mRNA sequence: 5’ AUGAAUGAGCUGAAC 3’

Translation

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Each active ribosome has 3 different binding sites for tRNA molecules: the

P

(peptide) site, which holds one tRNA and the growing chain of amino

acids; the A (acceptor) site, which holds the tRNA bringing the next

amino

acid to be added to the chain; and the E (exit) site, which releases the

tRNA

molecules back into the cytoplasm.

Ribosomes

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34Ribosomes

The anticodon of an aa-tRNA molecule binds to the mRNA codon exposed in the A site.

Enzymes catalyze the formation of a bond between the last aa on the lengthening polypeptide and the new aa. The polypeptide chain is transferred from the tRNA in the P site to the tRNA in the A site.

The ribosome moves down the mRNA strand, shifting the binding site a distance of 3 nucleotides (1 codon), this is called translocation. A new A site is exposed as the tRNA that was in the P site is moved to the E site and released.

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35Ribosomes

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36Termination of Protein Synthesis

Translocation of the ribosome exposes a stop codon in the A site. Stop codons do not code for an aa, there are no corresponding tRNAs.

A protein called a release factor binds to the exposed A site causing the polypeptide to separate from the remaining tRNA molecule.

Ribosome falls of the mRNA and translation stops.

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37Termination In Protein Synthesis

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38Recent Advances

Recent advances in producing and selecting functional proteins by using cell-free translation.

Recent advances of protein microarrays.

Recent advances in dynamic m6A RNA modification. Recent advances of DNA Microarray Technology.

References Genes: VII - Benjamin Lewin - Google Books ,oxford university,edition-VII,pp.156-158

Rusmini, F., Zhong, Z. and Feijen, J., 2007. Protein immobilization strategies for protein

biochips. Biomacromolecules, 8(6), pp.1775-1789.

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Susman, M., Genes: Definition and Structure. eLSLaland, K.N. and Brown, G.R., 2011.  Sense and nonsense: Evolutionary perspectives on human behaviour. Oxford University Press Horowitz, N.H., 1995. One‐gene‐one‐enzyme: Remembering biochemical genetics.  Protein Science, 4(5), pp.1017-1019. Portin, P., 1993. The concept of the gene: short historyand present status. The Quarterly Review of Biology, 68(2), pp.173-223. Merrick. Hershey, J.W., 1996. 2 The Pathway and Mechanism of Eukaryotic Protein Synthesis. Cold Spring

Harbor Monograph Archive, 30, pp.31-69.

Hartl, D.L. and Ruvolo, M., 2011. Genetics. Jones & Bartlett Publishers.