4.1- DNA, GENES & CHROMOSOMES Flashcards
Aspartic acid and proline are both amino acids. Describe how two amino acids differ
from one another. You may use a diagram to help your description.
(1)
endding base triplet
ASPARTIC ACID- GAC GAU
PROLINE- CCA CCG CCC CCU
Have different R group.
Deletion of the sixth base (G) in the sequence shown in the diagram above would
change the nature of the protein produced but substitution of the same base would
not. Use the information in the table and your own knowledge to explain why. (3)
AUGCCGUACCGACU
ending base triplet
ASPARTIC ACID- GAC GAU
PROLINE- CCA CCG CCC CCU
- Substitution would result in CCA / CCC / CCU;
- (All) code for same amino acid / proline;
- Deletion would cause frame shift / change in all following codons /
change next codon from UAC to ACC.
Messenger RNA (mRNA) is used during translation to form polypeptides. Describe how mRNA is produced in the nucleus of a cell. (6)
- Helicase;
- Breaks hydrogen bonds;
- Only one DNA strand acts as template;
- RNA nucleotides attracted to exposed bases;
- (Attraction) according to base pairing rule;
- RNA polymerase joins (RNA) nucleotides together;
- Pre-mRNA spliced to remove introns.
Describe the structure of proteins. (5)
- Polymer of amino acids;
- Joined by peptide bonds;
- Formed by condensation;
- Primary structure is order of amino acids;
- Secondary structure is folding of polypeptide chain due to hydrogen
bonding;
Accept alpha helix / pleated sheet
6. Tertiary structure is 3-D folding due to hydrogen bonding and ionic /
disulfide bonds;
7. Quaternary structure is two or more polypeptide chains.
Describe how proteins are digested in the human gut. (4)
- Hydrolysis of peptide bonds;
- Endopeptidases break polypeptides into smaller peptide chains;
- Exopeptidases remove terminal amino acids;
- Dipeptidases hydrolyse / break down dipeptides into amino acids.
4
3.Read the following passage carefully.
A large and growing number of disorders are now known to be due to types of
mitochondrial disease (MD). MD often affects skeletal muscles, causing
muscle weakness.
We get our mitochondria from our mothers, via the fertilised egg cell. Fathers
do not pass on mitochondria via their sperm. Some mitochondrial diseases
are caused by mutations of mitochondrial genes inside the mitochondria.
Most mitochondrial diseases are caused by mutations of genes in the cell
nucleus that are involved in the functioning of mitochondria. These mutations
of nuclear DNA produce recessive alleles.
5
One form of mitochondrial disease is caused by a mutation of a mitochondrial
gene that codes for a tRNA. The mutation involves substitution of guanine for
adenine in the DNA base sequence. This changes the anticodon on the
tRNA.
This results in the formation of a non-functional protein in the mitochondrion.
10
There are a number of ways to try to diagnose whether someone has a
mitochondrial disease. One test involves measuring the concentration of
lactate in a person’s blood after exercise. In someone with MD, the
concentration is usually much higher than normal. If the lactate test suggests
MD, a small amount of DNA can be extracted from mitochondria and DNA
sequencing used to try to find a mutation.
Use information in the passage and your own knowledge to answer the following
questions.
(a) Mitochondrial disease (MD) often causes muscle weakness (lines 1–3). Use your
knowledge of respiration and muscle contraction to suggest explanations for this
effect of MD. (3)
- Reduction in ATP production by aerobic respiration;
- Less force generated because fewer actin and myosin interactions in
muscle; - Fatigue caused by lactate from anaerobic respiration.
A large and growing number of disorders are now known to be due to types of
mitochondrial disease (MD). MD often affects skeletal muscles, causing
muscle weakness.
We get our mitochondria from our mothers, via the fertilised egg cell. Fathers
do not pass on mitochondria via their sperm. Some mitochondrial diseases
are caused by mutations of mitochondrial genes inside the mitochondria.
Most mitochondrial diseases are caused by mutations of genes in the cell
nucleus that are involved in the functioning of mitochondria. These mutations
of nuclear DNA produce recessive alleles.
5
One form of mitochondrial disease is caused by a mutation of a mitochondrial
gene that codes for a tRNA. The mutation involves substitution of guanine for
adenine in the DNA base sequence. This changes the anticodon on the
tRNA.
This results in the formation of a non-functional protein in the mitochondrion.
10
There are a number of ways to try to diagnose whether someone has a
mitochondrial disease. One test involves measuring the concentration of
lactate in a person’s blood after exercise. In someone with MD, the
concentration is usually much higher than normal. If the lactate test suggests
MD, a small amount of DNA can be extracted from mitochondria and DNA
sequencing used to try to find a mutation
Two couples, couple A and couple B, had one or more children affected by a
mitochondrial disease. The type of mitochondrial disease was different for each
couple.
None of the parents showed signs or symptoms of MD.
• Couple A had four children who were all affected by an MD.
• Couple B had four children and only one was affected by an MD.
(b) Use the information in lines 5–9 and your knowledge of inheritance to suggest why:
• all of couple A’s children had an MD
• only one of couple B’s children had an MD.
COUPLE A=….
COUPLE B=….
(4)
Couple A,
1. Mutation in mitochondrial DNA / DNA of mitochondrion affected;
2. All children got affected mitochondria from mother;
3. (Probably mutation) during formation of mother’s ovary / eggs;
Couple B,
4. Mutation in nuclear gene / DNA in nucleus affected;
5. Parents heterozygous;
6. Expect 1 in 4 homozygous affected.
Suggest how the change in the anticodon of a tRNA leads to MD (lines 10–13). (3)
- Change to tRNA leads to wrong amino acid being incorporated into
protein; - Tertiary structure (of protein) changed;
- Protein required for oxidative phosphorylation / the Krebs cycle, so less /
no ATP made.
If someone has MD, the concentration of lactate in their blood after exercise is
usually much higher than normal (lines 15–17). Suggest why. (3)
- Mitochondria / aerobic respiration not producing much / any ATP;
- (With MD) increased use of ATP supplied by increase in anaerobic
respiration; - More lactate produced and leaves muscle by (facilitated) diffusion.
A small amount of DNA can be extracted from mitochondria and DNA sequencing
used to try to find a mutation (lines 18–19).
From this sample:
• how would enough DNA be obtained for sequencing?
• how would sequencing allow the identification of a mutation?
- Enough DNA using PCR;
2. Compare DNA sequence with ‘normal’ DNA.
The Amish are a group of people who live in America. This group was founded by 30 Swiss
people, who moved to America many years ago. The Amish do not usually marry people
from outside their own group.
One of the 30 Swiss founders had a genetic disorder called Ellis-van Creveld syndrome.
People with this disorder have heart defects, are short and have extra fingers and toes.
Ellis-van Creveld syndrome is caused by a faulty allele.
In America today, about 1 in 200 Amish people are born with Ellis-van Creveld syndrome.
This disorder is very rare in people in America who are not Amish.
(b) The faulty allele that causes Ellis-van Creveld syndrome is the result of a mutation
of a gene called EVC. This mutation leads to the production of a protein that has
one amino acid missing.
(i) Suggest how a mutation can lead to the production of a protein that has one
amino acid missing. (2)
Loss of 3 bases / triplet = 2 marks;;
‘Stop codon / code formed’ = 1 mark max unless related to
the last amino acid
Loss of base(s) = 1 mark;
eg triplet for last amino acid is changed to a stop codon /
code = 2 marks
3 bases / triplet forms an intron = 2 marks
Accept: descriptions for ‘intron’ eg non-coding DNA
‘Loss of codon’ = 2 marks
The faulty allele that causes Ellis-van Creveld syndrome is the result of a mutation
of a gene called EVC. This mutation leads to the production of a protein that has
one amino acid missing.
Suggest how the production of a protein with one amino acid missing may lead
to a genetic disorder such as Ellis-van Creveld syndrome. (2)
- Change in tertiary structure / active site;
Neutral: change in 3D shape / structure - (So) faulty / non-functional protein / enzyme;
Accept: reference to examples of loss of function eg fewer
E-S complexes formed
The genetic code is described as degenerate.
What is meant by this? Use an example from Table 1 to illustrate your answer.
[2 marks]
- More than one codon codes for a single amino
acid; - Suitable example selected from Table 1;
