SB3 - Genetics

Question 1

What is sexual reproduction and what are its advantages and disadvantages?

Answer 1

Sexual reproduction involves the fertilisation of a female sex cell by a male sex cell. The offspring produced are different (i.e. there is genetic variation in the species), which is an advantage as if there is an environmental change (i.e. a new disease) some of the species are likely to be better adapted to coping with it. However, one disadvantage is that it takes longer than asexual reproduction.

Question 2

What is asexual reproduction and what are its advantages and disadvantages?

Answer 2

Asexual reproduction is when offspring are produced from just one parent, without the fertilisation of gametes. Advantages include that it is faster than sexual reproduction, however disadvantages are that there is no genetic variation in a species so it will be less likely to survive environmental change.

Question 3

What is meiosis and why is it necessary?

Answer 3

Meiosis is a type of cell division which produces four non-identical daughter cells, which are haploid gametes. Gametes need to be haploid in order to maintain the correct number of chromosomes over multiple generations (if they were diploid the zygote would have double the number of chromosomes it should have).

Question 4

What are the stages of meiosis?

Answer 4

Interphase 1 - DNA and organelles replicate
Prophase 1 - chromosomes condense, spindle fibres grow, nuclear membrane breaks down
Metaphase 1 - chromosomes line up in the middle of the cell
Anaphase 1 - spindle fibres pull chromatids to opposite poles of the cell
Telophase 1 - nuclear membranes form at each pole of the cell
Cytokinesis 1 - cytoplasm spilts and cell divides.
These are all repeated exactly the same except that DNA is not replicated a second time.

Question 5

What measures are taken to ensure genetic variation during meiosis?

Answer 5

Crossing over - during prophase 1, the chromosomes of each homologous pair twist and look tangled. Every place they touch is called a chiasma and DNA it swapped between chromosomes at each chiasma. The genetic composition of each chromosome is now different.
Independent segregation of chromosomes - during metaphase 1 when the homologous chromosomes line up, they do so randomly to ensure that the genetic composition of the daughter cells is also random.

Question 6

Describe the structure of DNA.

Answer 6

DNA has two strands, which form a double helix. They are connected by bases.

Question 7

Describe the structure of a nucleotide.

Answer 7

A nucleotide contains a sugar, a phosphate and a base. The sugar connects to the phosphate to form the sugar-phosphate backbone which protects the bases. The bases connect to the sugar and their complementary base with hydrogen bonds (2 bonds for A and T, 3 for C and G)

Question 8

What are the complementary base pairs in DNA and mRNA?

Answer 8

DNA - cytosine bonds to guanine
adenine bonds to thymine
mRNA - cytosine bonds to guanine
adenine bonds to uracil

Question 9

Core practical - extracting DNA

Answer 9

Grind the sample with 10ml salt solution (clumps DNA). Pour into a 100ml beaker and stir for 5 minutes with a few drops of washing up liquid (dissolves cellular and nuclear membranes). Filter the solution into a test tube and add a few drops of protease (breaks down the DNA as DNA is a type of protein). Pour ice-cold ethanol down the side of the test tube (precipitates the DNA so you can see it). You will see white strands floating to the top, this is DNA.

Question 10

What are codons/triplet codes?

Answer 10

Groups of 3 bases. They each code for a specific amino acid.

Question 11

Describe the process of protein synthesis.

Answer 11

There are two stages: transcription and translation.
Transcription - In the nucleus, the enzyme RNA polymerase binds to a non-coding region of DNA, just before a gene. It separates the two strands of DNA and uses one as the template strand. RNA nucleotides match up to their complementary DNA bases (all the same except thymine is replaced by uracil). These RNA nucleotides link to form an mRNA strand.
Translation - The mRNA strand leaves the nucleus through a nuclear pore. In the cytoplasm, it attaches to a ribosome. The ribosome moves along the mRNA strand, decoding it 3 bases at a time. tRNA molecules bring down codons with complementary bases to the codons of mRNA, carrying the amino acid coded for by the mRNA codon. The amino acids are joined together to form a polypeptide chain, which folds in a specific way to form a protein.

Question 12

What is a chromosome?

Answer 12

Chromosomes determine what the cell codes for, each cell has 23 pairs of chromosomes, one from each pair from each parent.

Question 13

What is a gene?

Answer 13

A gene is a small section of DNA which coded for your characteristics.

Question 14

What is an allele?

Answer 14

An allele is a different form of the same gene (e.g. eye colour has alleles for blue, brown and green).

Question 15

What is a dominant allele?

Answer 15

Dominant alleles will always be expressed regardless of the identity of the other allele. They are represented with a capital letter.

Question 16

What is a recessive allele?

Answer 16

Recessive alleles are only expressed if you have two of the same recessive allele. They are represented with lower case letters.

Question 17

What does heterozygous mean?

Answer 17

It means that the alleles for a trait are different (e.g. Bb).

Question 18

What does homozygous mean?

Answer 18

It means that the alleles for a trait are the same (e.g. BB).

Question 19

What is a genotype?

Answer 19

The genotype is the combination of alleles which make up a trait. (e.g. BB).

Question 20

What is a phenotype?

Answer 20

The physical appearance/observable characteristics of an organism. (e.g. brown eyes).

Question 21

What is a genome?

Answer 21

The genome is the entire of an organism’s genetic material.

Question 22

What are mutations?

Answer 22

Mutations are changes in the bases of a gene. Some mutations cause a change in the organism’s phenotype. Mutations are what cause the existence of different alleles for a specific trait and can cause genetic diseases.

Question 23

What do mutations in non-coding DNA cause?

Answer 23

Mutations in non-coding DNA can cause a change in the amount of certain proteins that is made, as it can affect how well RNA polymerase binds to the non-coding regions.

Question 24

What did Mendel do?

Answer 24

He cross-bred pea plants with different characteristics and observed the characteristics of the offspring. He created 3 laws of inheritance:
- each gamete receives only one factor for a characteristic
- the version of the factor they receive is random and does not depend on other factors
- some versions of a factor are more powerful than others and always have an effect in the offspring.
Mendel’s ‘factors’ were later called genes.

Question 25

What are Punnett Squares?

Answer 25

Punnett squares show the possible genotypes and phenotypes of offspring based on the parents’ genotypes and how likely each outcome is.

Question 26

What is cystic fibrosis?

Answer 26

A genetic disorder caused by recessive alleles. It causes mucus to build up in the body which sits in the lungs and hinders breathing. It is abnormally thick which makes it difficult for the cilia to move it away from the lungs. They get frequent infections and permanent lung damage and there is no cure.

Question 27

What is Huntington’s disease?

Answer 27

A genetic disorder which is caused by dominant alleles. It doesn’t show signs until you’re about 40 and causes a loss of motor control.

Question 28

What is sickle cell anaemia?

Answer 28

A genetic disorder caused by recessive alleles which alters the shape of red blood cells, reducing their oxygen capacity. This causes weakness and anaemia.

Question 29

What are family pedigree charts?

Answer 29

They show the inheritance of particular traits over generations and can be used to predict the genotype of members of the family.

Question 30

Explain the inheritance of blood groups.

Answer 30

There are 4 blood groups: A, B, AB, O and are either positive or negative. The blood group determines which cell surface markers are on the surface of red blood cells (O doesn’t have any). O is recessive and A and B are codominant (so if someone has both the A and B alleles, they have both A and B markers - this is the AB blood group). When writing the genotypes for blood groups, always use the letter I then the allele above (e.g. group AB would be I^A I^B).

Question 31

What are sex-linked disorders?

Answer 31

Sex-linked disorders are a type of genetic disorder with its genes carried on a sex chromosome (X or Y). For example, the gene for colourblindness is carried on the X chromosome and is recessive, which is why men are more likely to be colourblind than women as if men have even one colourblindness allele then they will be colourblind as they only have one X chromosome, but if women have one dominant normal allele, they will not be colourblind.

Question 32

What is the human genome project and what are its uses?

Answer 32

The HGP successfully mapped an entire human genome. This can be used for genetic screening, personalised medicine, gene therapy and to study evolution.

Question 33

What is genetic variation?

Answer 33

Differences between individuals caused by the alleles they inherit. (e.g. eye colour)

Question 34

What is environmental variation?

Answer 34

Differences between individuals caused by their surroundings (e.g. a scar). Characteristics which are changed by the environment are called acquired characteristics.

Question 35

What are the two types of variation?

Answer 35

Discontinuous variation is where the data can only take a limited number of values (e.g. shoe size)
Continuous variation is where the data can be any value on a range (e.g. foot length).

Question 36

What is a normal distribution?

Answer 36

Continuous data for variation often forms a bell-shaped curve, known as a normal distribution. The mean, median and mode all lie in the middle of the range. The further a value is from the middle, the fewer individuals display it.