PROTEIN SYNTHESIS

• The formation of new proteins using the code carried on DNA

Gene is Selected

• A gene is a section of DNA coding for a particular polypeptide or protein.

• When a protein is required by the cell or the organism as a whole, the gene for that protein is activated.

Gene

Introns and Exons

• DNA is made up of coding sequences called exons and non coding sequences called introns.

Exon Exon Exon Exon

Intron Intron Intron Intron

Transcription

• An enzyme RNA Polymerase unwinds the section of DNA carrying the desired gene.

• Only one strand of the DNA (the template) is recognised.

• RNA Polymerase then binds to a promoter site and begins to form an RNA copy of the template strand (called messenger RNA (mRNA)).

Naming the Strands

• An mRNA copy of the top coding strand is made by base pairing with the bottom template strand.

DNA

Primary RNA

Coding strand

Template strand

• The DNA strand is unwound exposing the bases that carry the genetic code.

Messenger RNA

• Free nucleotides match up with the coded sequence of DNA.

• When bases pair, Uracil is present in RNA instead of Thymine.

• A single stranded primary transcript that is complementary to the template strand is formed.

• DNA template TAC GCC AAA TCC • mRNA codons AUG CGG UUU AGG

Removal of Introns

• The primary transcript contains a copy of both introns and exons.

• Introns are removed and exons are spliced (joined) to form mature mRNA.

Movement of mRNA

• The completed single strand of mRNA migrates out of the nucleus through pores in the nuclear membrane.

• mRNA binds to ribosomes.• The mRNA is divided into triplets of

bases called codons.• Different codons carry the code for a

particular amino acid (AA)• Other codons carry start and stop

codes.

Important Codon Facts

• AUG is the start codon on all viable mRNA molecules. It also codes for the amino acid Methionine.

• UAA, UAG, UGA are stop codes.• Many codons are degenerate --more

than one codon codes for an AA.• The code is universal. The same

codes work in all organisms.

• 43 different combinations give 64 different codons.

Amino Acid Codons

Transfer RNA

U A C

AA Methionine

• Transfer RNA (tRNA) is a carrier molecule which picks up amino acids from the cytoplasm and carries them to the mRNA / ribosome complex.

anticodon

Translation

• A triplet of bases on the tRNA called an anticodon matches with the mRNA codon.

• tRNA carrying its AA binds to the codon

• The ribosome moves along the mRNA activating new codons.

• AA’s are joined by peptide bonds to form long polypeptide chains.

Ribosome Action

A U G C G G U U U A G G C C C U A G

• The ribosome acts as catalyst holding all the components together while bonds are formed.

G C C

AA2

U A C

AA1

Peptide Bond Formation

A U G C G G U U U A G G C C C U A G

• As the ribosome moves along the mRNA, amino acids are added until a stop signal is reached.

A U C

stopAA1 AA4AA3AA2 AA5

Translation speed

• Rapid protein formation can be achieved when the mRNA is attached to more than one ribosome.

Electron Microscope View

• mRNA (purple) binds to a ribosome.• Beads of AA’s (yellow) are joined to

form a polypeptide.

Questions• What name is given to a section of DNA coding for a

polypeptide?• Gene.• The coding sequences of DNA are known as what?• Exons.• Name the enzyme responsible for unwinding the DNA

helix.• Helicase• Which DNA strand is used in the base pairing process?• The template. (the parental strand)

• In RNA, which base binds with Adenine?• Uracil.• What is the codon for the start of an mRNA?• AUG.• What does the term “codons are degenerate” mean?• More than one codon may code for an amino acid.• Why is the codon a three base sequence?• 43 different combinations gives codes for all 20 amino

acids, start and stop codes plus degenerate codes.

• What does the term, “the genetic code is universal” mean?• The same code works in all organisms.• What name is given to the process of forming mRNA?• Transcription.• What is the full name for tRNA?• Transfer Ribonucleic acid.• What is the role of tRNA?• Carries amino acids from the cytoplasm to the ribosomes.• mRNA has a codon, tRNA has an?• Anticodon.• What holds amino acids together in a polypeptide?• Peptide bonds.

• What is the role of the ribosomes?• They act as catalysts, holding the various components together.• How do the ribosomes operate that allows long amino acid chains

to form?• They move along the mRNA allowing tRNA’s to bring in new

amino acids.• What name is given to the process of converting an mRNA code

into an amino acid sequence?• Translation.• Describe one way in which the rate of translation can be

increased?• By attaching the mRNA to more than one ribosome.