Topic 3b: Genetics (Protein synthesis + Inheritance) - PAPER ONE

Question 1

How can proteins with different functions be produced?

Answer 1

• Proteins are made up of chains of molecules called amino acids
• The order of bases in a gene decides the order of amino acids in a protein
• Each different protein has its own number and order of amino acids
• Amino acid chains fold up to give each protein a different, specific shape meaning each protein has a different function

Question 2

How are proteins made? (long answer)

Answer 2

2 stages..

TRANSCRIPTION:
- RNA polymerase (enzyme) binds to a region of non-coding DNA in front of a gene
- RNA polymerase moves along one of the strands and produces a complementary mRNA strand from the coding DNA of the gene
- Once made, the mRNA molecule moves out of the nucleus and joins with a ribosome

TRANSLATION:
- Amino acids are brought to the ribosome by transfer RNA
- (The order that they are brought to the ribosome matches order of base triplets in the mRNA known as codons)
- The triplet code from the mRNA (codon) is matched by a complementary tRNA anticodon at the ribosome
- The amino acids are then joined together by the ribosome in the correct order, making a polypeptide (protein)

(then amino acid chain folds up to form a protein)

Question 3

What’s mRNA?

Answer 3

Messenger RNA
A copy of a single gene
- much shorter than DNA
- Only a single strand
- Contains the base uracil instead of thymine

Question 4

What are Non-coding regions?

Answer 4

Regions of DNA that don’t code for any amino acids

Question 5

Explain the affects of mutations occurring in the non-coding regions of DNA.

Answer 5

• If a mutation happens in the non-coding region of DNA where RNA polymerase binds to before transcription, it affects the ability of RNA polymerase to bind to it (could make it easier or harder to bind to)
• How well the RNA polymerase binds to the region of DNA affects how much mRNA is transcribed and therefore how much protein is produced.
• The phenotype of the organism may be affected by how much of it is made.

Question 6

Explain how a gene can code for a specific protein.

Answer 6

• The order of bases in a gene determines the order of amino acids in a protein
• Each gene contains a different order of bases which the gene can code for a particular protein

Question 7

Explain how a genetic variant can result in a protein with a low/high level of activity.

Answer 7

• Alters the sequence of amino acids coded for by a gene
• This affects the shape of the protein, decreasing/increasing its activity

Question 8

What did Mendel do to discover the basics of genetics?

Answer 8

Pea Plants
- In one experiment he bred together 2 pea plants of different heights and the offspring produced were all tall
- He then bred two of the (tall) offspring together and found that for every 3 tall offspring there was 1 dwarf (3:1 ratio)
- So, the unit for tall plants was dominant over the unit for dwarf plants

Question 9

What did Mendel Discover?

Answer 9

• Characteristics in plants are determined by hereditary units
• These units are passed onto offspring unchanged with one unit from each parent
• 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 10

Why did it take people a while to understand Mendel’s work?

Answer 10

• At the time, scientists didn’t know about genes, DNA and chromosomes
• After his death, people recognised the significance of his work

Question 11

Define Homozygous/Heterozygous

Answer 11

Homozygous - Organism has 2 alleles for a particular gene that are the same

Heterozygous - Organism has 2 alleles for a particular gene that are different

Question 12

Define Genotype and Phenotype

Answer 12

Genotype - alleles that an organism has for a particular characteristic
Phenotype - physical characteristic of an organism

Question 13

Define Zygote and Chromosome

Answer 13

Zygote - a diploid cell formed by the fusion of the nucleus of a male gamete with the nucleus of a female gamete

Chromosome - tightly coiled DNA that carries genetic information in the form of genes

Question 14

Define Dominant/Recessive

Answer 14

Dominant - Allele always expressed if present in a genotype
Recessive - Allele only expressed if two are present in a genotype

Question 15

Define Gene and Allele

Answer 15

Gene - a section of DNA
Allele - a version of a gene

Question 16

How many pairs of chromosomes are there in a human body cell? - how is sex determined?

Answer 16

23 pairs (46 total) - 23rd pair is either XX (female) or XY (male)

Question 17

Are males or female more likely to have sex-linked genetic disorders? - why?

Answer 17

• Males have an X and Y chromosome
• The Y chromosome is smaller so it carries fewer genes
• Males have one allele for sex linked genes as they have one X chromosome
• So, the characteristic is shown even if their one allele on the X chromosome is recessive
• This means they are more likely than females to show recessive characteristics for genes that are sex linked (e.g. colour blindness)

Question 18

Explain how different blood groups are produced (and how to write them) - e.g. which ones are recessive?

Answer 18

Blood group A (I^A)
Blood group B (I^B)
Blood group O (I^O)
———————
A and B are both dominant over O
They are also codominant (meaning if they have both alleles, they’ll have the blood type AB where not one is dominant over the other)
Blood group O is recessive

Question 19

Explain how a mother with blood group ‘A’ and a father with blood group ‘B’ can have a child with blood group ‘O’

Answer 19

The mother would have a genotype of I^A I^O
The father would have a genotype of I^B I^O
This would give the child a 25% chance of having the genotype I^O I^O (phenotype group O)

Question 20

What are the 2 causes of variation that influence a phenotype?

Answer 20

Genetic
Environmental

Question 21

Describe how genetic variation within a species occurs

Answer 21

Caused by organisms having different alleles which leads to difference in phenotype.

Different alleles caused by:
- Sexual reproduction results in alleles being combined in different ways in offspring
- New alleles could arise through mutations

Question 22

Describe how environmental variation within a species occurs

Answer 22

e.g. plants grown on sunny windowsill could grow luscious and green

Plants grown in darkness would grow tall and spindly and leaves would turn yellow

Question 23

What are environmental variations also known as?

Answer 23

Acquired characteristics

Question 24

Explain how both genetic and environmental factors contribute to variation together

Answer 24

e.g. the maximum height a plant could grow in determined by its genes. If it actually grows tall depends on its environment (e.g. how much food it gets)

Question 25

Explain how genetic mutations can effect the phenotype of an organism.

Answer 25

- When mutations occur within a gene, they result in an allele - Some mutations slightly alter an individual's characteristic - rarely, they result in the production of a protein that is so different it can no longer carry out its function e.g. cystic fibrosis - New combinations of alleles can also interact with each other to produce new phenotypes

Question 26

What are the 3 applications of the Human Genome Project's research?

Answer 26

Prediction and prevention of diseases: If we know what genes cause what diseases, we could get checked regularly to ensure early treatment if we develop diseases we are susceptible to Testing and treatment for inherited disorders: Scientists can now identify the genes and alleles that could cause an inherited disorder faster than in the past (due to HGP). Once the allele has been identified, people can be tested for it New and better medicines: Some variations affect how our individual bodies react to certain diseases and possible treatments. With this knowledge, we can design new drugs specific to people with particular gene variations

Question 27

What are the drawbacks of the Human Genome Project's research?

Answer 27

1) If people know that they are susceptible to a brain disease they may panic every time they get a headache 2) People with known genetic problems may come under pressure to not have children 3) If you are known to have a genetic likelihood of a serious disease, life insurance would become difficult to get and expensive. Employers may discriminate against these people.

Question 28

Describe what effect a mutation could have on the action of an enzyme.

Answer 28

* a mutation is a change in a gene (sequence) / base pairs (1) * (change in DNA) causes a change in amino acid(s) /order of amino acids (1) * change in shape of {active site / protein / enzyme} (1) * prevent / reduce binding to substrate (1) * enzyme can no longer function / reduced function (1) * enzyme could be more effective (1)

Question 29

Explain how two parents with a dominant phenotype can produce offspring expressing a recessive characteristic.

Answer 29

both parents must be heterozygous for the recessive allele (1), so the offspring must inherit the recessive allele from each parent (1)

Question 30

Explain how the order of amino acids in a protein is determined by the sequence of the gene.

Answer 30

- a single strand of messenger RNA is transcribed from the gene in the nucleus (1) - messenger RNA molecule binds to the ribosome (1) - the triplet code from the mRNA is matched by a complementary tRNA anticodon at the ribosome (1) - tRNA transfers amino acids to the polypeptide chain in a specific order (1)