unit 1 bio genes and mutations

What is a gene?

A sequence of bases on a DNA molecule that codes for a sequence of amino acids in a polypeptide chain.

Nature of genetic code non overlapping

non-overlapping meaning that each triplet is only read once and triplets don't share any bases

Contain start and stop codons which either start or stop protein synthesis.

nature of genetic code degenrate

Degenerate meaning that more than one triplet codes for the same amino acid. This reduces the number of mutations which are mistakes in the base sequence

Contain start and stop codons which either start or stop protein synthesis.

what is genetic code

The order of nucleotide bases on DNA makes up the genetic code which consists of triplets of bases; each triplet of bases codes for a particular amino acid and is known as a codon.

Not all of the genome codes for proteins - the non-coding sections of DNA are called introns and the coding regions are called exons.

genetic mutations

Since proteins are coded for by the sequence of bases on DNA, any errors made in copying the DNA during replication can affect the polypeptide produced, since a different amino acid means different folding and bonding so a different tertiary structure and 3D shape.

Mutations

The majority of mutations are not harmful however, due to the nature of the genetic code, and the presence of non-coding introns. As introns do not code for proteins so if a mutation occurs here, no polypeptide will be affected.

subsitution mutation

Where one base is substituted for another.

In some cases, the mutation may not change the amino coded for, since the degenerate nature of the genetic code means multiple codons code for the same amino acid. Sometimes however, it does result in a different amino acid

define gene

A sequence of bases on a DNA molecule that codes for a sequence of amino acids in a polypeptide chain

define allele

A form of a gene

insertion mutation

Where an extra base (or bases) is inserted into the DNA sequence.

This mutation of then results in a non-functional protein as all of the codons after the insertion are affected as the code is non-overlapping. This is known as frame shift.

deletion mutation

Where a base (or bases) is removed from the sequence.

This results in frame shift and a likely non-functional protein as all codons after the deletion are changed.

define phenotype

Physical characteristic that is expressed due to its genotype

define recessive allele

allele that is not expressed in the phenotype in the presence of an alternate allele

what is dominant allele

expressed in the phenotype even in the presence of an alternate allele

Define the term codominance

Alleles in the genotype that both contribute to the phenotype.

define homozygote

When an organism has 2 of the same alleles for a characteristic

define hetrozygote

When an organism has 2 different alleles for a characteristic

define genotype

combination of alleles present for a given characteristic

sex-linkage on the X chromosome

• A sex-linked gene is a gene located on a sex chromosome.

• Most sex-linked conditions are found on the X chromosome.

• The Y chromosome is much shorter and lacks many corresponding alleles.
  
  therefore men are more likely to get genetic disorders as they have only one X chromosome which may express the recessive allele whereas females will need 2 recessive alleles to show the condition

red green colourblindness

Red–green colour blindness is an X-linked recessive condition in which males are more commonly affected because they have only one X chromosome, while females require two recessive alleles to express the condition.