Transcription and Translation

The story so far…..

DNA replication

- DNA double helix unwinds and with the help of many enzymes to make a new semiconservative daughter strand.

Proteins are formed from amino acids into 4 different structures

RNA is a key player in getting the genetic code from DNA to the protein.

mRNA – takes information from DNA to ribosomes

tRNA – carries amino acids to the ribosomes to make polypeptides

rRNA – joins with proteins in the cytoplasm to form ribosomes

The next chapter in the story….

There are 2 stages in protein synthesis:

1. Transcription -DNA is copied onto mRNA and taken out of the nucleus

2. Translation - Instructions are followed from mRNA to assemble polypeptides

Transcription

DNA is used as a template to form a molecule of single-stranded mRNA

Three steps of Transcription1) DNA unzips

2) Complimentary base pairing of ribonucleotides

3) Termination

1) DNA Unzipping

One section of DNA is unzipped (H-bond between nitrogen bases are broken)

DNA Polymerase binds at the TATA box

Enzyme helicase – helps to break the Hydrogen bonds

2) Complimentary Base Pairing

Free-floating ribo-nucleotides temporarily hydrogen bond with their complimentary nucleotides on the DNA template

Ex: DNA – A T C G RNA –

Only one of the DNA strands is used as the template.

RNA polymerase helps to bond the sugar-phosphate backbone

3) Termination

RNA polymerase reaches termination sequence

RNA polymerase detaches from DNA, releasing

single-stranded mRNA strand

DNA rewinds into its double helix

mRNA is spliced (extrons and introns) and capped

mRNA is transported out of the nucleus – through

the nuclear pore

Translation

mRNA goes into the ribosome and is used as a template to synthesize a protein molecule.

Three Steps of Translation

1) Initiation – start (first amino acid)

2) Elongation – amino acid chain grows longer

3) Termination – stops (release factor)

1) Initiation

need start codon AUG

The first tRNA, with anticodon UAC and amino acid methionine temporarily binds to the first site

2) Elongation

The second tRNA molecule attaches to the second site of translation

The amino acid from the first tRNA is transferred to the amino acid on the second tRNA.

The first tRNA exits, the ribosome moves, a new tRNA enters, and the process is repeated.

The process is repeated many times, and a polypeptide or strand of amino acids, is formed.

3) Termination

The release factor enters.

Translation, or protein synthesis, ends.

The completed poly peptide is released.

Translation - Application

What does this diagram represent?

Connection between different RNAs

mRNA binds with tRNA – at the ribosome

tRNA bring the amino acid to the ribosome

mRNA has triplet codons that match with triplet anticodons on the tRNA

The codons/anticodons and amino acids – all correspond to each other

If GAA is the mRNA codon – what is the:

◦ tRNA anticodon:

CUU

◦ Amino acid:

Glutamate

Which of the following is an anticodon of a molecule of tRNA carrying glycine?

A. G G A

B. C C U

C. C C T

D. C G C

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Translation - Application

DNA Sequence:

T A C G C A A A A T G G C C T A C T

mRNA Sequence:

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

Amino acid sequence:

methionine – arginine – phenylalanine – threonine – glycine - stop