Protein Synthesis

Biology Unit 4

Protein Synthesis

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Biology Unit 4
Revision Notes
Topic 6: Infection, Immunity and Forensics
3. Explain the process of protein synthesis
(transcription, translation, messenger RNA, transfer
RNA, ribosomes and the role of start and stop codons)
and explain the roles of the template (antisense) DNA
strand in transcription, codons on messenger RNA,
anticodons on transfer RNA.
The basic steps of protein synthesis are as follows:
DNA mRNA amino acid chain protein
Transcription means "to make a full copy of". The mRNA copy is not direct, like a photocopy, but
complementary. mRNA is made in the nucleus, but polypeptides are assembled in the cytoplasm, so
the mRNA must move out of the nucleus through pores in the nuclear membrane.
The DNA that carries the genetic message is in the nucleus, but proteins are made on the
ribosomes. The message of the DNA is transcribed to form an mRNA molecule.
The breakdown of the hydrogen bonds between the bases making up the two strands of
DNA is catalysed by enzymes called RNA polymerase. The 5' prime DNA strand is transcribed
to give a single strand of mRNA.
This DNA strand is the template strand as it provides a template for ordering the sequence
of nucleotides in an RNA transcript.
Every triplet code on the DNA gives rise to a complementary codon on the mRNA.
Scientists usually refer to the DNA strand that has the same sequence as the resulting RNA as
the sense strand while the strand which acts as a template is the antisense strand.
Text Book: p.74 - 76

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Biology Unit 4
Revision Notes
Once in the cytoplasm, the mRNA attaches to a ribosome. tRNAs carrying specific amino acids bind to
the complementary codons on the mRNA. A peptide bond is formed between neighbouring amino
acids to produce a polypeptide.
Molecules of tRNA carry individual amino acids to the surface of the ribosome. The tRNA lines
up its anticodons alongside a complementary codon in the mRNA.…read more

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Biology Unit 4
Revision Notes
Ribosomes ­ Cell organelle that is the site of protein synthesis
Transcription ­ production of mRNA from a DNA strand
Translation ­ The conversion of the information in mRNA into a polypeptide or protein
mRNA ­ RNA that carries information from the DNA in the nucleus out into the cytoplasm to
be translated into proteins on the ribosomes
DNA-directed RNA polymerase ­ enzyme involved in making an RNA copy from a DNA
template using transcription in the nucleus
Template…read more


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