Download Topic 4.1: Chromosomes, genes, alleles, and mutations

4.1.1: State that eukaryotic
chromosomes are made of DNA and
proteins
 4.1.2: Define gene, allele and genome
 4.1.3: Define gene mutations
 4.1.4: Explain the consequence of a
base substitution mutation in relation to
the processes of transcription and
translation, using the example of sickle
cell anaemia


Chromosomes are bundles of
long strands of DNA
› If you could unwind a
chromosome, it would be like
unraveling a ball of string

In eukaryotes that reproduce
sexually, chromosomes always
come in pairs
› Humans have 46 chromosomes
in 23 pairs
› The DNA un eukaryotes is
associated with proteins which
helps to keep the DNA organized

A gene is a heritable factor
that controls a specific
characteristic
› Heritable means passed on
from parent to offspring and
characteristic refers to genetic
traits such as your hair color or
your blood type
› The estimated 30,000 genes
which you possess are
organized into chromosomes

The genes which determine
eye color have more than one
form
› Variations (like in eye color) of a
gene are called alleles
 An allele is one specific form of a
gene, differing from other alleles
by one or a few bases
 Alleles of the same gene occupy
a corresponding place (locus) on
each chromosome of a pair

In order to find out which gene does
what, a list must be made showing the
order of all the letters in the DNA code
(sequence of bases)
› A complete set of an organism’s base
sequence is called its genome
› The complete genome of a few organisms
have been fully written out.

A mutation is a random, rare change in
genetic materials
› One type involves a change of the sequence
of bases of DNA
 If DNA replication works correctly, this should
not happen

Base substitution mutation
› The consequence of changing one base
could means that a different amino acid is
placed in the growing polypeptide chain
 This may have little or no effect on the
organisms or it may have a major influence on
the organisms physical characteristics
 http://highered.mcgrawhill.com/sites/9834092339/student_view0/chap
ter15/mutation_by_base_substitution.html

In fruit flies
› One base substitution mutation can cause:
 Extra pair of wings
 Change in eye color
 The number of legs
 Shape of the wings

In humans:
› one base substitution mutation can cause:
 Mutations is sometimes found in the gene
which creates hemoglobin for red blood cells
 This mutation gives a different shape to the
hemoglobin molecule
 The mutated red blood cell with the characteristic
curved shape made its discoverers think of a sickle (a
curved knife used to cut tall plants)
 Called sickle cell anaemia

Sickle cell anemia
› One base is substituted for
another so that the codon
GAG becomes GTG
› During translation instead
of adding glutamic acid it
added valine instead
 This results in a different
shape of the polypeptide—
hemoglobin molecule has
a different shape which
causes the red blood cell
have a different shape

Symptoms of sickle cell anemia
› Weakness, fatigue, and shortness of breath
› Oxygen cannot be carried as efficiently by
the irregularly-shaped red blood cells
› Hemoglobin tends to crystallize within the
red blood cells, causing them to be less
flexible
 The affected red blood cells can get stuck in
capillaries so blood flow can be slowed or
blocked

People affected by sickle cell anima
have a risk of passing the mutated gene
to their offspring
› Mutated gene is mostly found in populations
originating from West Africa or from the
Mediterranean