Genes and Protein Synthesis

INSTRUCTIONS
DNA contains Genes which are Instructions for Making Proteins.

• A Gene is a Sequence of DNA Nucleotides that Codes for a Polypeptide.
• The Sequence of Amino Acids (AAs) in a Polypeptide chain forms the Primary Structure of a Protein.
• Different Proteins have a Different Number and Order of AAs
• The Order of the Nucleotide Bases in a Gene that determines the Order of AAs in a particular Protein.
• Each AA is Coded for by a Sequence of 3 Bases (called a Triplet) in a Gene.
• Different Sequences of Bases code for Different AAs, So the Sequence of Bases in DNA is a Template used to make Proteins during Protein Synthesis

DNA IS COPIED INTO RNA

• DNA is found in the Nucleus of the cell. However, the Organelles that make Proteins are found in the Cytoplasm.
• DNA is Too Big to move Out Of The Nucleus.
• A Section of DNA is Copied into mRNA. This bit is called Transcription.
• The mRNA Leaves the Nucleus and Joins with a Ribosome in the Cytoplasm, where it can Synthesise a Protein

RNA

RNA is a Single Polynucleotide Strand and is similar to a single DNA strand, but instead of Thymine (T) it contains Uracil (U). There are 3 types of RNA we need to know about:

MESSENGER RNA (mRNA)

• It is Made in the Nucleus.
• Three Adjacent Bases are called a Codon.
• It Carries the Genetic Code from the DNA in the Nucleus to the Cytoplasm where it is used for Protein Synthesis (Translation)

TRANSFER RNA (tRNA)

• Found in the Cytoplasm
• It has an AA Binding Site at one end and a Sequence of 3 Bases at the other end, which is called an Anticodon.
• It Carries the AAs that are used to make Proteins with the Ribosomes during Translation.

RIBOSOMAL RNA (rRNA)

• Forms the Two Subunits in a Ribosome.
• The Ribosome Moves along the mRNA strand during Protein Synthesis. The rRNA in the Ribosome helps to Catalyse  the Formation of Peptide Bonds between the AAs.

THE GENETIC CODE

The Genetic Code is the Sequence of Base Triplets (Codons) in DNA or mRNA. It Codes for Specific AAs.

Within the genetic code, each Codon is Read in Sequence, separate from the codon before and after it.

Base Triplets don't share their Bases, so they are 'Non-Overlapping'.

There are More Possible Combinations of Triplets Than there are AAs (20AAs = 64 Possible Triplets). We call this Degenerate. Because of this, some AAs are coded for by More Than One Base Triplet, i.e. Tyrosine can be coded for by either UAU or UAC

Some triplets tell the cell when to Start/Stop Production of the Protein, unsurprisingly, these are called Start and Stop Signals. (or Start/Stop Codons). They're found at the Beginning and End of the Gene. An example is UAG (Stop)

The genetic code is also Universal. The Same Specific Triplets Code for the Same AAs in All living organisms.