Protein Synthesis

RNA vs. DNA

• Both nucleic acids– Chains of nucleotides

• Different: – Sugar– Types of bases– Numbers of bases– Number of chains– Site of action– Function

Types of RNA Messenger RNA- mRNA- Dictates the order of amino acids in a protein using a series of three bases codons.

Ribosomal RNA- rRNA- Major component of the ribosome

Transfer RNA- tRNA- Transports the correct amino acid to the ribosome based on the codon in the mRNA

Protein Synthesis • Gene → Protein

– Transcription: DNA → RNA– Translation: RNA → Protein

TranscriptionNucleotide → nucleotide

– DNA → RNA• Template strand or coding

strand of DNA• RNA polymerase

Transcription• RNA polymerase

– Finds “promoter” region

– Stops at “terminator” sequence

• Produces pre-mRNA• Where must this occur?

– Adds complementary nucleotides to template strand making the mRNA

Why thymine vs. uracil? Not on testWhy thymine rather than uracil in DNA?•Cytosine is easily changed to uracil

by removing a amine group: • Occurs about 100 times per cell, per

day. • If DNA used uracil the cell would not

know if that uracil was supposed to be there or if it was supposed to be cytosine.

• DNA has thymine so the cell can easily recognize that when cytosine is changed to uracil there is a problem because uracil should not be in the DNA.

Cytosine Uracil

This would cause major

mutations!

Modification • Introns – internal

sequences that will be cut out

• Exons – sequences that will not be cut out

• RNA splicing – cuts out introns and rejoins exons

• This is not part of the model we are using

Translation

Nucleotide → amino acid– Ribosomes are within

cytosol – Requires mRNA, tRNA

and rRNA – Final product is a

polypeptide

The mRNA Strand is Read as a Series of Codons to Make a

Protein

• Each unique sequence of three bases is called a codon

• There are 64 codons that make up the information in the genetic code

Genetic Code

• Codon – sequence of three (3) nucleotides• Genetic Code – translates codons into amino acids

Translation• mRNA

– template for translation

• tRNA – attaches to

amino acids– Anticodon:

complementary binds to the codon of mRNA and

Initiation• Ribosome, made of rRNA and proteins, binds to mRNA• The ribosome travels down the mRNA until it finds the

start codon• Methionine tRNA binds to start codon

Elongation• The next codon

signals its respective tRNA to enter the ribosome and alines with the previous tRNA

• The two amino acids are brought close together so that they can be bound together.

Elongation• Once the two amino

acids are bond together the mRNA and tRNAs are moved

• The next tRNA is brought in

• This allows the correct amino acids to be brought close contact so they can be bound together in the correct sequence.

• This process continues down the length of the mRNA resulting in a complete polypeptide with amino acids in the correct order.

• The process ends when a stop codon enters the ribosome

Elongation

http://www.youtube.com/watch?v=41_Ne5mS2ls

Practice: Translate & Transcribe

Coding Sequence: TAC CCG GAT ACC

Another Way to Display at the Genetic Code

Types of Mutations

• Substitution – replacement of one base for another– Can be bad– Can be good– Could not effect

protein (silent)– Could add a start or

stop codon!

Types of Mutations: Frameshift Mutation

• Insertion or deletion of nucleotides not in multiples of three

• Sequence is “non-sense”

• Alters amino acids “downstream” from site

Original The fat cat ate the wee rat.

Frame Shift The fat caa tet hew eer at.

Example: Tay-Sachs Disease • Insertion or a deletion

(there are many forms of Tay-Sachs) in the HEXA gene

• Shown is one of the mutations with insertion of four base pairs results in frameshift mutation

• Lack enzyme that breaks down lipids in brain

• Results in death of brain cells, poor muscles, etc

• This form of Tay-Sachs usually causes death by 4 years old

Review: Steps of Transcription

1. RNA polymerase travels along the DNA until it hits a promoter that is turned on

2. The DNA unzips3. RNA polymerase brings in complementary

RNA nucleotides to make the RNA strand4. When RNA polymerase hits the terminator

sequence on the DNA it stops making the RNA

Review: Steps of Translation• mRNA and the ribosome bind together• The ribosome travels down the mRNA until it

finds the start codon• tRNA, with its attached amino acid, comes into

the ribosome and the anticodon on the tRNA binds to the codon on the mRNA

• This brings the amino acids together in the correct order for a peptide bond to occur

• This continues down the mRNA stringing together amino acids to make a protein.