Genetic Diversity

Question 1

What Is A Mutation?

Answer 1

Any change to the quantity or the base sequence of the DNA of an organism is known as mutation.

Mutations occurring during the formation of gametes may be inherited, often producing sudden and distinct differences between individuals.

Question 2

What Is A Gene Mutation?

Answer 2

Any change to one or more nucleotide bases, or a change in the sequence of the bases, in DNA is known as a gene mutation.

Gene mutations can arise spontaneously during DNA replication and include base substitution and bass deletion.

Question 3

Simplistic Formation Of Polypeptide.

Answer 3

A sequence of triplets on DNA is transcribed into MRN a and is then translated into a sequence of amino acids that make up a polypeptide.

Any changes to one or more bases in the DNA triplet could result in a change to the amino acid sequence of the polypeptide.

Question 4

Substitution Of Bases?

Answer 4

Gene mutation where a nucleotide and a DNA molecule is replaced by another nucleotide that has a different base is known as a substitution.

E.g. DNA triplet, GTC is changed to GTG. The amino acids formed is changed. Depending on what this amino acid coded for and how significant the change is will determine the changing of the polypeptide produced.

Sometimes the nucleotide base in the DNA that is changed can code for the same amino acid. This means the protein coded for is the exact same.

Question 5

Deletion Of Bases?

Answer 5

A gene mutation by 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 correctly. This is because the sequence of bases in DNA is read in units of three bases (triplet).

One deleted nucleotide causes all triplets in a sequence to be read differently because each has been shifted to the left by one base.

Question 6

Chromosome Mutations?

Answer 6

Changes in the structure or number of whole chromosomes are called chromosome mutations.

Chromosome mutations can arise spontaneously and take two forms:

• Changes in pairs of chromosomes occur when organisms have three or more sets of chromosomes rather than the usual two. This condition is called polyploidy and occurs mostly in plants.
• Changes in the number of individual chromosomes. Sometimes individual homologous pairs of chromosomes fail to separate during meiosis. This is known as NON-DISJUNCTION and usually results in a gamete having either one more or one fewer chromosome. On fertilisation with a gamete that has been through this, the resultant offspring have more or less chromosomes than normal in all their body cells. An example of non-disjunction in humans is down syndrome, where individuals have an additional chromosome.

Question 7

Mitosis Definition?

Answer 7

Mitosis produces two daughter cells with the same number of chromosomes as the parent cell and as each other.

Question 8

Meiosis Definition?

Answer 8

Meiosis usually produces for daughter cells, each with half the number of chromosomes as the parent cell.

Occurs only in reproduction of sex cells.

Question 9

Importance Of Meiosis?

Answer 9

In sexual reproduction, two gametes fuse to give rise to new offspring.

In humans, the diploid number of chromosomes in a cell is 46.

In order to maintain a constant number of chromosomes in the adults of a species, the number of chromosomes must be hard at some stage in the life-cycle. This having occurred as a result of meiosis.

During meiosis, homologous pairs of chromosomes separate, so that only one chromosome from each pair enters a daughter cell.

This is known as the haploid number of chromosomes which, in humans, was 23. Went to haploid gametes fuse at fertilisation, the diploid number of chromosomes is restored.

Question 10

What is a gamete?

Answer 10

A gamete is the sperm cells in males and the egg cells in females.

They join together at fertilisation to form a zygote, which divides and grows into an organism.

Normal body cells have the diploid number of chromosomes (46).

Gametes have a haploid number of chromosomes (23).

Fertilisation is random (any sperm can fertilise any egg) and so zygotes are produced with different combinations of chromosomes. This mixing of genetic material in sexual reproduction increases genetic diversity within a species.

Question 11

What are homologous pairs?

Answer 11

Chromosomes that are in pairs. They are the same size and have the same jeans, although they could have different versions of those jeans (called alleles).

Question 12

Two important reasons for meiosis?

Answer 12

1. When gametes fuse at fertilisation to form a zygote, the diploid number is restored. This enables a constant chromosome number to be maintained from generation to generation.
2. Variation is introduced into the genetic material through independent assortment of chromosomes and crossing over.

Question 13

Stages of meiosis?

Answer 13

Meiosis is the type of cell division. It takes place in the reproductive organs.

Cells that divide by mitosis are diploid to start with (they’re not sex cells - they’re a type of stem cell. We don’t need to know this. They’re diploid to start, don’t question it Hav), but the cells that are formed from meiosis are haploid.

1. To start with, there is a diploid cell with a paternal chromosome (Dad) and maternal chromosome (Mam). DNA unravels and replicates so there are two copies of each chromosome, called chromatids. Once DNA replicates, there are 46 PAIRS of homologous chromosomes.
2. The DNA condenses showing chromosomes, each made from two sister chromatids. The sister chromatids are joined in the middle by a centromere.
3. Meiosis 1 (division 1) - the chromosomes arrange themselves into homologous pairs. They line up.
4. These homologous pairs are then separated, halving the chromosome number.
5. Meiosis 2 (division 2) - the pairs of sister chromatids that make up each chromosome are separated (the centromere is divided).
6. Four haploid cells (gametes) that are genetically different from each other are produced. There’s only chromatids in these cells now.

Question 14

3 ways that genetic variation in meiosis can occur?

Answer 14

1. Independent segregation,
2. Crossing over.
   (3. Random fertilisation is basically which sperm reaches the egg first - its random. However, it doesn’t occur IN meiosis, it occurs after the gametes have formed so after meiosis).

Question 15

What is genetic diversity?

Answer 15

The total number of different alleles in a population (group of individuals of the same species that living in the same place and can interbreed).

Genetic diversity is reduced when there are fewer alleles.

Genetic diversity enables natural selection to occur.

Question 17

What is independent segregation?

Answer 17

This is what during meiosis that allows for genetically different gametes to be made.

During division 1, each chromosome lines up alongside its homologous partner so there’s 23 pairs of chromosomes side by side.

During independent segregation, the combination of the pairs is changed. So the homologous partners change.

There will always be a maternal and paternal in a pair.

The changing of these pairs change the chromatids that are created at the end.

Question 18

Crossing over?

Answer 18

Another way that during meiosis, genetic variation occurs.

• The chromatids of each pair become twisted around one another.
• During this twisting, tensions are created and portions of the chromatid break off.
• These broken potions might then rejoin with the chromatids of its homologous partner (they swap small sections of their genes). This is recombination.
• New genetic combinations of maternal and paternal alleles are produced.

Crossing over is the most rare way of genetic variation.

Sometimes, when the chromatids break off, they do not recombine with their partner. This will cause only 2 gametes to be produced because a pair has been broken.

Question 19

Gametes, zygotes and fertilisation?

Answer 19

Gametes are the sperm cells in males and egg cells in females. They have haploid number of chromosomes - only one copy of each.

They join together at fertilisation to form a zygote, which divides and develops into a new organism.

Normal body cells have the diploid number of chromosomes - meaning each cell contains two of each chromosome, one from the mum and one from the dad.

At fertilisation, the haploid sperm cells fuses with haploid egg cell, making it diploid. Half chromosomes are from mum and half are from dad.

Question 20

Products of meiosis compared to mitosis?

Answer 20

Mitosis - produces cells with the same number of chromosomes as the parent cell.

• Daughter cells are genetically identical to each other and the parent cell.
• There are 2 daughter cells.
• One division.

Meiosis - produces cells with half the number of chromosomes as the parent cell.

• Daughter cells are genetically different.
• 4 daughter cells.
• 2 divisions (which separate the homologous pairs and the sister chromatids).

Question 21

Chromosome mutations?

Answer 21

Caused be errors in cell division.

When meiosis works properly, all four daughter cells will end up with 23 chromosomes. When meiosis goes wrong, the daughter cells may have 24/22 chromosomes.

This is a chromosome mutation and is caused my meiosis errors.

They lead to inherited conditions because the errors are present in the gametes (when fertilised).

Question 22

How is genetic diversity increased?

Answer 22

Lots of alleles = higher genetic diversity.

It’s increased by:

• mutations and
• different alleles being introduced into a population (migration). This is called gene flow.

Genetic diversity allows natural selection to occur.

Question 23

How can we predict combinations in meiosis and sexual reproduction (mathematical)?

Answer 23

We can work out how many combinations can be made during meiosis.

N = number of pairs of homologous chromosomes.

Use: 2^n when calculating the number of combinations that can be made during meiosis.

So an organism with 4 homologous pairs of chromosomes can produce 2^4 which is 16 different combinations of chromosomes.

Use: (2^n)^2 when we’re talking about sexual reproduction (fertilisation).

So an organism with 4 homologous pairs of chromosomes can produce ((2^4)^2 )) which is 256 combinations.

Question 23

Genetic bottleneck?

Answer 23

A genetic bottleneck is an event that causes a big reduction in a population (when a large number of organisms within a population die, for example).

This reduced the number of alleles in the gene pool and reduces genetic diversity as a result.

The survivors reproduce and a large population is created from a few individuals.

Question 23

Founder effect?

Answer 23

Is a type of genetic bottleneck.

Describes what happens when just a few organism in a population start a new colony and there are only a small number of different alleles in the initial gene pool.

The frequency of each allele in the new colony may be very different to the frequency in the original colony - e.g. a rare allele in original is now common in new.

This may lead to a higher incidence of genetic disease.

The founder effect can occur as a result of migration leading to geographical separation or if a new colony is speedster from the original for another reason (e.g. religion).

Question 24

Why do mutated alleles which aren’t beneficial not increase in allele frequency?

Answer 24

Because the mutated allele is not beneficial, and if harmful, then it will die out of the population quickly.

Question 26

How does reproductive succession change allele frequency?

Answer 26

1. Variation caused by mutation.
2. Selection pressure.
3. One organism is better adapted to survive due to favourable phenotype.
4. They can reproduce and pass on advantaged allele onto offspring.
5. That causes an increase in the allele in the population.

Question 27

How to interpret data on effects of selection?

Answer 27

You’ll be given a graph showing changing alleles.

Describe the graph:
E.g. over the first two years (time), the average fur length is 21mm. The average length increases from 21mm to 24mm. This shows directional selection.

Suggest a cause:
E.g. the rabbits with longer fur are more likely to survive in the cold winters than the short-fur. This makes them more likely to reproduce because they have an advantaged allele, and then pass their allele onto offspring. The less advantaged phenotype will die and not reproduce. Over time, the allele for longer fur becomes more common and the average fur length increases.

Question 28

Investigating selection: practical?

Answer 28

You can investigate the effects of antimicrobial substances (antibiotics) on microbial growth, using aseptic techniques.

1. The bacteria is likely to be grown in a liquid broth (mixture of distilled water, bacteria culture and nutrients).
2. Use a sterile pipette to transfer the bacteria from the broth to an agar plate (a petit dish containing agar jelly).
3. Spread the bacteria over the plate evenly using a sterile plastic spreader.
4. Use sterile forceps to place paper docs soaked with different antibiotics spaced apart on the plate. Make sure you add a negative control disc soaked only in sterile water.
5. Lightly tape a lid on, invert, and incubate the plate at about 25* for 48 hours. This allows the bacteria to grow. Anywhere the bacteria can’t grow can be seen as a clear patch in the ‘lawn’ of bacteria. The clear space is the inhibition zone.
6. The size of an inhibition zone tells you how well an antibiotic works. Larger = the more bacteria were inhibited from growing.
7. A similar technique can be used to test the effects of antiseptics or disinfectants on microbial growth.

If there is no inhibition zone, the bacteria is resistant to that antibiotic up to that mg.

Question 29

Using aseptic techniques in the practical?

Answer 29

Aseptic techniques are used to prevent contamination of cultures by unwanted microorganisms.

This is important because contamination can affect the brother of the microorganisms that your working with. It’s also important to avoid contamination with disease-causing microbes because they can make you ill af.

Aseptic techniques to take into consideration:
1. Disinfect work surfaces. Don’t put any utensils on the work surface. Contaminated utensils should be placed in a beaker of disinfectant.

2. Use sterile equipment and discard safely after use. E.g. glassware can be sterilised before and egret use in an autoclave (steams equipment at high pressure). Pre-sterilised plastic instruments are used once then discarded.
3. Work near a Bunsen flame. This makes an microbes in the air to rise and be drawn away from culture. Hot air rises.
4. Minimise the time spent with the lid off the agar plate, to reduce the change of airborne microorganisms contaminating the culture.
5. Briefly flame the neck of the glass container of broth just after it’s opened and just before it’s closed. This causes air to move out of the container, preventing unwanted organism from falling in.
6. Wash hands.

Question 30

Meiosis stage?

Answer 30

Meiosis 1: Starts with a diploid cell (2n), DNA is replicated (2x2n). The first division separates homologous chromosomes.

• Independent segregation happens here
• Crossing over happens here

Meiosis 2: Starts with the cells from the last division, each cell contains one copy of each chromosome in its duplicated form. This stage is a bit like mitosis, where the arms of the duplicated chromosomes (sister chromatids) are separated. Four genetically different haploid (n) cells are formed.