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Starter – talk for one minute about the diagram
Tuesday, May 23, 2017
Chapter 3: Genetics
Title: Genes
Keywords:
• gene
• chromosome
• homologous
• allele
• sickle cell anemia
Learning Objectives:
We are learning….
• What is a gene?
• What is the relationship between genes and chromosomes?
• What is an allele?
• What is the relationship between genes, mutation and sickle
cell anemia?
• What is the human genome project?
Starter:
How are these words linked?
Chromosome, DNA, amino acid, meiosis, nucleus, mitosis
Introduction to genetics
Chimpanzees are set apart from all other organisms because their
parents wee chimpanzees and their offspring will also be
chimpanzees.
Every living organism inherits its own blueprint for life in the
chromosomes and genes that are passed to it from its parents.
The study of genetics attempts to explain this process of heredity and
it also plays a very significant role in the modern world, from plant and
animal breeding to human health and disease.
What is DNA?
Chromosomes and their genes are made of a molecule
called DNA.
DNA stands for
deoxyribonucleic
acid.
Each chromosome
is a very long molecule
of tightly coiled DNA.
DNA molecules carry the code that controls what your cells
are made of and what they do.
Which part of a DNA molecule holds this information?
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© Boardworks Ltd 2004
Genes
• Sections of DNA
• Code for the manufacture of proteins
• The position of a gene on a chromosome is its
locus
Different versions of genes
The chromosomes in a matching pair contain the same type
of genes that code for the same characteristics.
gene for
petal colour
gene for
petal colour
version for
red petals
version for
yellow petals
Each chromosome may have a different version of a gene.
Different versions of a gene, that code for different versions
of a characteristic, are called alleles.
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Pairs of alleles – homozygous
If the alleles in a matching pair are the same,
they are called homozygous alleles.
allele for
yellow petals
allele for
yellow petals
allele for
red petals
allele for
red petals
What colour are the flowers with these
homozygous pairs of alleles?
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(Click twice on each bud
to reveal the flower;
click again to close them.)
© Boardworks Ltd 2004
Representing alleles
Letters are used to represent different alleles.
A dominant allele is always a capital letter.
allele for
red petals
= R
A recessive allele is always the corresponding small letter.
allele for
yellow petals
=
r
The allele pair for each characteristic is called the genotype.
What colour are flowers with the genotype Rr?
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© Boardworks Ltd 2004
Allele
• Each gene exists in 2 or more forms
• Each of these forms is an allele
• A homologous pair may have different alleles
of the same gene
• If there are different alleles of a gene on
homologous chromosomes, the organism is
heterozygous for that gene (e.g. B,b)
• If the alleles are the same, the organism is
homozygous (B, B)
Multiple alleles and blood groups
In humans, the inheritance of the ABO blood groups is
determined by a gene, I, which has three different alleles.
Any two of these can occur at a single locus at any one
time.
Blood group
Possible genotypes
A
IA IA or IA IO
B
IB IB or IB IO
AB
IA IB
O
IO IO
Multiple alleles and blood groups
Allele A causes the production of antigen A
on red blood cells
Allele B causes the production of antigen B
on red blood cells
Allele O causes no production of antigens on
red blood cells (universal donor)
Alleles A and B are codominant and allele O is
recessive to both.
Blood group
Possible genotypes
A
IA IA or IA IO
B
IB IB or IB IO
AB
IA IB
O
IO IO
Try a cross
between
individuals with
blood group A and
an individual with
blood group B
Protein synthesis summary – how
many mistakes can you spot?
A
C
C
A
A
A
C
C
G
A
translation
cytoplasm
T
G
G
T
T
T
A
G
ribosome
Nucleus:
Leu
T
G
RNA
Leu
C
T
A
C
tRNA
transcription
Phe
Ser
Gly
PROTEIN
Guess the mutation – match the mutation
to the pictures
Mutation type
Addition
Inversion
Substitution
An example of how mutagens cause damage to the
genes controlling the normal cell cycle is shown below.
After exposure to UV light, adjacent thymine bases in
DNA become cross-linked to form a ‘thymine dimer’
(lesions form between bases).
This disrupts the normal base pairing and throws the
controlling gene’s instructions into chaos.
So what are mutations?
Any change in the structure or the amount of DNA of an
organism is called a mutation.
Most mutations occur in somatic (body) cells and are not
passed from one generation to the next.
Only those mutations which occur in the formation of gametes
can be inherited.
These mutations produce sudden and distinct differences
between individuals.
Michael Berryman is an American actor who has
appeared in many horror films. He was born with a
rare genetic condition which prevents him from
developing hair, sweat glands or fingernails and
claims to have had "twenty-six birth defects."
Changes in gene structure – point mutations
Changes in the structure of DNA which occur at a single
locus on a chromosome are called gene
mutations or point mutations.
Any change in the sequence
of nucleotides will produce
the wrong sequence of amino
acids in the protein it makes.
This protein is often an enzyme.
Why might this be a problem?
Complete the card sort to find out how sickle cell
anaemia is caused.
Problems with the code
A mutation in the code, or if part of the code is missing, can
lead to a lack of an enzyme or a faulty enzyme being formed.
Sickle cell anaemia is a hereditary condition caused by a
gene mutation.
The replacement of just one base in the DNA
molecule results in the wrong amino acid being
incorporated into two of the polypeptide chains
which make up the haemoglobin molecule.
The abnormal haemoglobin
causes red blood cells to become
sickle-shaped, resulting in anaemia
and possible death.
Over 12 500 people in the UK have
sickle cell anaemia. The majority of
them are of African or Caribbean
descent.
Types of gene mutation – the causes
Duplication
A portion of the nucleotide becomes repeated.
Addition (insertion)
An extra nucleotide sequence becomes inserted in the chain.
Deletion
A portion of the nucleotide chain is removed from the sequence.
Inversion
A nucleotide sequence becomes separated from the chain. It
rejoins in its original position, only inverted (backwards). The
nucleotide sequence of this portion is therefore reversed.
Substitution
One of the nucleotides is replaced by another which has a
different organic base.
Draw a diagram for each followed by a simple explanation.
Types of gene
mutations – the causes
The human genome project
Why is determining an organism’s genome sequence important?
Most types of cell in an organism contain a complete copy of its genome.
The organisation is quite complicated, but the simplest fact about any
genome is that it is a collection of DNA sequences – long strings of the
chemical ‘letters’ A, T, G and C (adenine, thymine, guanine and cytosine)
in a particular order.
Learn to read an organism’s genome sequence, and compare it with that
of other organisms, and it can tell you lots of different things.
The human genome sequence contains a
wealth of information about human biology,
in both health and disease. Our DNA is a
window on evolution and recent human
history – including the migration of people
around the world.
DNA sequencing – the Sanger
technique
The human genome project
Why is determining an organism’s genome sequence important?
The genome sequences of other species have many other uses. The
genomes of organisms used in farming, from rice and wheat to pigs and
cattle, are being sequenced to help to breed improved strains. But the
vast majority of the many thousands of genomes already completed are
from bacteria. Some are species that cause diseases in people, as well
as in agriculturally important animals or plants.
Others are important for maintaining health or have potential uses in the
industrial production of biologically active chemicals and enzymes.
Genomic information is used to track harmful microbes such as those that
cause infection in hospitals, as well as to aid the development of new
drugs. New influenza strains have their genomes read quickly to
understand how the virus spreads and to speed up vaccine production.
Knowledge of genome sequences also speeds up developments in
biotechnology and is finding uses in tracking biodiversity and policing
trade in protected species.
Main Activity:
Success Criteria: What I’m looking for…..
Plenary:
Your little sister is playing in the living room whilst
your parents watch the news. She hears an item
about genetically inherited illness. She knows you
are studying genetics at school so she asks you,
“What is a gene?”
You have five minutes in pairs to write out an
answer she will understand.
How successful were we this lesson?
Learning Objective
We were learning…..