Genetic Code

• The genetic code of an organism is the sequence of the bases
• A polypeptide (lots of these nucleotides), made from amino acids, are a sequence of bases in a gene code
• These codes are almost universal; the same codes for the same amino acids across all organisms (AUG = methionine in all organisms)
• The order of the four bases make up the genetic code

Codon/Triplet Code

A set of 3 in the genetic code is called a triplet on the DNA strand, and called a codon on the mRNA strand These codons code for a specific amino acid

Confirmed by Francis Crick; each triplet is degenerate, so that 1 or more triplets can code for each amino acid)

mRNA (messenger RNA)

mRNA is a single strand of replicated DNA; it has the sugar ribose instead of deoxyribose and instead of Thymine, it has Uracil¹ When a polypeptide is required, the triple code of its gene is converted into mRNA strand which goes to a ribosome to code for the amino acid

Steps to protein synthesis²

Transcription

DNA is too big to leave the nucleus, so it has to be transcribed with only the necessary gene information RNA polymerase unwinds the DNA around 17 bases at a time, and copies the DNA using the 3’ to 5’ strand of the DNA.

Adenine bonds with Uracil; Guanine bonds with Cytosine} uracil replaces thymine

RNA processing

Splicing

• In the gene, there are two types of “chunks”
• Introns: useless, exons: useful
• In DNA splicing, spliceosome cuts up the mRNA strand and sticks the exons together, while leaving the introns out

5’ methylated Guanine cap

• A modified methylated guanine cap is added to the 5’ end of the mRNA strand (think of it as a helmet)
• This stops it from being degraded by enzymes in the cytoplasm, and essentially protects it

Poly-A tail

• This is a tail added at the end of the mRNA strand, consisting of around 30-200 Adenine bases
• This protects it and makes it more stable to exit the nucleus
• Adenine is better to use as a tail because the mRNA strand will have to degrade eventually, and Adenine is easily degradable; another cap would take longer to get rid of

Translation

Once the mRNA is transcribed, it moves out of the nucleus via the nuclear pores

In the cytoplasm, mRNA combines with a ribosome {this is where mRNA codes for a polypeptide chain}

Transfer RNA looks like T’s that carry an amino acid

Each tRNA has a complememtary codon to the codon on the RNA, the ribosome finds the tRNA with the matching anti-codon and brings it; the amino acid the tRNA is carrying will be dropped off (E.g. AUG codes for methionine)