Download DNA and cell division exam questions B2

DNA and cell division (B2 Revision)
Chromosomes are made from DNA. Genes are short sections of DNA. Genetically identical
cells are produced by a type of cell division called mitosis. In sexual reproduction, a male
gamete fuses with a female gamete to produce a new cell. This is called fertilisation.
Gametes are produced by a type of cell division called meiosis. They contain a single set
of chromosomes, whereas body cells contain two sets of chromosomes.
DNA, genes and chromosomes
DNA
DNA (deoxyribose nucleic acid) molecules are large and complex.
They carry the genetic code that determines the characteristics of a
living thing.
Except for identical twins, each person’s DNA is unique. This is why
people can be identified using DNA fingerprinting. DNA can be cut up
and separated, forming a sort of 'bar code' that is different from one
person to the next.
Genes
A gene is a short section of DNA. Each gene codes for a specific
protein by specifying the order in which amino acids must be joined
together.
Chromosomes
The cell’s nucleus contains chromosomes made from long DNA
molecules.
The diagram shows the relationship between the cell, its nucleus,
chromosomes in the nucleus, and genes.
Mitosis
The chromosomes contain the cell’s genetic information. They are
normally found in pairs in body cells. Body cells divide by a process
called mitosis.
The diagram summarises how this happens. Note that you do not need to know or understand the
different stages in mitosis for the examination.
When a body cell divides by mitosis:
1. The genetic material is copied
2. The cell divides once to form two genetically identical body cells
Mitosis occurs during growth or to produce replacement cells.
Gametes and fertilisation
Gametes
Cells in the reproductive organs (testes and ovaries in humans) divide to form gametes. Gametes
are sex cells:


Male gametes are sperm (produced in the testes)
Female gametes are eggs (produced in the ovaries)
Gametes contain one set of genetic information, while body cells contain two sets of genetic
information. The type of cell division that produces gametes is called meiosis.
Fertilisation
Fertilisation is the joining or fusion of a male gamete and a female gamete. When fertilisation
happens, a single body cell with new pairs of chromosomes is formed. The new cell then divides
over and over again by mitosis. This creates the many cells that eventually form a new individual.
Differentiation and stem cells
Differentiation
Most types of animal cells differentiate at an early stage - they become specialised for a
particular function and cannot change into different types of cell. In mature animals, cell division is
mainly restricted to replacement and repair. However, many plant cells keep the ability to
differentiate.
Stem cells
Stem cells can be made to differentiate to form different types of cell, such as nerve cells.
Human stem cells can come from human embryos or from adult bone marrow. They are able to
develop into any kind of human cell. Note that you do not need to know any details of how this
happens for the examination.
Meiosis - Higher tier
A cell divides by meiosis to form
gametes. When this happens:
1. Copies of the genetic
information are made
2. The cell divides twice to form
four gametes
Each gamete has a single set of
chromosomes.
The diagram summarises how this
works. Note that you do not need
to know or understand the
different stages in meiosis for the
examination.
Alleles
Some characteristics, such as eye colour and the shape of the earlobe, are controlled by a single
gene. These genes may have different forms.
Different forms of the same gene are called alleles (pronounced 'al-eels'). The gene for eye colour
has an allele for blue eye colour and an allele for brown eye colour.
Alleles are dominant or recessive:


The characteristic controlled by a dominant allele develops if the allele is present on one or
both chromosomes in a pair
The characteristic controlled by a recessive allele develops only if the allele is present
on both chromosomes in a pair
For example, the allele for brown eyes is dominant, while the allele for blue eyes is recessive. An
individual who inherits one or two alleles for brown eyes will have brown eyes. An individual will
only have blue eyes if they inherit two copies of the allele for blue eyes.
The cells of offspring produced by asexual reproduction are produced bymitosis from the
parental cells. They contain the same alleles as the parent. However, sexual reproduction causes
variation because when two gametes join together one of each pair of alleles comes from each
parent.
Note that if you are doing the Foundation tier paper you are expected to be able to interpret
genetic diagrams. If you are doing the Higher tier paper, you are expected to be able to draw
genetic diagrams for any combination of dominant and recessive alleles.
Embryo screening
Embryos can be screened for the alleles that cause polydactyly, cystic fibrosis and other genetic
disorders. Note that you do not need to know or understand how embryo screening works for the
examination.
1. Cystic fibrosis is an inherited disorder that can seriously affect health.
(a)
Which one of these is affected by cystic fibrosis?
Draw a ring around your answer.
blood
cell membranes
kidneys
nervous system
(1)
(b)
The diagram shows the inheritance of cystic
fibrosis in a family. The allele that produces cystic
fibrosis is recessive.
(i)
Explain why Alice inherited cystic fibrosis.
.............................................................
.............................................................
............................................................
..........................................................
.............................................................
..........................................................
(2)
(ii)
Explain why Ted did not inherit cystic fibrosis.
..........................................................................................................................
..........................................................................................................................
..........................................................................................................................
........................................................................................................................ (2)
(c)
Bob and Carol know that there is a risk that their next baby will have cystic fibrosis.
Embryos can be screened for the allele that produces cystic fibrosis. Many people
support the screening of embryos, but others do not.
(i)
Suggest one reason why many people support the screening of embryos for the
cystic fibrosis allele.
..........................................................................................................................
........................................................................................................................ (1)
(ii)
Suggest one reason why many people are against the screening of embryos for
the cystic fibrosis allele.
..........................................................................................................................
.......................................................................................................................... (1)
(Total 7 marks)
2. The diagram shows a family tree in which some individuals have an inherited disorder, which
may cause serious long-term health problems.
(a)
What proportion of the children of A and B have the disorder?
................................................................................................................................... (1)
(b)
Explain the evidence from the diagram which shows that the allele for the disorder is
dominant. Use the appropriate letters to identify individuals in your answer.
You may use genetic diagrams in your explanation. There is space for you to draw a
genetic diagram at the top of the facing page.
.....................................................................................................................................
.....................................................................................................................................
.................................................................................................................................. (3)
3. In humans, sex chromosomes control whether a person is male or female.
(a)
Use letters X and Y to complete the Punnett square for sex inheritance.
Eggs
or
XX..........
............................
............................
............................
...........
Sperm
or
(3)
(b)
A couple already have three boys. What is the probability that their next child will be a
girl?
........................................................................................................................ (1)
(Total 4 marks)
4. A woman gives birth to triplets.
Two of the triplets are boys and the third is a girl.
The triplets developed from two egg cells released from
the ovary at the same time.
The diagram shows how triplets A, B and C developed.
(a)
Which stages on the diagram show gametes?
Draw a ring around your answer.
1 and 2
(b)
2 and 3
3 and 7
1 and 7
(1)
Embryo B is male.
Which of the following explains why embryo B is
male?
Tick ( ) one box.
Cell P has an X chromosome; cell R has an X chromosome.
Cell P has a Y chromosome; cell R has an X chromosome.
Cell P has an X chromosome; cell R has a Y chromosome.
(1)
(c)
Single cells from an embryo at Stage 7 can be separated and grown in a special
solution.
(i)
What term describes cells that are grown in this way?
Draw a ring around your answer.
lleles
screened cells
stem cells
(1)
(ii)
What happens when the cells are placed in the special solution?
Tick ( ) two boxes.
The cells divide
The cells fertilise
The cells differentiate
The cells separate
(2)
(iii)
Give one use of cells grown in this way.
...........................................................................................................................
.......................................................................................................................... (1)
1(a)
cell membranes
1
(b)
(i)
two recessive / cystic fibrosis / faulty / diseased / the allele(s) / genes
two can be implied by second marking point
ignore chromosomes
1
from Bob and Carol / both parents / the parents
if no other marks awarded ‘Carol is a carrier’ gains 1 mark
1
(ii)
(inherited) dominant / normal allele / gene
1
from Carol / mother
ignore references to recessive allele / gene from father / Bob
if no other marks awarded he has just / only one recessive
allele gains 1 mark
1
(c)
(i)
reduce number of people with cystic fibrosis (in population)
or
reduce health-care costs
or
expensive to have baby with cystic fibrosis
accept to allow decision / emotional argument qualified
eg allows abortion
or
allows people to make choices about termination
or
help to prepare financially / emotionally etc
1
(ii)
any one from:
•
possible damage / risk to embryo / fetus / baby
allow possible harm / risk to mother
•
screening / it is expensive
•
(may) have to make ethical / moral / religious decisions
ignore not natural / playing God / unethical / immoral /
religious unqualified
•
right to life
1
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2 (a)
1 in 4 / 1/4 / 1: 3 / 25% / 0.25
do not accept 3:1 / 1:4 / 2:6
1
(b)
either from C and D
accept synonyms for dominant / recessive eg
Normal / faulty
accept genetic diagram if clearly referring to correct individuals
or genotypes on family tree
allow ‘gene’ for ‘allele’
any three from:
•
C and D have disorder
ignore ‘C & D are carriers’
•
I/J don’t have disorder
•
C and D have dominant and
recessive alleles
•
recessive alleles from C and D passed to I/J
or I/J have two recessive alleles
NB if allele was recessive then all offspring of C and D would
have the disorder = 3 marks
or from A and B
assume response refers to A + B unless contradicted
•
A is homozygous recessive / rr, and B is heterozygous / Rr can be shown
in words or symbols
allow any symbol
•
offspring can be rr or Rr described
allow without key
3
(c)
(i)
(embryos) checked for inherited / genetic disorders / conditions
accept diseases for disorders
1
(ii)
any three from:
•
C/D have disorder / have dominant allele
accept disease / condition
accept ‘gene’ for ‘allele’
ignore reference to ‘carriers’
•
chance of embryo / foetus / child having disorder
or may pass on alleles for disorder to their offspring
•
C/D might want to decide on termination or prepare
for child with disorder
•
G and H don.t have disorder / both homozygous
recessive / have no dominant alleles (for this disorder)
•
so offspring (of G and H) cannot / don.t have disorder
3
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3 (a)
(X)
(XX)
XX
Y
XY
XY
1 mark for Y in sperm box
1 mark if XX box correct
1 mark if both XY boxes correct
3
(b)
1 : 1 or 50% or ½ or 0.5 or 1 in 2 or 1 out of 2 or 50 : 50
do not accept 50/50
accept equal (probability)
1
[4]
4
(a)
2 and 3
1
(b)
cell P has an X chromosome; cell R has a Y chromosome
1
(c)
any two from:
•
(formed from) different egg / 2 eggs
•
(formed from) different sperm / 2 sperm
•
have different genes / alleles / chromosomes / DNA
allow genetics
2
(d)
(i)
stem cells
1
(ii)
the cells divide
1
the cells differentiate
1
(iii)
(medical) research / named eg growing organs
or
medical / patient treatment
allow (embryo) cloning
do not allow designer babies / more babies
1
(iv)
any one from:
•
ethical / moral / religious objections
ignore cruel / not natural / playing God
•
potential harm to embryo
allow deformed
ignore harm to mother
1
[9]