1.4 The genetic code and protein synthesis

What is the genetic code?

A representation of genetic information/instructions contained in a gene that tells a cell how to make a specific protein.

What is coded and decoded information?

• Coded information are the nucleotide sequences in a DNA strand

• Decoded information are the order of amino acids in polypeptides

How is the genetic code a universal code?

The same sequence of nucleotides codes for the same amino acids

• example CCA codes for proline implants, animals and bacteria

What does it mean for a genetic code to be degenerate?

This refers to how more than one codon can code for the same amino acid

What is a codon?

A sequence of three consecutive nitrogenous bases

What is a gene?

A sequence of DNA which is transcribed into RNA

What are the steps in gene expression?

involves

• transcription

• RNA processing

• translation

What is the template strand?

The strand of DNA which produces a complementary mRNA strand during transcription

What is the coding strand or the (non-template strand)?

A strand complementary to the template strand and cantina the genetic code

How does DNA get transcribed into mRNA?

1. an enzyme (RNA polymerase) attaches to a specific promoter sequence of DNA

2. DNA unzips and exposes the bases of the template strand

3. The RNA polymerase enzyme moves along in a three’ to five’ direction building a complementary mRNA strand with the help of free ribonucleotides

4. Once past the coding region transcription stops and mRNA is released from the template.

What is RNA processing?

A process which occurs after transcription and is involved in modifying pre-mRNA to form mature mRNA known as post transcription modification

What are the steps to RNA processing?

1. Capping

   The front end of the pre-mRNA is capped with an altered guanine base offering protection and helps to attach it to the ribosome

2. Adding a tail

   The back end of the pre-mRNA gets added to a long chain of adenine bases (poly-adenine tail) offering stability and facilitates mRNA export out of the nucleus

3. Splicing

   Not all parts of the RNA code is useful, therefore some parts are spliced.

   Introns - ‘bad’ parts which get cut out as they don’t code

   Exons - ‘good parts which contain instructions and are spliced together by spliceosomes.

What is translation?

A process where mRNA sequence is decoded and translated into a protein chain - primary structure

What is the process of mRNA being translated to a protein chain?

1. mRNA attaches to a ribosome

2. Amino acids are brought to the mRNA via transfer RNA with three bases that make up the anticodon during translation and detached when the following tRNA attaches

3. The ribosome continues to move along the mRNA and tRNA molecules

4. A codon representing stop and a polypeptide is released from the ribosome

What polymer is synthesised during transcription?

mRNA

In which direction does a coding/non-template strand move?

A 5’-3’ direction

In which direction does a template/non-coding strand move?

A 3’ to 5’ direction

In which direction does mRNA move"?

A 5’-3’ direction

What is another name for RNA processing?

Post transcription modification