Genetics, Variation & Relationships between Organisms

Question 1

How is DNA stored?

Answer 1

• chromosomes are made of one long DNA molecule and proteins, which is in the nucleus
• DNA is wound around proteins called histones, which are coiled up tightly to make compact chromosomes. Histones allow DNA to be switched on/off depending on how tightly they are wrapped
• in prokaryotes, DNA is shorter and in a loop in the cytoplasm with no histones as well as in mitochondria and chloroplasts, as their DNA supercoils to fit in them

Question 2

What are genes?

Answer 2

Genes are sequences of DNA that code for polypeptides (form protein primary structure and the different orders of amino acids determine the bases of genes) or tRNA and rRNA

Question 3

What is the difference between a genome and a proteome?

Answer 3

A genome is the complete set of genes in a cell and a proteome is the full range of proteins produced by that cell.

Question 4

What is the genetic code?

Answer 4

It is the sequence of base triplets in mRNA, which code for amino acids. The base triplets are non-overlapping so each triplet is read separately from the one before and after it. The genetic code is universal in all living organisms.

Question 5

What is the degenerate code?

Answer 5

Degenerate amino acids can be coded for by multiple base triplets as there are more possible combinations of triplets than amino acids. There are also other triplets which don’t code for amino acids like the “stop” (UAG) and “start” (AUG) signals

Question 6

What are introns and exons?

Answer 6

• Introns are non-coding regions of genes which don’t code for amino acids. They are only in eukaryotic cells and their purpose is unknown. Introns are removed during protein synthesis so they don’t affect the order of amino acids.
• Exons are coding regions of genes which code for amino acids. However, eukaryotic DNA contains regions of multiple repeats which don’t code for amino acids too.

Question 7

What is transcription?

Answer 7

Transcription the first stage of protein synthesis when an mRNA copy of a gene is made from DNA and it takes place in the nucleus of a eukaryotic cell and in the cytoplasm of a prokaryotic cell.

Question 8

What happens during transcription?

Answer 8

1. RNA polymerase attaches to DNA double helix at the start of a gene. In eukaryotes, hydrogen bonds between the two DNA strands are broken by DNA helicase attached to RNA polymerase. This makes the strands separate and the DNA uncoils, exposing bases. One of the separated DNA strands is used as a template for mRNA copy.
2. RNA polymerase lines up the free RNA nucleotides with exposed bases on template strand and the free bases are attracted and joined by RNA polymerase to the complementary ones. This forms the mRNA strand, which is now a complementary copy of the DNA template.
3. RNA polymerase moves along DNA and hydrogen bonds reform after RNA polymerase has passed by and made the mRNA strands. This coils back into a double helix.
4. After RNA polymerase reaches the stop signal, it stops making mRNA and detaches from DNA. In eukaryotes, mRNA moves out of the nucleus through the nuclear pore and attaches to a ribosome in the cytoplasm.
5. In eukaryotes, introns and exons are both in mRNA during transcription. Splicing occurs which removes the introns and joins exons together by spliceosome which forms mRNA strands.

Question 9

What is translation?

Answer 9

It is the second stage of protein synthesis which occurs in ribosomes in the cytoplasm, when amino acids are joined to make polypeptide chains.

Question 10

What happens during translation?

Answer 10

1. mRNA attaches itself to a ribosome and tRNA molecules carry amino acids to it. ATP provides the necessary energy for the bond between the amino acid and tRNA molecule.
2. tRNA molecule with an anti-codon is complementary to the first codon on the mRNA and attaches and this continues with the next tRNA molecule and codon.
3. Two amino acids attached to tRNA molecules are joined together by peptide bonds and the first tRNA molecule moves away and leaves its amino acid behind.
4. The next tRNA molecule binds to the next mRNA codon and its amino acid binds to the first two tRNA molecules and the second moves away. This process continues until reaching a stop codon.
5. The polypepide chain moves away from the ribosome and translation is complete.

Question 11

What is the structure of tRNA?

Answer 11

tRNA is a single polynucleotide strand folded into a clover shape with hydrogen bonds between base pairs which holds its structure together. Each tRNA molecule has a specific sequence of 3 bases called an anticodon and an amino acid binding site.

Question 12

What is a mutation?

Answer 12

It is any change to a quantity of bases sequences of DNA like a gene mutation (change to nucleotide bases or sequences of bases) or chromosome mutation (change in structure or number of chromosomes).

Question 13

What causes mutations?

Answer 13

• random mistake in DNA replication which isn’t repaired
• mutagens like radiation, chemical carcinogens, ageing, heredity and viruses

Question 14

How are mutations repaired?

Answer 14

• DNA polymerase proof reads and checks for mutations
• Proteins look for mismatches in base pairs during G2
• Enzymes remove mutations and DNA ligase repairs gaps
• The cell cycle stops, genes shut down and apoptosis (cell death) occurs

Question 15

What is the difference between base substitution and deletion?

Answer 15

Base substitution is when a nucleotide in a DNA molecule is replaced by another so the polypeptide will only differ in a single amino acid. Base deletion is when a nucleotide is lost from the DNA sequence, which will change every triplet that follows so the polypeptide is unlikely to function.

Question 16

What is meiosis?

Answer 16

It is the process of cell division for sexual reproduction, which introduces genetic diversity by randomly dividing a cell’s chromosomes in two diploid cells then splitting each chromosome into chromatids which result in four haploid daughter cells.

Question 17

What is the process of meiosis?

Answer 17

1. metaphase 1 - homologous pairs of chromosomes line up in the centre of the cell
2. anaphase 1 - homologous pairs of chromosomes separate
3. telophase 1 - the chromosomes on either side of the cell are put into nuclei, which are haploid
4. metaphase 2 - the chromosomes line up in the centre of the cell
5. anaphase 2 - the chromosomes are separated into chromatids
6. telophase 2 - the chromatids are put into nuclei in groups of 2
7. cytokinesis - the cytoplasm divides to create 4 different diploid cells

Question 18

How does meiosis cause genetic variation?

Answer 18

• crossing over of chromosomes during prophase 1
• independent segregation of homologous chromosomes during metaphase 1
• random fertilisation (it is random which gametes will fertilise eachother)

Question 19

Why do chromosome mutations happen?

Answer 19

If meiosis goes wrong and cells contain variations in the number of whole chromosomes or parts of chromosomes (non-dysjunction)

Question 20

What are some types of chromosome mutations?

Answer 20

• duplication - a section of chromosomes is copied
• inversion - a section of chromosomes is swapped between two DNA strands
• deletion - a section of chromosomes is removed
• insertion - a section of chromosomes is moved from one DNA strand and put into another
• translocation - two different sections of chromosomes are swapped between two different strands

Question 21

What is non-dysjunction?

Answer 21

It is a type of chromosome mutation where chromosomes fail to separate properly, which can lead to inherited conditions

Question 22

How is Down’s syndrome caused?

Answer 22

It is caused by an extra copy of chromosome 21 through non-dysjunction. This means that one of the gametes gets no copy of the chromosome while the other gets an extra one. When the gamete with an extra copy fuses with another gamete during fertilisation, the zygote will have 3 copies of chromosome 21, causing Down’s syndrome.

Question 23

What is an allele?

Answer 23

It is a different version of a gene, which codes for a different version of a characteristic. They are caused by mutations.

Question 24

What is genetic diversity?

Answer 24

High genetic diversity is a large number of different alleles in a population, causing a large variety of different characteristics. Low genetic diverisity populations can’t adapt to changes in the environment and can be wiped out by a single event.

Question 25

How does genetic diversity increase?

Answer 25

- mutations in DNA which form new alleles - different alleles introduced into a population when individuals migrate and reproduce (gene flow)

Question 26

What is a genetic bottleneck?

Answer 26

It is an event that causes a big reduction in a population so organisms die before reproducing. This reduces the number of different alleles in a gene pool, reducing genetic diversity. Therefore, the survivors reproduce from a smaller gene pool and a larger population is created from a few individuals with similar alleles.

Question 27

What is the founder effect?

Answer 27

It is what happens when a few organisms from a population start a new colony, so there are only a small number of different alleles in the initial gene pool. This means that the frequency of each allele in the new colony could be very different to the original population, which could lead to a high incidence of a specific genetic disease.

Question 28

What is the process of natural selection?

Answer 28

1. randomly occurring mutations sometimes result in the formation of a new allele 2. while some mutations are harmful, the ones that have beneficial alleles to an organism 3. this means the organism is more likely to survive and reproduce, which increases the frequency of the allele within the population

Question 29

How does natural selection work?

Answer 29

1. differential reproductive success means individuals with a beneficial allele have an increased chance of survival and so reproduce and pass on their genes 2. a greater proportion of the next generation inherits the beneficial allele 3. these individuals are more likely to survive, reproduce and pass these genes on 4. the frequency of beneficial alleles in a population increases between generations 5. this leads to evolution as advantageous alleles become more common when isolated from other populations

Question 30

What is evolution?

Answer 30

The gradual change in species over time, which leads to the huge diversity of living organisms

Question 31

What are types of adaptations?

Answer 31

- behavioural - ways an organism acts to increase its chance of survival - physiological - processes inside an organism's body that increase its chance of survival - anatomical - features of an organism's body that increases its chance of survival

Question 32

What are the different types of natural selection?

Answer 32

- directional selection - where individuals with alleles for a characteristic of an extreme type are more likely to survive and reproduce in response to an environmental change - stabilising selection - where individuals with alleles for characteristics in the middle of the range so they are more likely to survive and reproduce, which occurs when the environment isn't changing

Question 33

What is an example of directional selection?

Answer 33

Antibiotic resistance: 1. some bacteria individuals have random mutations that are resistant to antibiotics 2. when the population is exposed to this antibiotic, the resistant alleles remain alive when the rest die 3. the resistant bacteria reproduce without competition so their offspring are also antibiotic resistant 4. after some time, most organisms in a population will carry the antibiotic resistant allele

Question 34

What is an example of stabilising selection?

Answer 34

Human birth weight: 1. very small babies are less likely to survive as they can't maintain body temperature 2. very large babies are less likely to survive due to complications in pregnancy and childbirth 3. therefore, conditions are more favourable for medium-sized babies so weight shifted towards the middle of the range