Genetics: protein synthesis

1. Transcription of DNA
2. Movement of mRNA from the nucleus to the cytoplasm
3. Amino acid activation
4. Translation of mRNA to make a polypeptide

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Background information
DNA and RNA

Thymine-------------Uracil
helix-----------------single chain
deoxyribose----------ribose
DNA is much too large to be soluble, or to pass through the pores in the nuclear envelope. DNA stay in the nucleus while mRNA and tRNA are small enough to be soluble and move around the cytoplasm of the cell.

mRNA
Shape

• single polynucleotide strand

Feature

• carries copies of genetic information(base sequence), explained as codon for 1 amino acid to ribosome in cytoplasm
  

Function

• act as a template to determine the sequence of amino acids in translation

tRNA
Shape

• single strand, folded into a 'clover leaf'
  

Feature
---amino acid binding site

• Attach specific amino acid

---mRNA binding site = anticodon

• bind specific codon

Function

• (aa binding site)combine various amino acid molecules present in the cytoplasm
• (anticodon)recognize the correct codons on mRNA molecules
• transfer amino acid to the ribosome
  

rRNA

• component of ribosome
• provides site for assembly of amino acids to make protein

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Transcription

1. Section of DNA molecule unwound by breaking of the hydrogen bond between base pair. This action occurs due to RNA polymerase
   
2. One strand is the template
3. RNA polymerase move along, complementary ribonucleotide is attracted to the template.
4. With the help of RNA polymerase adjacent complementary ribonucleotides are linked until a mRNA is formed. Energy is needed in this process.

Movement of mRNA
The mRNA strand leaves the nucleus through a nuvlear pore and attaches to a ribosome at rough endoplasmic reticulum in the cytoplasm.

Amino acid activation
Enzyme attach amino acid to their specific tRNA molecue. This needs energy supplied by ATP.
Anticodon, which is complementary to the codon on mRNA, specifies particular protein to be attached.

Translation
Ribosome
---Structure

• Non-membrane bound organelles
• Consist of rRNA and protein molecules

---Function

• Sites of protein synthesis
• Act as a translational machinery which hold the mRNA and tRNA-amino acid complex in position

Process
Translation is the synthesis of polypeptide chain using an mRNA molecule as a template. This occurs at the ribosomes by matching the codons on the mRNA molecule with the anticodons on tRNA molecules.

1. In cytoplasm, rRNA bond to the surface/cleft of ribosome
2. mRNA codon read, tRNA anticodon attached to the codon in ribosome
   
3. Next codon read, ......
4. Peptide bond form for 2 amino acid
5. 1st tRNA released,ribosome move along, next codon read
6. Peptide bonds are continuously formed likewise.
   
7. Process stop when the ribosome reach stop condon
8. Polypeptide chain release
   

Primary structure of the produced polypeptides coil and fold to obtain 3-D structure of protein.
mRNA is broken down to regenerate nucleotide.

Polysome
During translation, a mRNA molecule may associate with a number of ribosomes. Such a chain of ribosomes attaching to an mRNA is called a polysome.
Polysome enables a large number of identical polypeptides to be synthesized on a single mRNA strand at a comparatively short period.

reference and resources form
Simon Chiang, Y.K HO