Protein synthesis and gene variants

From base order to protein shapeSpec 3.7B

A gene is a section of DNA that codes for a specific protein. Proteins are made of chains of amino acids. The order of bases in a section of DNA decides the order of amino acids in the protein.

1. The sequence of bases in the gene decides the order of amino acids.
2. The chain of amino acids folds up.
3. The way the chain folds depends on the order of the amino acids, so it produces a specifically shaped protein.

The shape of a protein matters because it decides what the protein can do. For example, an enzyme has an active site with a shape that fits its substrate. A different order of bases would give a different order of amino acids, a differently folded protein and so a different shape.

Protein synthesis: transcription and translationSpec 3.8B

Protein synthesis has two main stages. In transcription, a copy of the gene is made as a strand of mRNA. In translation, the mRNA is used to build a polypeptide at a ribosome.

Transcription (in the nucleus)

1. RNA polymerase binds to the non-coding DNA located in front of a gene.
2. RNA polymerase produces a complementary mRNA strand from the coding DNA of the gene.
3. The mRNA moves out of the nucleus into the cytoplasm.

Translation (at a ribosome)

1. The mRNA attaches to a ribosome.
2. The mRNA is read in triplets of bases called codons. Each codon codes for a specific amino acid.
3. tRNA molecules transfer amino acids to the ribosome. Each tRNA carries the amino acid that matches a codon on the mRNA.
4. The amino acids are linked together to form a polypeptide, which folds into a protein.

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Genetic variants in non-coding DNASpec 3.9B

A genetic variant is a difference in the DNA sequence. Variants in the non-coding DNA in front of a gene can affect the phenotype without changing the protein itself.

1. The variant is in the non-coding DNA, where RNA polymerase binds.
2. This can change how well RNA polymerase binds.
3. More or less mRNA is made, so the quantity of protein produced is altered.
4. The change in the amount of protein can change the phenotype.

Genetic variants in coding DNASpec 3.10B

Variants in the coding DNA of a gene can change the base sequence that codes for the protein.

1. A variant in the coding DNA can alter the sequence of amino acids.
2. The polypeptide may then fold into a different shape.
3. A protein with a different shape may have different activity. For example, an enzyme's active site may no longer fit its substrate.
4. The change in the activity of the protein can affect the phenotype.