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Chapter – 10 Part II
Molecular Biology of the Gene Genetic Transcription and
Translation
The Structure of Proteins
• Made up of amino acids
• There are 20 amino acids involved in protein
synthesis
• Amino acids combine together to form the
peptide linkage (link between amino group of
one amino acid with the carboxyl group of
adjacent amino acid)
• Many peptide linkages form a polypeptide chain
• Polypeptide chains fold to form a protein.
The Structure of Proteins
Stages of protein synthesis
Two major stages involved:
• The first stage is called transcription
– The 2 strands of the DNA molecule unwind
and mRNA copies the genetic code (letters A,
C, G and T) from DNA, the master molecule.
Stages of protein synthesis
• mRNA then comes out from the nucleus to
cytoplasm
• The second stage is called translation
– mRNA and tRNA (carrying amino acids) reach
the cytoplasmic ribosome (composed of two
protein subunits and rRNA) where the mRNA
code is used to assemble amino acids
– These amino acid chains elongate and fold to
form proteins.
Protein Synthesis
Summary: The process
of copying DNA into RNA
is called transcription and
the Process of
assembling proteins from
RNA instructions is called
translation
Transcription
Triplet Code
• Three mRNA bases code for a specific
amino acid is called code / codon ( three
nucleotides).
Triplet Code
• Three mRNA bases code for a specific amino acid is
called code / codon ( three nucleotides).
Translation
• mRNA (carrying instructions for sequence of
amino acid in a protein) comes out of the
nucleus and reaches the cytoplasm to the site of
ribosome, the location of protein synthesis.
• rRNA a structural component of ribosome
Translation
• tRNA (translator molecule)
– Anticodon of tRNA attracts a specific amino acid
– tRNA matches the codon in the mRNA
– Part of the tRNA binds an amino acid and the other
end has three nucleotides, anticodon that forms a
base pair with the codon in the mRNA)
– Amino acids of neighboring tRNA’s link to form the
polypeptide chains.
– Lengthened amino acid chains fold to form the protein
molecule.
Types of RNA
Translation
Steps in Translation
• mRNA binds to the small subunit of the
ribosome. Start codon AUG, brings in initiator
tRNA with the amino acid methionine, and then
large subunit binds.
• tRNA that matches the next codon brings in the
appropriate amino acid in to the “A” site.
Steps in Translation
• Then the ribosome joins the mRNA to the
next codon, moving the newly formed peptide
to the “P” site. This continues until a stop
codon is reached.
• At the stop codon, no new tRNA comes into
the “A” site and the whole complex falls
apart, releasing the new protein.
Steps in Translation
Editing out Genetic Material
• Intron is a segment of DNA, (or the mRNA
transcript complementary to it), that does not
encode information for the sequence of amino
acids in protein.
• Exon is the segment of DNA or mRNA ,that
encodes information for the sequencing of
amino acids in protein.
• A segment of DNA that makes transcript is
called gene
• Genome size for human is 3 billion.