Ribosomes and Protein Synthesis

13.23/18/2011

How Genes Are Expressed

DNA

RNA

PROTEIN

TRAN

SCRIP

TION

TRAN

SLATIO

N

Protein REVIEW

• Proteins are polymers of amino acids• Proteins are also called peptides or

polypeptides because the amino acids that make them up are held together by peptide bonds

• There are 20 different amino acids which make up all the proteins in living things– Different numbers & combinations = different

proteins with individual properties

The Genetic Code

• mRNA transcribed from DNA contains a code for making proteins• How is the mRNA genetic code of As, Us, Cs, and Gs “read” into a specific sequence of amino acids?• How can a code with only 4 letters

contain instructions for 20 different amino acids?

Reading the Genetic Code of mRNA

• The genetic code is read THREE letters at a time

• Each “word” is three letters long and corresponds to one specific amino acid

• Each of these three letter “words” is called a codon

How to Read mRNA Codons

• Read the Genetic Code chart from the inside out

• Start & stop codons–Think of as “punctuation”–Translation begins at start codon• AUG, the codon for methionine

–Translation ends at one of three stop codons•UGA, UAA, UAG

Transfer RNA

• Two sided: one side has anti-codon, the other is carrying an amino acid–tRNA anti codon matches mRNA codon

Protein Synthesis (Translation

• During translation, or protein synthesis, the cell uses information from mRNA to produce proteins• The ribosomes of the cell read the instructions (the mRNA) and assemble the parts (the amino acids) into proteins

Steps of Translation1. Initiation– Ribosome attaches to mRNA in the cytoplasm

2. Elongation– As each mRNA codon passes through the ribosome,

the tRNAs bring the appropriate amino acid– One at a time, the ribosome attaches the amino

acid from the tRNA to the growing chain of amino acids (this is forming the protein)

– This “assembly line” continues until…

3. Termination1. STOP codon reached, then…2. mRNA and protein released from the ribosome

• http://www.youtube.com/watch?v=5bLEDd-PSTQ