DNA & Protein synthesis

What is a gene?

A gene is a sequence of DNA bases that contains the coded information for making peptides and functional RNA.

Degenerate code

When the different triplets code for the same amino acid.

Features of the genetic code

• The code is known as degenerate, because most amino acids are coded for by more than one triplet

• Some triplets act as ‘stop’ and ‘start’ code.

• The code is non-overlapping, as each base in the sequence is only read once and belongs to one triplet

• The code is universal, meaning each triplet codes for the same amino acid in all organisms

Introns and Exons

There are regions in eukaryotic DNA that do not code for polypeptides. These regions are called introns.

The coding sequences are called exons.

Introns are removed during proteins synthesis so they do not affect the amino acid order.

Proteins synthesis - Transcription

1. In the nucleus, the DNA strand unwinds and DNA helicase breaks the hydrogen bonds between the nitrogenous bases.

2. Free RNA nucleotides in the nucleus align with their complementary DNA base on the exposed sense strand.

3. RNA polymerase joins the RNA nucleotides together via a condensation reaction to form a pre-mRNA strand

4. Introns on the pre-mRNA strand are spliced out and the exons rejoin, forming mRNA.

Protein synthesis - Translation

1. mRNA exits the nucleus through a nuclear pore into the cytoplasm. A ribosome associates with the mRNA strand.

2. tRNA aligns its anticodon region to the complementary triplet codon on the mRNA strand, while carrying a specific amino acid that is bound to the tRNA by ATP

3. Step 2 repeats, and multiple tRNA molecules with specific amino acids will be paired with their complementary anticodon on the mRNA strand. The amino acids are joined by peptide bonds via a condensation reaction, forming a polypeptide chain.

4. The first tRNA molecules move away from the mRNA strand, leaving the amino acid, and returning to the cytoplasm to retrieve another amino acid.

5. The ribosome moves along the mRNA molecule, reading 2 codons at a time until the whole sequence has been read and the polypeptide chain is complete.

Chromosome structure (eukaryotes)

• Chromosomes are only visible as distinct structure when a cell is dividing

• Chromosomes appears 2 threads, each thread called a chromatid, as the replicated DNA gives 2 identical (sister) DNA molecules.

• They are associated with proteins called histones which holds the DNA together.

• The DNA is highly coiled and folded.