Objectives

• Identify that amino acids are coded by mRNA base sequences and are linked to become proteins

• Describe how mRNA codons are translated into amino acids

• Summarize the process of protein synthesis

Review

• What was the purpose of transcription?– To transcribe (copy) DNA into a disposable copy

called mRNA to leave the nucleus.• What are the nitrogen bases of mRNA?

– A, U, G, C• What is the sugar for mRNA?

– Ribose• What are the 3 differences between DNA and

RNA?– Ribose sugar, single stranded, Uracil instead of

Thymine• What are the 3 different types of RNA?

– mRNA, rRNA, tRNA

Translation

• TRANSLATION: process that converts, or translates, an mRNA message into a protein (amino acid chain).

• How is mRNA translated??– By reading the nitrogen base sequences in

groups of 3. These groups of 3 are called Codons.

Codons

• Codon- a group of three nucleotides on mRNA that code for a particular amino acid

codon formethionine (Met)

codon forleucine (Leu)

• If you had the mRNA sequence:

UCGCACGGU

• The codons would be:

UCG CAC GGU

• What amino acids would be coded for?– Let’s find out!

Finding Amino acids from Codon

• Reading the mRNA codons left to right write the amino acid sequence of the protein translated from the mRNA.

Codon to Amino Acid

UCG = – Serine

CAC = – Histidine

GGU = – Glycine

Amino Acids• There are 64 possible three-base codons that

can be created using the four nitrogen bases (4 x 4 x 4 = 64)

• There are 20 different amino acids– There are multiple codons that code for the

same amino acids• AUG= start codon for protein synthesis or

methionine. This means that translation always begins with methionine (AUG).

• Three codons do not code for an amino acid so are considered stop codons that signals the end of an amino acid chain. - UAA, UAG, UGA

How does a cell interpret DNA?

• Write the following DNA sequence:– GAC AAG TCC ACA ATC

• Write the mRNA sequence that would be transcribed from this gene:– CUG UUC AGG UGU UAG

• Reading the mRNA codons, write the amino acid sequence of the polypeptide translated from the mRNA

Codon to Amino Acid

CUG= – Leucine

UUC= – Phenylalanine

AGG= – Arginine

UGU= – Cysteine

UAG= – Stop

• So proteins are determined by the chain of amino acids that make them up

• But how do our protein makers know which amino acids to add?

What’s Next…Translation!

– After mRNA LEAVES the NUCLEUS, it takes DNA’s message to the RIBOSOME (rRNA)

– Proteins are made on the rRNA using mRNA as a template.

– So translation occurs in the rRNA, which is located in the cytoplasm of the cell.

• Three steps to translation– Initiation– Elongation– Termination

REMEMBER: mRNA is READ 3 letters at a time in order to determine what protein is to be madeo These three letters are called a CODONCODONo The The CODONCODON on the mRNA matches up with on the mRNA matches up with

the the ANTICODONANTICODON on the on the tRNA moleculetRNA molecule

)

Initiation: The start of translation

• The small ribosomal subunit attaches to the strand of mRNA at the 5` end.

• A tRNA molecule with the start or initiator anticodon (UAC) attaches to the mRNA.

• The large ribosomal subunit attaches, locking the initiator tRNA in the P (peptide) site.

ElongationThe amino acid chain gets longer

• The second tRNA molecule, with an anticodon that is complementary to the second codon on the mRNA, attaches in the A site of the large subunit.

• A peptide bond is formed between the amino acid of the tRNA in the A site and that of the P site.

• The tRNA molecule in the P site is kicked out and the one in the A site (with the growing polypeptide chain) shifts into the P site.

• The process repeats many times until tRNA reaches a stop codon.

TerminationThe final step

• Toward the end of the coding sequence of the mRNA, a termination (stop) codon is encountered (UAG, UAA, or UGA).

• There are no tRNA molecules that match the stop codons, but a special protein called a release factor does recognize the sequence, and attaches.

• This special protein causes the newly created amino acid chain to be released.

• The ribosomal subunits disassemble.• Translation is complete!!!!!!!!!!!!!!!!!

Amino Acid Chain

Review • 1. Initiation

• 2. Elongation

• 3. Termination

**Protein Synthesis in a Nutshell**

Transcription

-In NUCLEUS

-DNA copied to mRNA

-mRNA to Ribosome

Translation

-In RIBOSOME

-Codon/Anticodon pair

-codon determines what AA comes in

-AA combine-proteins

RNADNA

RNApolymerase

Adenine (DNA and RNA)Cystosine (DNA and RNA)Guanine(DNA and RNA)Thymine (DNA only)Uracil (RNA only)

Transcription

mRNA Start codon

Ribosome

MethioninePhenylalanine tRNA Lysine

Nucleus

mRNA

Translation

Ribosome Translation direction

Lysine tRNA

tRNA

Growing polypeptide chain

mRNA

Ribosome

Translation Continued