Define semi-conservative replication .

Semi-conservative replication is DNA copying in which each new molecule contains one original strand and one newly made strand .

Why must a cell replicate its DNA before dividing?

So that both daughter cells receive a full copy of the DNA. This ensures genetic continuity between generations of cells.

What is the role of helicase in DNA replication?

It unwinds the double helix. It breaks the hydrogen bonds between base pairs to expose the two template strands.

What is the role of DNA polymerase?

It joins free nucleotides together along each template strand. It catalyses the formation of phosphodiester bonds in the new sugar-phosphate backbone.

How do free nucleotides line up on the template strand?

By complementary base pairing . Each base pairs with its complement: A with T, and G with C.

Why is DNA replication described as semi-conservative?

Each new molecule conserves half of the original DNA. One strand is from the original molecule and one is newly made.

During replication, each original DNA strand acts as a for building a new complementary strand.

During replication, each original DNA strand acts as a template for building a new complementary strand.

What did Meselson and Stahl's experiment set out to determine?

Whether DNA replication is conservative or semi-conservative .

How did Meselson and Stahl use nitrogen isotopes in their experiment?

Bacteria were first grown in heavy ¹⁵N so their DNA was heavy. They were then moved to light ¹⁴N and allowed to replicate once.

What result showed that replication is semi-conservative?

After one round in light nitrogen, all the DNA settled in the middle of the tube. This meant each molecule had one heavy and one light strand.

What result would have supported conservative replication instead?

Two separate bands — one heavy (original DNA) at the bottom and one light (new DNA) at the top.

True or False? In semi-conservative replication, both strands of a new DNA molecule are newly made.

False. One strand is from the original molecule and only the other is newly made.

Define gene mutation .

A gene mutation is a change in the sequence of base pairs in a DNA molecule, which may result in an altered polypeptide.

When do gene mutations most often occur?

Spontaneously and continuously. Errors often arise during DNA replication .

Why do most mutations not change the polypeptide produced?

The genetic code is degenerate . So a changed triplet may still code for the same amino acid .

What is a substitution mutation?

One base is swapped for a different base. It only affects the one triplet where it occurs, with no knock-on effect.

What is a frameshift mutation, and which mutations cause it?

A mutation that shifts the reading of every triplet after the change. It is caused by insertion or deletion of a nucleotide.

Name the three types of substitution mutation.

Silent (no change to the amino acid). Missense (one amino acid changed). Nonsense (creates a premature stop codon).

Why can an insertion or deletion be more damaging than a substitution?

It causes a frameshift , changing every triplet downstream. A substitution only changes the single triplet where it occurs.

Which gene is faulty in cystic fibrosis, and what does it normally code for?

The CFTR gene on chromosome 7. It codes for chloride ion channels in cell membranes.

How does a faulty CFTR gene lead to thick, sticky mucus?

It produces non-functional chloride channels . This reduces the movement of water into secretions by osmosis, so mucus becomes thick and sticky.

How does cystic fibrosis impair the gas exchange system?

Thick mucus builds up and blocks the airways . This reduces the surface area for gas exchange and traps microorganisms, causing frequent lung infections.

How does cystic fibrosis impair the digestive system?

Mucus blocks the tube to the pancreas , so digestive enzymes cannot reach the small intestine. Mucus on the intestine lining also reduces absorption of nutrients.

How does cystic fibrosis impair the reproductive system?

In men , the tubes of the testes can be blocked, stopping sperm being released. In women , thick cervical mucus can stop sperm reaching the egg.

Cystic fibrosis is caused by a allele, so a person must be homozygous to have the disorder.

Cystic fibrosis is caused by a recessive allele, so a person must be homozygous to have the disorder.

True or False? There is only one mutation of the CFTR gene that can cause cystic fibrosis.

False. There are many possible mutations of the large CFTR gene — around 1000 have been identified.

A gene is a length of DNA that codes for a single polypeptide .

An allele is a different version of a gene, with a slightly different nucleotide sequence, found at the same locus.

The genotype is the combination of alleles an organism possesses for a gene.

The phenotype is the observable characteristics of an organism, resulting from its genotype and the environment.

What is the difference between homozygous and heterozygous?

Homozygous means having two identical alleles of a gene. Heterozygous means having two different alleles of a gene.

Define dominant allele.

A dominant allele is one that is always expressed in the phenotype, whether the organism is homozygous or heterozygous.

Define recessive allele.

A recessive allele is one that is only expressed when no dominant allele is present, i.e. in a homozygous recessive individual.

Define incomplete dominance .

Incomplete dominance is when neither allele is fully dominant, so a heterozygote shows a third, intermediate phenotype.

A heterozygous individual carrying a recessive allele that is not expressed in their phenotype. They can still pass the allele to their offspring.

A locus is the specific position of a gene on a chromosome.

A pair of chromosomes that match in size and carry the same genes at the same loci are called a pair.

A pair of chromosomes that match in size and carry the same genes at the same loci are called a homologous pair.

True or False? In incomplete dominance, snapdragons with genotype Rr are pink.

True. With incomplete dominance, red (RR) and white (rr) give a heterozygous pink (Rr) phenotype.

Define pedigree diagram .

A pedigree diagram is a chart that traces the inheritance of a trait through the generations of a family .

How are males and females represented on a pedigree diagram?

Males are shown as squares . Females are shown as circles .

How are affected individuals shown on a pedigree diagram?

By shading , colour or cross-hatching of their symbol.

What do horizontal and vertical lines show on a pedigree diagram?

A horizontal line between two individuals shows they have produced children. A vertical line shows the link between parents and their children.

What information can a pedigree diagram provide?

Whether an allele is dominant or recessive . The likely genotypes of individuals and the probability of a child inheriting a trait.

How can you tell an allele is recessive from a pedigree diagram?

Two unaffected parents produce an affected child . The parents must both be carriers of the recessive allele.

If two unaffected parents have a child with a recessive disorder, what are the parents' genotypes?

Both parents must be heterozygous (carriers). Each passed a recessive allele to the affected child.

What is the genotype of an individual affected by a recessive disorder?

Homozygous recessive — they have two copies of the recessive allele.

On a pedigree diagram, an unaffected individual who carries a recessive allele is called a .

On a pedigree diagram, an unaffected individual who carries a recessive allele is called a carrier .

True or False? A useful first step when reading a pedigree is to write in the genotypes of affected individuals.

True. Affected individuals with a recessive disorder are homozygous recessive , which gives a starting point for deducing the others.

What are the two possible genotypes of an unaffected child of two carrier parents?

Homozygous dominant or heterozygous . They have at least one dominant allele, but may also carry a recessive one.

Define monohybrid inheritance .

Monohybrid inheritance is the inheritance of the alleles of a single gene from one generation to the next.

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Inheritance (Edexcel A Level Biology (A) SNAB): Flashcards

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Define semi-conservative replication.

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Cards in this collection (86)

Define semi-conservative replication.
Semi-conservative replication is DNA copying in which each new molecule contains one original strand and one newly made strand.
Why must a cell replicate its DNA before dividing?
So that both daughter cells receive a full copy of the DNA.
This ensures genetic continuity between generations of cells.
What is the role of helicase in DNA replication?
It unwinds the double helix.
It breaks the hydrogen bonds between base pairs to expose the two template strands.
What is the role of DNA polymerase?
It joins free nucleotides together along each template strand.
It catalyses the formation of phosphodiester bonds in the new sugar-phosphate backbone.
How do free nucleotides line up on the template strand?
By complementary base pairing.
Each base pairs with its complement: A with T, and G with C.
Why is DNA replication described as semi-conservative?
Each new molecule conserves half of the original DNA.
One strand is from the original molecule and one is newly made.
During replication, each original DNA strand acts as a for building a new complementary strand.
During replication, each original DNA strand acts as a template for building a new complementary strand.
What did Meselson and Stahl's experiment set out to determine?
Whether DNA replication is conservative or semi-conservative.
How did Meselson and Stahl use nitrogen isotopes in their experiment?
Bacteria were first grown in heavy ¹⁵N so their DNA was heavy.
They were then moved to light ¹⁴N and allowed to replicate once.
What result showed that replication is semi-conservative?
After one round in light nitrogen, all the DNA settled in the middle of the tube.
This meant each molecule had one heavy and one light strand.
What result would have supported conservative replication instead?
Two separate bands — one heavy (original DNA) at the bottom and one light (new DNA) at the top.
True or False?
In semi-conservative replication, both strands of a new DNA molecule are newly made.
False.
One strand is from the original molecule and only the other is newly made.
Define gene mutation.
A gene mutation is a change in the sequence of base pairs in a DNA molecule, which may result in an altered polypeptide.
When do gene mutations most often occur?
Spontaneously and continuously.
Errors often arise during DNA replication.
Why do most mutations not change the polypeptide produced?
The genetic code is degenerate.
So a changed triplet may still code for the same amino acid.
What is a substitution mutation?
One base is swapped for a different base.
It only affects the one triplet where it occurs, with no knock-on effect.
What is a frameshift mutation, and which mutations cause it?
A mutation that shifts the reading of every triplet after the change.
It is caused by insertion or deletion of a nucleotide.
Name the three types of substitution mutation.
Silent (no change to the amino acid).
Missense (one amino acid changed).
Nonsense (creates a premature stop codon).
Why can an insertion or deletion be more damaging than a substitution?
It causes a frameshift, changing every triplet downstream.
A substitution only changes the single triplet where it occurs.
Which gene is faulty in cystic fibrosis, and what does it normally code for?
The CFTR gene on chromosome 7.
It codes for chloride ion channels in cell membranes.
How does a faulty CFTR gene lead to thick, sticky mucus?
It produces non-functional chloride channels.
This reduces the movement of water into secretions by osmosis, so mucus becomes thick and sticky.
How does cystic fibrosis impair the gas exchange system?
Thick mucus builds up and blocks the airways.
This reduces the surface area for gas exchange and traps microorganisms, causing frequent lung infections.
How does cystic fibrosis impair the digestive system?
Mucus blocks the tube to the pancreas, so digestive enzymes cannot reach the small intestine.
Mucus on the intestine lining also reduces absorption of nutrients.
How does cystic fibrosis impair the reproductive system?
In men, the tubes of the testes can be blocked, stopping sperm being released.
In women, thick cervical mucus can stop sperm reaching the egg.
Cystic fibrosis is caused by a allele, so a person must be homozygous to have the disorder.
Cystic fibrosis is caused by a recessive allele, so a person must be homozygous to have the disorder.
True or False?
There is only one mutation of the CFTR gene that can cause cystic fibrosis.
False.
There are many possible mutations of the large CFTR gene — around 1000 have been identified.
Define gene.
A gene is a length of DNA that codes for a single polypeptide.
Define allele.
An allele is a different version of a gene, with a slightly different nucleotide sequence, found at the same locus.
Define genotype.
The genotype is the combination of alleles an organism possesses for a gene.
Define phenotype.
The phenotype is the observable characteristics of an organism, resulting from its genotype and the environment.
What is the difference between homozygous and heterozygous?
Homozygous means having two identical alleles of a gene.
Heterozygous means having two different alleles of a gene.
Define dominant allele.
A dominant allele is one that is always expressed in the phenotype, whether the organism is homozygous or heterozygous.
Define recessive allele.
A recessive allele is one that is only expressed when no dominant allele is present, i.e. in a homozygous recessive individual.
Define incomplete dominance.
Incomplete dominance is when neither allele is fully dominant, so a heterozygote shows a third, intermediate phenotype.
What is a carrier?
A heterozygous individual carrying a recessive allele that is not expressed in their phenotype.
They can still pass the allele to their offspring.
Define locus.
A locus is the specific position of a gene on a chromosome.
A pair of chromosomes that match in size and carry the same genes at the same loci are called a pair.
A pair of chromosomes that match in size and carry the same genes at the same loci are called a homologous pair.
True or False?
In incomplete dominance, snapdragons with genotype Rr are pink.
True.
With incomplete dominance, red (RR) and white (rr) give a heterozygous pink (Rr) phenotype.
Define pedigree diagram.
A pedigree diagram is a chart that traces the inheritance of a trait through the generations of a family.
How are males and females represented on a pedigree diagram?
Males are shown as squares.
Females are shown as circles.
How are affected individuals shown on a pedigree diagram?
By shading, colour or cross-hatching of their symbol.
What do horizontal and vertical lines show on a pedigree diagram?
A horizontal line between two individuals shows they have produced children.
A vertical line shows the link between parents and their children.
What information can a pedigree diagram provide?
Whether an allele is dominant or recessive.
The likely genotypes of individuals and the probability of a child inheriting a trait.
How can you tell an allele is recessive from a pedigree diagram?
Two unaffected parents produce an affected child.
The parents must both be carriers of the recessive allele.
If two unaffected parents have a child with a recessive disorder, what are the parents' genotypes?
Both parents must be heterozygous (carriers).
Each passed a recessive allele to the affected child.
What is the genotype of an individual affected by a recessive disorder?
Homozygous recessive — they have two copies of the recessive allele.
On a pedigree diagram, an unaffected individual who carries a recessive allele is called a .
On a pedigree diagram, an unaffected individual who carries a recessive allele is called a carrier.
True or False?
A useful first step when reading a pedigree is to write in the genotypes of affected individuals.
True.
Affected individuals with a recessive disorder are homozygous recessive, which gives a starting point for deducing the others.
What are the two possible genotypes of an unaffected child of two carrier parents?
Homozygous dominant or heterozygous.
They have at least one dominant allele, but may also carry a recessive one.
What do Roman numerals usually indicate on a pedigree diagram?
The generations of the family.
Define monohybrid inheritance.
Monohybrid inheritance is the inheritance of the alleles of a single gene from one generation to the next.
Define Punnett square.
A Punnett square is a genetic diagram used to predict the genotypes and phenotypes of offspring from a cross.
Why is there an equal chance of a gamete carrying either allele?
During meiosis, the two alleles of a homologous pair separate into different gametes.
So each gamete is equally likely to carry either allele.
In a cross between two heterozygotes (Bb × Bb), what is the phenotype ratio?
3 dominant : 1 recessive.
In a cross between two heterozygotes (Bb × Bb), what is the genotype ratio?
1 BB : 2 Bb : 1 bb.
What gametes can a heterozygous individual (Bb) produce?
Two types: one carrying B and one carrying b.
Why might real offspring ratios differ from those predicted by a Punnett square?
The predictions are based on chance.
Which gametes actually fuse is random, so real results can differ, especially with small numbers.
What offspring result from crossing a homozygous dominant (BB) with a homozygous recessive (bb)?
All offspring are heterozygous (Bb).
All show the dominant phenotype.
In a cross between two heterozygotes, the expected phenotype ratio is 3 dominant to recessive.
In a cross between two heterozygotes, the expected phenotype ratio is 3 dominant to 1 recessive.
True or False?
A cross between a heterozygote and a homozygous recessive gives a 1:1 phenotype ratio.
True.
Bb × bb produces half Bb (dominant) and half bb (recessive) — a 1:1 ratio.
What information is needed to predict offspring from a monohybrid cross?
The genotypes of the two parents.
Which allele is dominant and which is recessive.
What is the first step in drawing a genetic cross?
Write the parental genotypes.
Then work out the possible gametes each parent can produce.
What does the chi-squared test determine?
Whether there is a significant difference between the observed and expected results.
What type of data is the chi-squared test used for?
Categorical data — data that can be sorted into groups or classes.
Define null hypothesis.
The null hypothesis states that there is no significant difference between the observed and expected results.
Outline how the chi-squared value is calculated.
For each class, find the difference between observed and expected, then square it.
Divide each squared difference by the expected value, and sum the results.
How are degrees of freedom calculated?
Number of classes minus 1.
For example, four phenotypes gives 4 − 1 = 3 degrees of freedom.
What probability level do biologists usually use?
0.05, or 5%.
This means a 5% probability that any difference is due to chance.
What does it mean if the chi-squared value is greater than or equal to the critical value?
There is a significant difference between observed and expected results.
The null hypothesis is rejected, and a factor other than chance is involved.
What does it mean if the chi-squared value is smaller than the critical value?
There is no significant difference between observed and expected results.
The null hypothesis is accepted, and any difference is due to chance.
What is the critical value compared against, and what does it depend on?
It is compared with the calculated chi-squared value.
It depends on the probability level and the degrees of freedom.
If the chi-squared value is greater than the critical value, the null hypothesis is .
If the chi-squared value is greater than the critical value, the null hypothesis is rejected.
True or False?
A significant difference between observed and expected results can suggest linkage between genes.
True.
A significant difference means something other than chance is acting — for example, linkage between the genes.
Why is each difference squared in the chi-squared calculation?
So that it does not matter whether the difference is positive or negative.
Squaring makes all the differences positive before they are summed.
Define genetic screening.
Genetic screening is testing an individual's DNA to determine whether they carry a particular allele or genetic disorder.
What are the three main uses of genetic screening?
Identifying carriers of an allele.
Preimplantation genetic diagnosis (PGD) of embryos.
Prenatal testing of a foetus.
What is the purpose of carrier testing?
To show whether a person with no symptoms carries a recessive allele.
This lets couples judge the chance of their children inheriting the disorder.
What is preimplantation genetic diagnosis (PGD)?
Analysing the DNA of an embryo produced by IVF before it is implanted.
Only unaffected embryos are implanted, avoiding abortion.
Describe chorionic villus sampling.
A small sample of cells is taken from the placenta with a fine needle.
The foetal DNA is tested for genetic disorders (around 11–14 weeks; ~1–2% miscarriage risk).
Describe amniocentesis.
A sample of amniotic fluid is taken with a fine needle.
The foetal cells in it are tested for genetic disorders (around 15–20 weeks; ~1% miscarriage risk).
What is a benefit of prenatal testing?
It lets parents make informed decisions about the pregnancy.
Results can help them prepare for the future care and treatment of the child.
What is the role of a genetic counsellor?
To discuss the probability and implications of a disorder before screening.
Afterwards, to explain the results and support the parents' decisions.
Give two ethical concerns about identifying carriers.
It can cause emotional stress and may reveal other genetic disorders.
Employers or insurers could use the information against the person.
What ethical concern is raised by preimplantation genetic diagnosis?
It could lead to 'designer babies'.
Parents might select embryos for traits such as eye colour or sex.
Because no test is 100% accurate, screening can give false positive or false results.
Because no test is 100% accurate, screening can give false positive or false negative results.
True or False?
Prenatal testing methods carry a risk of miscarriage.
True.
Both chorionic villus sampling and amniocentesis carry a small risk of miscarriage.

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