1. Bellwork: How does a cell interpret the genetic code
Bellwork: How does a cell interpret the genetic code? How does the genetic code affect the expressions of specific traits in an organism?

2. Ribosomes and protein synthesis
Section 13.2

3. The genetic code Step one - copy DNA to produce RNA
RNA contains instructions on how to make proteins
Proteins are chains of amino acids called polypeptides
Up to 20 different amino acids are commonly found in polypeptides
Specific amino acids and their arrangement determine properties of different proteins
Amino acid arrangement influences shape, which determines proteins function

4. The genetic code continued…
RNA contains four different bases
Essentially a language with 4 letters
Genetic code is read three letters at a time
Each ’word’ is three bases long and corresponds to an amino acid
In mRNA, each ‘three letter’ word is a codon
Three consecutive bases that specify a single amino acid

5. Reading codons There are 64 possible three base combinations in RNA
Due to the 4 different bases
Most amino acids can be specified by more than one codon
Leucine is coded by 6 different codons..
Genetic code table makes decoding codons easy

6. Start and Stop codons
These are essentially punctuation marks in the genetic code
Methionine codon serves as the initiation or start codon for protein synthesis
After this point, the mRNA is read three bases at a time
There are 3 different stop codons which mark the end of polypeptide formation

7. Translation Why are ribosomes so important?
mRNA = instruction manual
Ribosome reads the instruction manual and assembles the parts together
The assembly process performed by ribosomes is called translation

8. Translation overview

9. Steps of translation
Step 1: Ribosome attaches to mRNA molecule in the cytoplasm
Step 2: As each codon passes through the ribosome, tRNA brings the proper amino acids into the ribosome. The ribosome attaches amino acids together into a growing chain
Each tRNA molecule is three unpaired bases, called the anticodon
Each tRNA molecule anticodon is complementary to one mRNA codon
Step 3: Ribosomes help to form peptide bonds between amino acids and build the protein, like a production line
Step 4: Polypeptide chain grows until it reaches a stop codon
Here the newly formed polypeptide and mRNA molecule are released.

10. Roles of tRNA and rRNA in translation
All three types of RNA come together during translation
mRNA carries coded message
tRNA molecules carry the amino acids called for by each codon
Ribosomes are made of about 80 proteins and 3 or 4 different rRNA molecules
rRNA helps keep ribosomal proteins in place, and help locate start of mRNA message
They may also help in formation of peptide bonds

11. Molecular basis of heredity
Most genes simply contain instructions for making proteins
Proteins are key in controlling what traits are exhibited
Many proteins are enzymes, which catalyze and regulate chemical reactions
Skin colour can be controlled by a gene that codes for an enzyme to prdouce a specific pigment
Proteins act as microscopic tools, each specifically designed to build or operate a component of a cell

12. Central Dogma Information is transferred from DNA to RNA to protein
There are however many exceptions – e.g. viruses transfer information in the opposite direction, from RNA to DNA
Acts as a useful generalization to show how genes work
Genetic code operates virtually the same way in all living organisms
Some organisms may vary which amino acid applies to specific codons
Code is always read three bases at a time
Code is always read in the same direction
There is remarkable unity between all living organism with the molecular biology of the gene.