Protein synthesis in cell

Protein Synthesis in Cell

05/03/2023 Dept. of Physiology, MSRMC

Learning Objectives• Importance of proteins• Site of protein synthesis• Steps of protein synthesis• Regulation of protein synthesis• Applied aspects


Proteins: their importance• Membrane proteins• Structural proteins• Enzymes• Hormones• Antigens


Site of Protein Synthesis


DNA• DNA has two functions.

– self-renewing data repository that maintains a constant source of genetic information for the cell.

– serve as a template for the translation of genetic information into proteins, which are the functional units of the cell.


Basic building blocks of DNA• Phosphoric acid• Sugar• Nitrogenous base


Genetic Code• The gene consists of a segment of DNA that

is transcribed into RNA.• The genetic code consists of successive

"triplets" of bases on the DNA.• Each three successive bases is a code word.• The successive triplets eventually control the

sequence of amino acids in a protein molecule that is to be synthesized in the cell.


Pathway from Genes to Proteins


• Central dogma of molecular biology: genetic information flows unidirectionally from DNA to proteins.


Steps of Protein Synthesis• Transcription

• Translation

• Post translational modificaton


Transcription• Definition: Transcription is the synthesis

of RNA from a DNA template, mediated by an enzyme called RNA polymerase.

• Site: Nucleus


• Requirements:

– DNA template

– RNA polymerase

– Activated ribonucleotides

Transcription contd.


Building blocks of RNA• Phosphoric acid

• Sugar

• Nitrogenous base


• The basic building blocks of RNA form RNA nucleotides.

• RNA nucleotides are then activated by RNA Polymerase.


• RNA polymerase recognises the promoter region in DNA and binds to it.

• Unwinding of a segment of DNA

• Attachment of activated ribonucleotides to the DNA segment


• Polymerase moves along the DNA strand

• Breakage of 2 phosphate radicals from RNA nucleotides

• Covalent linkage of 3rd phosphate with ribose


• RNA polymerase reaches end of DNA gene (chain terminating sequence)

• Breaking away of polymerase & RNA chain

• Formation of RNA transcript


• The code that is present in the DNA strand is eventually transmitted in complementary form to the RNA chain.

• The ribose nucleotide bases always combine with the deoxyribose bases in a fixed combination.


DNA Base RNA BaseGuanine…………………………….……CytosineCytosine ………………………..…….… GuanineAdenine …………………………………… UracilThymine ………………………………… Adenine


• The RNA that is initially transcribed from a gene is called the primary transcript.

• Most eukaryotic genes contain exons, DNA sequences that are present in the mature mRNA, alternating with introns, which are not present in the mRNA.


• Splicing• Addition of 5’ methyl cap• Cleavage of RNA transcript downstream

from polyadenylation signal• Addition of poly A tail

Processing of The Primary Transcript


Types of RNA• mRNA• tRNA• rRNA• miRNA


mRNA• mRNA molecules are long, single RNA

strands that are suspended in the cytoplasm.

• They contain codons that are exactly complementary to the code triplets of the DNA genes.


• Carrier for specific amino acids from cytoplasm to mRNA

• Has sites for binding amino acid & mRNA

tRNA


rRNA• Functions in association with tRNA &

mRNA

• Present in ribosomes


miRNA• Non-coding RNA

• Regulate gene expression


Translation • Definition: Translation is the synthesis

of protein from RNA.

• Site: Ribosomes


• Requirements:– Amino acids– mRNA– tRNA– ATP

Translation contd.


• Begins at AUG• Ends at UAG, UAA, UGA

Translation contd.


• Amino acid + ATP

• Activated amino acid + AMP

• tRNA

• Amino acid tRNA complex

• Binds to mRNA


• Peptide bonds formed between successive amino acids

• Process stops at chain termination codon

• Release of polypeptide

Translation contd.


• It is common to have more than one ribosome on a given mRNA chain at a time.

• The mRNA chain plus its collection of ribosomes is visible under the electron microscope as an aggregation of ribosomes called a polyribosome.


Post-translational Modification

• Folding• Hydroxylation• Carboxylation• Glycosylation• Phosphorylation • Cleavage of peptide bonds


Regulation of Gene Expression

• Step 1: Chromatin remodeling• Step 2: Initiation of transcription• Step 3: Transcript elongation• Step 4: Termination of transcription• Step 5: RNA processing• Step 6: Nucleocytoplasmic transport• Step 7: Translation• Step 8: mRNA degradation


Applied physiology


Mutation • Definition: change in the DNA structure of

a gene• Types:

– Point mutation (transition, transversion)– Frame shift mutation(deletion, insertion)

• Mutations can lead to genetic diseases, cancers.

• Some mutations may be silent.


Genetic diseases• Sickle cell anaemia• Cystic fibrosis• Phenylketonuria• Huntington’s chorea• Haemophilia


• Gene therapy: methods to cure an inherited disease by providing a patient with correct copy of a defective gene.

• Gene therapy has been successful in– Cystic fibrosis– Severe combined immunodeficiency

Gene therapy


Summary

Protein synthesis in cell

Education

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