Topic 3 - Genetics

Question 1

Define Sexual Reproduction:

Answer 1

• Where genetic information from two organisms (a father and a mother) is combined to produce offspring which are genetically different to either parent.

Question 2

What happens at fertilisation?

Answer 2

• A male gamete fuses with a female gamete to produce a fertilised egg (zygote).
• The zygote undergoes cell division and develops into an embryo.
• This embryo inherits characteristics from both parents as it has received a mixture of chromosomes from its mother and father.

Question 3

Stage 1 of Meiosis (before cell division):

Answer 3

• Before the cell starts to divide, it duplicates its DNA. One arm of each x-shaped chromosome is an exact copy of the other arm.

Question 4

First Division Of Meiosis:

Answer 4

1) - The chromosomes line up in pairs at the centre of the cell.

2) - The pairs are then pulled apart, so each new cell only has one copy of each chromosome. Some of the father’s characteristics and some of the mother’s go into each new cell.

3) - This creates genetic variation in the offspring.

Question 5

Second Division of Meiosis:

Answer 5

1) - The chromosomes line up again in the centre of the cell.

2) - The arms of the chromosomes (chromatids) are pulled apart.

Question 6

Result of Meiosis:

Answer 6

• Four genetically different haploid daughter cells.

Question 7

Advantages of Asexual Reproduction:

Answer 7

1) - Produces lots of offspring very quickly because the reproductive cycle is so fast.

2) - Only one parent is needed which means organisms can reproduce whenever conditions are favourable without having to wait for a mate.

Question 8

Disadvantages of Asexual Reproduction:

Answer 8

1) - No genetic variation between offspring. So, if the environment changes and the conditions become unfavourable, the whole population may be affected.

Example - Black Sigatoka affects banana plants

Question 9

Advantages of Sexual Reproduction -

Answer 9

1) - Creates genetic variation within the population, which means different individuals have different characteristics. This means that if the environmental conditions change, it’s more likely that some individuals will have characteristics to survive the change.

2) - Over time, this can lead to natural selection and evolution as species become better adapted to their environment.

Question 10

Disadvantages of Sexual Reproduction -

Answer 10

1) - Takes more time and energy than asexual reproduction, so organisms produce fewer offspring in their lifetime.

2) - Organisms need to find and attract mates, which also takes time and energy.

3) - Two parents are needed for this. This can be a problem if individuals are isolated.

Question 11

What are DNA strands?

Answer 11

• Polymers made up of lots of repeating units called nucleotides.
• Each nucleotide consists of a sugar, a phosphate group and one ‘base’.
• The sugar and phosphate groups in the nucleotides form a ‘backbone’ to the DNA strings. The sugar and phosphate groups alternate.

Question 12

Complimentary Base Pairing

Answer 12

• A with T
• C with G
• Joined by weak hydrogen bonds.

Question 13

What is a DNA molecule made of?

Answer 13

• Two DNA strands coiled together in the shape of a double helix.

Question 14

Define Chromosomes

Answer 14

• Chromosomes are long, coiled up molecules of DNA found in the nucleus of eukaryotic cells.

Question 15

Define gene:

Answer 15

• A gene is a section of DNA on a chromosome that codes for a particular protein.

Question 16

Define Genome:

Answer 16

• All of an organism’s DNA makes up its genome.

Question 17

What does the detergent do in the DNA fruit cell practical?

Answer 17

• Breaks down the cell membranes to release the DNA.

Question 18

What does the salt do in the fruit cell DNA practical?

Answer 18

• Makes the DNA stick together.

Question 19

Why is the mixture filtered in the fruit cell DNA practical?

Answer 19

• To get the froth and large, insoluble bits of cell out.

Question 20

Why is ice-cold ethanol used in the fruit cell DNA practical?

Answer 20

• To precipitate the DNA.

Question 21

Gap fill: The order of the _______ in a _______ decide the order of the _______ ______ in a _______.

Answer 21

• The order of the bases in a gene decide the order of amino acids in a protein.

Question 22

Define a base triplet:

Answer 22

• Each amino acid being coded for by a sequence of three bases in the gene.

Question 23

Describe the process of transcription (4 marks)

Answer 23

1) - RNA polymerase binds to a region of non- coding DNA in front of a gene.

2) - The two DNA strands unzip and the RNA polymerase moves along one of the strands of the DNA.

3) - It uses the coding DNA in the gene as a template to make the mRNA. Base pairing between the DNA and RNA ensures that the mRNA is complimentary to the gene.

4) - Once made, the mRNA molecule moves out of the nucleus and joins with a ribosome.

Question 24

Describe the process of translation (4 marks):

Answer 24

1) - Amino acids are brought to the ribosome by another RNA molecule called transfer RNA (tRNA).

2) - The order in which the amino acids are brought to the ribosome matches the order of the base triplets in mRNA. Base triplets in mRNA are also known as codons.

3) - Part of the tRNA’s structure is called an anticodon - it is complimentary to the codon for the amino acid. The pairing of the codon and anticodon makes sure that the amino acids at brought to the ribosome in the correct order.

4) - The amino acids are joined together by the ribosome. This makes a polypeptide (protein).

Question 25

Define Codon:

Answer 25

Base triplets in mRNA are known as codons.

Question 26

Define a mutation:

Answer 26

• A rare, random change to an organism’s DNA base sequence that can be inherited.

Question 27

What happens as a result of a mutation in a gene?

Answer 27

1) - If a mutation happens in a gene, it produces a genetic variant - a different version of the gene.

2) - The genetic variant may code for a different sequence of amino acids, which may change the shape of the final protein and so its activity.

3) - For example, the activity of an enzyme may increase, decrease or stop altogether.

4) - This could end up changing the characteristics of an organism.

Question 28

What happens as a result of a mutation in non-coding regions of DNA?

Answer 28

1) - It could affect the ability of RNA polymerase to bind to the region of non-coding DNA. It might make it easier to bind to, or more difficult.

2) - How well RNA polymerase can bind to this region of DNA will affect how much mRNA is transcribed and therefore how much of the protein is produced. Depending on the function of the protein, the phenotype of the organism may be affected by how much of it is made.

Question 29

Define Alleles:

Answer 29

• Different versions of the same gene.

Question 30

Describe how Mendel carried out his experiment:

Answer 30

1) - In one experiment, Mendel crossed two pea plants of different heights - a tall pea plant and a dwarf pea plant. The offspring produced were all tall pea plants.

2) - He then bred two of these tall offspring together and found that when the offspring from the first cross were crossed with each other, three tall offspring were produced for every one dwarf offspring overall.

Question 31

What did Mendel’s experiment show?

Answer 31

1) - The height characteristics in pea plants was determined by separately inherited “hereditary units” passed on from each parent.

2) - Other characteristics of pea plants were inherited in the same way e.g purple flower colour was dominant over a white colour.

Question 32

What were the three conclusions Mendel reached?

Answer 32

1) - Characteristics in plants are determined by “hereditary units”.

2) - Hereditary units are passed on to offspring unchanged from both parents, one unit from each parent.

3) - Hereditary units can be dominant or recessive - if an individual has both the dominant and recessive unit for a characteristic, the dominant characteristic will be expressed.

Question 33

Why are males more likely to get sex linked disorders?

Answer 33

1) - Y Chromosome is smaller than the X Chromosome and carries fewer genes.

2) - Males only have one X chromosome, they often only have one allele for sex-linked genes.

3) - Because of this, the characteristic of the allele is shown even if it is recessive.

Question 34

Define sex-linked disorder:

Answer 34

• Disorders caused by faulty alleles located on sex chromosomes.

Question 35

Blood Types -

Answer 35

• A, B, AB, O
• A and B are co-dominant

Question 36

What causes genetic variation?

Answer 36

1) - Caused by organisms having different alleles which can lead to differences in phenotype.

2) - Can be caused by new alleles arising through mutations.

3) - Sexual reproduction also causes it since it results in alleles being combined in lots of different ways in offspring.

Question 37

How do alleles arise?

Answer 37

• Through mutations occurring in a gene.

Question 38

The big idea of the Human Genome Project:

Answer 38

• To find every single human gene.

Question 39

Medical Applications as a result of the Human Genome Project:

Answer 39

1) - Prediction and prevention of diseases such as cancers and heart disease as these diseases are caused by the interaction of different genes.

2) - Testing and treatment for inherited disorders as scientists can now identify genes and alleles suspected of causing an inherited disorder quicker than before.

3) - New and better medicines as scientists can tailor new drugs to people with a particular genetic variation. These drugs can also have less side effects.

Question 40

Drawbacks of the Human Genome Project:

Answer 40

1) - Increased stress knowing you’re susceptible to a bad disease.

2) - Gene-ism - people could become under pressure not to have children.

3) - Discrimination by employers and insurers - life insurance could become more expensive.