Genetic diversity and adaptation

Question 1

What is a mutation?

Answer 1

any change to the quantity or the base sequence of the DNA of an organism

Question 2

What is a gene mutation?

Answer 2

Any change to one or more nucleotide bases or change in the sequence of the bases in DNA

Question 3

how are amino acids made into polypeptide?

Answer 3

sequence of triplets on DNA is transcribed into mRNA then translated into a sequence of amino acids that make up a polypeptide
- any change to the bases results in a change in the amino acid sequence of the polypeptide

Question 4

Substitution of bases

Answer 4

• when a nucleotide in a DNA molecule is replaced by another nucleotide that has a different base
• With base substitution mutations, only a single nucleotide within a gene sequence is changed, so only one codon is affected
• however most amino acids have more than one codon so if the new triplet of bases still code for same amino acid as before

Question 5

Deletion of base

Answer 5

• deletion arises when a nucleotide is lost from the normal DNA sequence
• usually the amino acid sequence of the polypeptide is entirely different and so the polypeptide is unlikely to function properly this is because the sequence of bases in DNA read in triplets so if one nucleotide is deleted then all triplets are to be read differently because they have shifted to the left by one base

Question 6

Chromosome mutations

Answer 6

Two forms:

• Changes in whole set of chromosomes occur when organisms have three or more sets of chromosomes rather that two. This condition is called polyploidy and occurs mostly in plants
• Changes in the number of number of individual: sometimes homologous pairs fail to separate during meiosis, this is known as non-disjunction and results in a gamete having either one or more fewer chromosomes e.g Downsyndrome where individuals have an additional chromosome

Question 7

Importance of meiosis

Answer 7

• human diploid number of chromosomes is 46 which means the cell would mean that this cell would have 92 chromosomes
• diploid number is the cell that produce has double this number
• haploid number in humans is 23 so at fertilisation two haploid gametes fuse at fertilisation the diploid number of chromosomes is restored

Question 8

The process of meiosis

Answer 8

• first division (meiosis 1) homologous chromosomes pair up and their chromitids wrap around each other and portions of chromatids may be exchanged in a process called crossing over
• second division (meoisis 2) chromatids move apart and at the end of meiosis, four cells have been formed so in each cell contains 23 chromosomes

Question 9

Meiosis brings about this genetic variation in the following two ways:

Answer 9

• independent segregation of homologous chromosomes

- new combination of maternal and paternal alleles by crossing over

Question 10

3 stages of homologous pairs:

Answer 10

stage 1) one of the pair of chromosomes includes the gene for tongue rolling and carries one allele for roller and non roller. The other chromosomes includes the gene for blood group A and the allele for blood group B
stage 2) at the end of meiosis 1, homologous chromosomes have segregated into two separate cells
stage 3) at the end of meiosis 2 , the chromosomes have segregates into chromatids producing four gametes for arrangement

Question 11

Genetic recombination by crossing over

Answer 11

• chromatids of each pair become twisted around one another
• during this, tensions are created and portions of chromatids break off
• these broken portions might then rejoin with the chromatids of its homologous partner
• usually it’s the equivalent portions of homologous chromosome
• in this way the new genetic combinations of maternal and paternal alleles are produced

Question 12

Genetic diversity

Answer 12

• is described as the total number of different alleles in a population
• a population is a group of individuals of the same species that live in the same place and can interbreed
• a species consists of one or more populations
• the greater the number of different alleles that all members of a species posses the greater the genetic diversity of that species
• greater range of alleles meaning a wider range of characteristics

Question 13

Reproductive success and allele frequency

Answer 13

• there’s a gene pool containing a wide variety of alleles
• random mutation of alleles in this gene pool may result in a new allele of a gene which in most cases can be harmful
• as these new individuals have the ‘adventageous’ allele they’re more likely to survive and reproduce effectively

Question 14

Depending on which characteristics are favoured, selection will produce a number of different results

Answer 14

• selection may favour individuals that vary in one condition from the mean ot the population a.k. a directional selection and changes the characteristics of the population
• selection may favour average individuals a.k.a stabilising selection

Question 15

Directional selection

Answer 15

Directional selection is one type of natural selection in which the phenotype (the observable characteristics) of the species tends toward one extreme rather the mean phenotype or the opposite extreme phenotype.
- The directional selection phenomenon is usually seen in environments that have changed over time. Changes in weather, climate, or food availability can lead to directional selection

Question 16

Stabilising selection

Answer 16

When selective pressures select against the two extremes of a trait, the population experiences stabilizing selection. For example, plant height might be acted on by stabilizing selection. A plant that is too short may not be able to compete with other plants for sunlight. However, extremely tall plants may be more susceptible to wind damage. Combined, these two selection pressures select to maintain plants of medium height. The number of plants of medium height will increase while the numbers of short and tall plants will decrease.

Question 17

stabilising selection results in phenotypes around the mean of the population being selected for and those at both extremes being selected against:

Answer 17

Natural selection results in species that are better adapted to the environment they live in, these adaptations might be:

• anatomical such as shorter ears and thicker fur in arctic foxed compared to foxes in warmer climates
• physiological such as oxidising of rat rather that carbohydrate in kangaroo rats to produce additional water in a dry desert environment
• behavioural such as the autumn migration of swallows from the UK to africa to avoid food shortages in the uk winter