3.1 Genes
Understandings, Applications and Skills (This is what you may be assessed on)
Statement
Guidance
3.1.U1
A gene is a heritable factor that consists of a length of DNA and
influences a specific characteristic.
3.1.U2
A gene occupies a specific position on a chromosome.
3.1.U3
The various specific forms of a gene are alleles.
3.1.U4
Alleles differ from each other by one or only a few bases.
3.1.U5
New alleles are formed by mutation.
3.1.U6
The genome is the whole of the genetic information of an
organism.
3.1.U7
The entire base sequence of human genes was sequenced in the
Human Genome Project
3.1.A1
The causes of sickle cell anemia, including a base substitution
mutation, a change to the base sequence of mRNA transcribed
from it and a change to the sequence of a polypeptide in
hemoglobin.
[Students should be able to recall one specific
base substitution that causes glutamic acid to be
substituted by valine as the sixth amino acid in
the hemoglobin polypeptide.]
3.1.A2
Comparison of the number of genes in humans with other
species.
[The number of genes in a species should not be
referred to as genome size as this term is used
for the total amount of DNA. At least one plant
and one bacterium should be included in the
comparison and at least one species with more
genes and one with fewer genes than a human.]
3.1.S1
Use of a database to determine differences in the base sequence [The Genbank® database can be used to search
of a gene in two species.
for DNA base sequences. The cytochrome C
gene sequence is available for many different
organisms and is of particular interest because
of its use in reclassifying organisms into three
domains.]
3.4. U9
Radiation and mutagenic chemicals increase the mutation rate
and can cause genetic diseases and cancer
3.4.A4
Consequences of radiation after nuclear bombing of Hiroshima
and accident at Chernobyl.
[Deletions, insertions and frame shift mutations
do not need to be included.]
3.2 Chromosomes
Understandings, Applications and Skills (This is what you may be assessed on)
Statement
3.2.U1
Prokaryotes have one chromosome consisting of a
circular DNA molecule.
3.2.U2
Some prokaryotes also have plasmids but eukaryotes
do not.
3.2.U3
Eukaryote chromosomes are linear DNA molecules
associated with histone proteins.
3.2.U4
In a eukaryote species there are different chromosomes
that carry different genes.
3.2.U5
Homologous chromosomes carry the same sequence of
genes but not necessarily the same alleles of those
genes.
3.2.U6
Diploid nuclei have pairs of homologous chromosomes.
3.2.U7
Haploid nuclei have one chromosome of each pair.
3.2.U8
The number of chromosomes is a characteristic feature
of members of a species.
3.2.U10
Sex is determined by sex chromosomes and autosomes
are chromosomes that do not determine sex.
Guidance
[The two DNA molecules formed by DNA replication
prior to cell division are considered to be sister
chromatids until the splitting of the centromere at the
start of anaphase. After this, they are individual
chromosomes.]
3.2.A1
Cairns’ technique for measuring the length of DNA
molecules by autoradiography.
3.2.A2
Comparison of genome size in T2 phage,Escherichia
[Genome size is the total length of DNA in an organism.
coli, Drosophila melanogaster, Homo sapiens and Paris The examples of genome and chromosome number
japonica.
have been selected to allow points of interest to be
raised.]
Comparison of diploid chromosome numbers of Homo
sapiens, Pan troglodytes, Canis familiaris, Oryza
sativa, Parascaris equorum.
3.2.A3
3.2.S1
Use of databases to identify the locus of a human gene
and its polypeptide product.
Define the following:
Chromosome
(autosome)
(3.2.U10)
Gene
(3.1.U1,
3.1.U2 )
Allele
(3.1.U3)
Gene locus
Genome
(3.1.U6)
Chromatid
Mutation
Haploid
(3.2.U7)
Diploid
(3.2.U6)
Plasmid
Gamete
Sex
Chromosome
(3.2.U10)
Homologous
(3.2.U5)
3.2U3 Eukaryote chromosomes are linear DNA molecules associated with histone proteins.
State the components of a chromosome.
DNA & _____________
3.2.U8 The number of chromosomes is a
characteristic feature of members of a species.
3.2.A3 Comparison of diploid chromosome numbers
of Homo sapiens, Pan troglodytes, Canis familiaris,
Oryza sativa, Parascaris equorum.
State the number of chromosomes present in a
single human diploid cell.
Identify structures a. and b. on the line drawing of a chromosome in prophase shown to the right.
a.
b.
3.1.A2 Comparison of the number of genes in humans with other species.
The number of genes in a species should not be referred to as genome size as this term
is used for the total amount of DNA. At least one plant and one bacterium should be
included in the comparison and at least one species with more genes and one with fewer
genes than a human.
Comparing Numbers of Genes: Page 142
Group
Name of Species
Brief Description
Prokaryotes
Haemophilus influenzae
Escherichia coli
Trichomonas vaginalis
Saccharomyces cerevisae
Oryza sativa
Arabidopsis thaliana
Populus trichocarpa
Drosophila melanogaster
Caenorhabditis elegans
Homo sapiens
Daphnia pulex
Pathogenic bacterium
Gut bacterium
Unicellular parasite
Unicellular fungus
Crop grown for wood
Small annual weed
Large tree
Larvae consume ripe fruit
Small soil roundworm
Large omnivorous biped
Small pond crustacean
Protoctista
Fungi
Plants
Animals
Numbers of
Genes
1700
3200
60000
6000
41000
26000
46000
14000
19000
23000
31000
3.2.A2 Comparison of genome size in T2 phage,Escherichia coli, Drosophila melanogaster, Homo
sapiens and Paris japonica.
Genome size is the total length of DNA in an organism.
The examples of genome and chromosome number have been selected to allow points of interest to be raised.
 Comparing Genome Size page 153 – 154.
3.2.U4 In a eukaryote species there are different chromosomes that carry different genes.
3.2.U5 Homologous chromosomes carry the same sequence of genes but not necessarily the same
alleles of those genes.
 Complete the DBQ on page 153 and 156
Page 153 DBQ: Comparing the chromosomes of mice and humans:
Deduce the number of types of chromosomes in mice and in humans. [2]
Identify the two human chromosome types that are most similar to mouse chromosomes. [2]
Identify mouse chromosomes which contain sections that are not homologous to human
chromosomes [2]
Suggest reasons for the many similarities between the mouse and human genomes. [2]
Deduce how chromosomes have mutated during the evolution of animals such as mice and humans
[2]
DBQ Page 156: Differences in chromosome number
There are many different chromosome numbers in the table, but some numbers are missing, for
example 5, 7, 11, 13. Explain why none of the species has 13 chromosomes. [3]
Discuss, using the data in the table, the hypothesis that the more complex an organism is, the more
chromosomes it has. [4]
Explain why the size of the genome of a species cannot be deduced from the number of
chromosomes. [1]
Suggest, using the data in table 1, a change in chromosome structure that may have occurred during
human evolution. [2]
3.2.A1 Cairns’ technique for measuring the length of DNA molecules by autoradiography.
 Read 150 – 151 in the textbook
3.2.S1 Use of databases to identify the locus of a human gene and its polypeptide product.
 Finding the Loci of Human Genes: Page 154
3.2.U1 Prokaryotes have one chromosome consisting of a circular DNA molecule.
3.2.U2 Some prokaryotes also have plasmids but eukaryotes do not.
3.1.U3 The various specific forms of a gene are alleles.
Give two examples of genes and some of their possible alleles.
Gene
Possible alleles
Eye colour
Blue, brown, green, hazel
3.1.U5 New alleles are formed by mutation.
List factors that increase the chance of a genetic mutation.



3.4.U9 Radiation and mutagenic chemicals increase the mutation rate and can cause genetic
diseases and cancer
Outline two types of factors that increase the mutation rate:


3.4.A4 Consequences of radiation after nuclear bombing of Hiroshima and accident at Chernobyl.
 Consequences of Nuclear Bombing and Accidents at Nuclear Power Stations:
Read 184 – 186 and complete the DBQ questions below:
Calculate the percentage of excess deaths over control groups due to leukemia in people exposed to
(a) 0.005-0.02 Sv (Sieverts) of radiation (b) >1 Sv of radiation. [4]
Construct a suitable type of graph or chart to represent the data in the right-hand column of the table,
including the two percentages that you have calculated. There should be two y-axes, for the leukemia
deaths and the cancer deaths. [4]
Compare and contrast the effect of radiation on deaths due to leukemia and deaths due to cancer. [3]
Discuss, with reasons, what level of radiation might be acceptable in the environment. [4]
3.1.A1 The causes of sickle cell anemia, including a base substitution mutation, a change to the base
sequence of mRNA transcribed from it and a change to the sequence of a polypeptide in hemoglobin.
Students should be able to recall one specific base substitution that causes glutamic acid to be substituted by
valine as the sixth amino acid in the hemoglobin polypeptide.
mRNA
amino acid
Distinguish between the two DNA strands above in terms of:
a. DNA base sequence.
b. Amino acid sequence in the resulting polypeptide.
Compare the following types of base-substitution mutation.
Silent mutation
Mis-sense
mutation
Nonsense
mutation
Number of bases
1
substituted
Stop codon
produced early –
Effect on polypeptide
polypeptide
shortened
Example illness
Sickle cell disease
Describe the effects of sickle cell disease on sufferers in terms of:
a. Hemoglobin production

b. Symptoms and mortality

Identify parts of the world where a single sickle cell (Hbs) allele could be beneficial

Explain your answer
Define evolution.

Outline how mutations lead to evolution by natural selection.






Outline how the spread of the sickle cell gene is an example of natural selection in action.



How could this be an example of a correlation which has a strong element of causality?