protein synthesis

Protein synthesis

(Transcription and Translation)

 Define protein synthesis.

Protein synthesis is the process by which the cells produce proteins using the genetic

information carried by genes (DNA).

 Protein synthesis consists of two processes: 1. Transcription

2. Translation

Transcription

 Define transcription.

It is the first process of protein synthesis. Transcription is copying the sequence of

nucleotides of the transcribed DNA strand of a gene to produce m-RNA.

 Where does transcription take place?

Transcription occurs in the nucleus.

 What is the product of transcription?

The product of transcription is m-RNA

Note: during transcription, many copies of m-RNA are formed depending on the concentration

of protein needed.

 What are the steps of transcription?

1. Unwind the double helix

2. Unzip the double strand by RNA polymerase

3. Addition of complementary bases to the transcribed strand with T replaced by U to

form m-RNA.

4. m-RNA leaves the nucleus and DNA rewinds

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 Table of difference between DNA and RNA

DNA RNA

Location Nucleus Nucleus and cytoplasm

Sugar Deoxyribose Ribose

Number of strands 2 strands 1 strand

Nitrogenous bases A,T,C,G A,U,C,G

Base pairing A-T C-G A- U G-C

Size Longer Shorter

Presence Permanent Temporary

 Refer to doc. c p 54 and formulate a hypothesis tested in the experiment

Hypothesis: m-RNA is synthesized in the nucleus and is exported toward the cytoplasm.

 Refer to the following document and answer the following questions

a. Compare the sequence of nucleotides of m-RNA to that of transcribed (template)

DNA strand.

b. Compare the sequence of nucleotides of m-RNA to that of non transcribed strand.

a. The nucleotides of m-RNA are complementary to the nucleotides of (Template DNA

strand) transcribed strand with T replaced by U.

b. The nucleotides of m-RNA are identical to the nucleotides of (non-template) non transcribed

strand DNA strand with T replaced by U.

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Translation

 Define translation.

Translation is translating m-RNA to a sequence of amino acids in order to produce a

polypeptide chain.

 Where does translation occur?

Translation occurs in the cytoplasm.

 What is the product of translation?

The product of translation is a peptide chain.

 Tools needed for translation:

1. m-RNA : has linear shape and it is a transcript (copy) of the genetic information of

a gene. It starts with a start codon AUG and ends with a stop codon (UGA/UAA/

UAG)

2. t-RNA (transfer RNA): coiled in way to form a t-shape. It has an anti-codon arm

and an amino acid arm.

3. Ribosome: small organelles which have 2 subunits (large ribosomal subunit and

small ribosomal subunit) .The large ribosomal subunit has two sites A and P sites.

The steps of translation:

I. Initiation:

1. The small ribosomal subunit binds to the m-RNA at the start codon.

2. Initiator t-RNA carrying amino acid methionine binds facing to the start codon of m-

RNA (AUG).

3. The large ribosomal subunit then binds to the m-RNA and the initiator t-RNA will

occupy the P-site of the large ribosomal subunit.

II. Elongation

4. A new t-RNA arrives and occupies A-site of the large ribosomal subunit facing the 2nd

codon of m-RNA.

5. A peptide bond is formed between the two amino acids

6. The ribosme is translocated (moves) so that the 2nd t-RNA carrying peptide will occupy

P-site .The A- site will become free again.

7. Another t-RNA will occupy A-site facing the 3rd codon. And the process will continue.

III. Termination

8. The ribosome reaches the stop codon (UAA, UGA, UAG). So translation stops.

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9. Peptide is released and ribosomes dissociates.

10.The 1

st amino acid (methionine is removed)

Note: During translation, a polysome is formed to produce many molecules of the same peptide

chain from the same m-RNA depending on the concentration of protein needed.

A polysome is a string of ribosome simultaneously translating different regions of m-RNA to

produce many peptides.

Note :

t-RNA carrying the peptide binds to P-site of the large ribosomal subunit.

t- RNA carrying amino acid occupies A-site of the large ribosomal subunit.

 Properties of genetic code:

1. Genetic code is universal: a codon codes for the same amino acid in all living

things.

2. Genetic code is redundant: more than one codon codes for the same amino acid.

 Protein synthesis:

DNA nucleotides transcription m-RNA nucleotides translation amino acids