Molecular Biology2.7 DNA Replication, transcription and translation

DNA to Protein Synthesis The function of most genes is to specify

the sequence of amino acids in a particular polypeptide.

Two processes are needed to produce a polypeptide using the base sequence of a gene. Transcription Translation

Transcription Transcription is the synthesis

of mRNA copied from the DNA base sequences by RNA polymerase.

The enzyme RNA polymerase binds to a site on the DNA at the start of a gene

The RNA polymerase separates the double strand of DNA and pairs up RNA nucleotides with complementary bases.

Transcription The RNA polymerase

forms covalent bonds between the nucleotides

Transcription stops at the end of the gene. The completed mRNA is released.

Transcription The completed mRNA has

a base sequence that is complementary to the template DNA strand. Note: Uracil replaces

Thymine

Complete the transcription portion of the practice sheet

Translation Translation is synthesis

of polypeptides on ribosomes

Ribosomes are complex structures that consist of a small and large sub-unit with binding sites for each of the molecules used in translation.

Translation The amino acid

sequence of polypeptides is determined by mRNA according to the genetic code

Translation Codons of three bases

on mRNA correspond to one amino acid in a polypeptide

“Genetic Code” = The translation “dictionary” that enables the cellular machinery to convert the base sequence on mRNA into the amino acid sequence

Translation There are 20 amino

acids and only 4 nucleotide bases.

Living organisms use a triplet code with 3 bases coding for an animo acid.

The sequence of 3 bases on the mRNA = CODON

Examine the list of codons

Translation There are 3 “stop”

codons that code for the end of translation.

Different codons can code for the same amino acid. The code is

“degenerate”

Translation Translation depends on

complementary base pairing between codons on mRNA and anticodons on tRNA

The amino acids are carried on tRNA

The tRNA has a 3 base ANTICODON complementary to the mRNA codon

Complete the translation portion of the worksheet