mutation and gene editing

gene mutation

structural changes to genes at the molecular level

• substitution - one base in coding sequence of a gene is replaced by a different base

• insertion - a nucleotide inserted, so extra base in the sequence of the gene - requires break in sugar - phosphate backbone

• deletion - a nucleotide removed, requires 2 break in sugar phosphate backbone

consequence of base substitution

In coding regions:

• same sense : the change in the base does not cause a change in the amino acid

• nonsense : the change causes a stop codon to replace an amino aciod

• mis sense : the change causes a different amino acid to be inserted in the chain

• SNPS are result of base substitution mutations bc of degeneracy of the genetic code that may or may not change a single amino acid in a polypeptide

consequence of insertions and deletions

• inserting or deleting a base changes the ‘reading frame’ of a codon

• usually harmful

• if 3 bases added/ deleted, reading frame stays the same but it creates a different amino acid sequence, usually harmful

cause of gene mutation

mutations are random and caused by errors in replication and mutagens

mutagens cause chemical changes to DNA : radiation (UV, x-rays), chemical substances (tobacco smoke, mustard gas)

Randomness in mutation

• mutations occur in individuals and are random

• occur anywhere in base sequence of genome

• whether or not they are beneficial does not change the likelihood of them occuring

• there is no natural mechanism for creating a mutation

• mutations can occur anywehre in the genome but some are more likely than others

• most mutations acquired during an organism’s lifetime are not passed to offspring even if they are beneficial

consequences of mutation in germ cells and somatic cells

somatic cell : body cell

• not passed to offspring

• example : oncogene mutations that cause cancer

Germ cell - gamete producing cell

• can be passed to offspring

• most are harmful

• important to minimise exposure

mutations are a source of genetic variation

allele - different versions of a gene

• new alleles arise from mutations

• increase in genetic variation

• most are either silent or harmful

• rare, beneficial mutations have an advantage - natural selection

• environments change → organisms must change → mutations

gene knockout

• investigate function of a gene by changing it to make it inoperative

• open reading frame - characteristic base sequences that help identify where genes are located

• gene knockout - a technique used to determine the function of a gene

1. use an embryo

2. delete on copy of the target gene

3. grow into an adult

4. breed 2 adults together that each have one missing gene

5. compare the offspring to see what the gene does (two copies, one copy, no copies)

use of CRISPR sequences adn the enzyme Cas9 in gene editing

1. method of finding and altering genes

• target DNA identified

• cut and separate target DNA RNA is reversed transcribed into DNA

• inserted into the cut

• organisms has a new/alterred section of DNA

2. Eliminating genetic diseases

• sickle cell anemia

• modifying plants to be more nutritious/tolerant

• infertile mosquitoe

hypotheses to account for conserved or highly conserved sequences in genes

• conserved sequences - base sequences that are identical across a species or group of species

• highly conserved sequences - base sequences that are identical over long periods of time and across a wider range of species

• code for things with unchanging functions (rRNA or tRNA)

• there are some highly conserved non coding elements but the functions are unclear, hypothesis is that they are found in regions of genome where mutations are rare