DNA, Mutations and Natural Selection

Structure of DNA

• Arranged as a double-helix

• Two strands that run in opposite directions

• Each strand consists of a chain of nucleotides

• Nucleotides consist of deoxyribose sugar, phosphate group and a nitrogenous base

• Nitrogenous bases on opposite strands are complementary

• Adenine pairs with thymine and cytosine pairs with guanine

• Complementary bases held together by hydrogen bonds

Function of DNA

A molecule in the nucleus of a cell that determines the types of protein a cell can make and codes for these proteins.

Describe how DNA replicates

• Occurs in the nucleus of cells during interphase before mitosis

• The enzyme Helicase unzips the DNA strand

• Free DNA nucleotides pair up using DNA as template

• Due to complementary base pairing

• DNA polymerase joins the nucleotides forming a sugar-phosphate backbone

• Ligase joins the nucleotides to the sugar phosphate backbone,

• Two identical copies are formed

Nuclear DNA vs Mitochondrial DNA

• Most of DNA molecules found in the nucleus of each cell, known as nuclear DNA

• Small amount of DNA found in the mitochondria, known as mitochondrial DNA (mtDNA)

• Makes up less than 1% of total DNA in humans

• Important for the functioning of the mitochondria and therefore the cell

Evolution definition

The change in characteristics of a species over time.

Phenotype definition

Observable characteristics of the individual - the way the genotype is expressed.

Genotype definition

Genetic information carried by an individual.

Allele definition

Different forms of the same gene.

Population definition

A group of organisms of the same species living together in a particular place at a particular time.

Gene pool definition

Sum of all the alleles in a given population.

Allele frequency

How often each allele of a gene occurs in the gene pool for that population.

 

Give an example of how gene pools can be used to compare populations at different locations or times.

Mutation definition

Mutations are changes in the DNA resulting in variation in the associated trait. They may occur spontaneously, but are often due to exposure to a mutagen.

Mutagen definition

Agent which is known to increase the rate at which mutations occur.

Mutant defintion

An organism which a characteristic resulting from a mutation is called a mutant.

List mutagens

• Formaldehyde

• Mustard Gas

• Sulfur dioxide

• Antibiotics

• Cigarette smoke

• Pesticides

• Ionising Radiation

  • Ultraviolet light X-ray

  • Cosmic rays

  • Fallout from atomic and nuclear explosions

  • Radiation from radioactive waste

Induced mutations defintion

Increased chance of a mutation occurring due to the presence of a mutagen (mutagenic agent)

Spontaneous mutations definition

A mutation which occurs due to a random error in a biological process such as mitosis or meiosis.

Gene mutation defintion

Changes in a single gene - traits usually produced by that gene are changed or destroyed. Example: sickle cell anaemia

Chromosome mutation defintion

 All or part of the chromosome is affected, therefore a number of genes are affected. Often causing abnormalities so severe that miscarriage can occur in early pregnancy. Example: Trisomy 21 (Down syndrome)

Somatic mutation

• A persons body cells (somatic cells) are involved

• Only the individual with the somatic mutation is affected

• Mutation is passed onto daughter cells each time the body cell divides

• Reproductive cells are not affected - once the individual dies the mutation is lost

• Example: cancerous growths that may be the result of a mutagenic agent

 

Germinal/Germline Mutations

• Reproductive cells are affected (germline cells)

• The individual is not typically affected but produces gamete with changed DNA

• Mutations can be passed on to the next and subsequent generations

• An affected gamete and subsequent embryo can lead to a miscarriage during pregnancy

• Example: PKU phenylketonuria

Point mutation definition

A point mutation is a type of genetic mutation that involves changes in a single nucleotide within the DNA sequence. This means that only one base is changed.

3 different ways mutations can be classified

Heritability of the mutation, effect of the mutation, extent of the mutation.

Heritability of the mutation

Somatic and Germinal/Germline Mutations

Effect of mutation

Missense Mutations, Nonsense Mutations, Neutral Mutations, Silent Mutations

Missense mutations definition

Cause a change in the amino acid, and therefore in the protein produced.

Nonsense mutations definition

Change the base sequence to the code to STOP. This means that the synthesis of the protein will stop, and so a shorter protein is produced that is unlikely to be able to fulfill its function.

Neutral mutation definition

Cause a change in an amino acid; however, the amino acid id of the same type and does not change the structure of the protein enough to change its function.

Silent mutation definition

Do not cause any change in the amino acid, and therefore in the protein produced. This is possible, as most amino acids are coded for by more than one base sequence.

3 types of point mutations

• Inserted - a new nucleotide is added to the DNA strand

• Substituted - an existing nucleotide is replaced with another one, with a different base

• Deleted - a nucleotide is removed from the DNA strand

Frameshift definition

• Occurs when bases have been added or removed

• Results in the series of three bases that code form an amino acid starting at a different base

• Frameshift mutations affect the outcome for all the DNA from that point on

List 5 types of chromosomal mutations

Duplication, Insertion, Deletion, Inversion, Translocation

Duplication mutation definition

A portion of the chromosome is copied twice and the gene sequence is repeated. This could cause the overproduction of a protein which could have a negative effect on the organism.

Insertion mutation definition

A segment of one chromosome is inserted into another chromosome. This can disrupt the genes at the insertion site and lead to various genetic disorders.

Deletion mutation definition

This occurs when the chromosome breaks and a fragment of the DNA, a gene is lost. This means that the chromosome is missing part of the genetic code, which can be lethal if it contained the code for an essential protein such as an enzyme.

Inversion mutation definition

A deletion occurs but this time the fragment reattaches in an inverted (upside down) position. The correct genes are present but in the wrong order so code for an incorrect protein.

Translocation mutation definition

A fragment of the chromosome becomes deleted and rejoins at a different position on the same chromosome. 

Non-Disjunction mutation definition

• During meiosis, a chromosome pair does not separate and so one daughter cell has an extra chromosome and one daughter cell has one less than the normal number.

• These are sometimes referred to not as 'mutations', but as aneuploidy - a change in the chromosome number.

Aneuploidy definition

Aneuploidy refers to a change in the chromosome number. It is a term used to describe situations where a cell has too many or too few chromosomes, rather than a typical gene mutation.

Trisomy definition

Trisomy is a result of non-disjunction, failure of one or more chromatids to separate in the second division of meiosis. The eggs or sperm formed when non-disjunction occurs have one chromosome too many, or one chromosome is missing.

Monosomy defintion

Monosomy is where an individual is missing a chromosome. If an autosome is completely missing, monosomy usually results in severe malformations and miscarriage. If only part of a chromosome is missing, it is referred to as partial monosomy. Part of the chromosome has two copies, but part has only one copy.

Karyotype defintion

Explain what is meant by a lethal recessive and the impact these can have on a gene pool.

Explain the significance of mutations in the process of evolution – they are the ultimate source of variation introducing new alleles into a population: new alleles may be favourable or unfavourable to survive.