Download DNA, chromosomes and Genes

DNA, Chromosomes
and Genes
K. Chamberlain 2008
Part One
Genes
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Many of you traits are determined by Genes
A gene is the length of DNA
(Deoxyribonucleic Acid) that contains
instructions
There are various forms of genes called
alleles
This diversity is what causes some
differences between individuals (other
differences are caused by your environment)
All the different genes and alleles that a
species has is called the gene pool
K. Chamberlain 2008
Genes
Genes also play a part in diseases and
behaviour
 For example,
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– Breast cancer
– Colour blindness - inability to see certain
colours
• Test
• How deficient people see the world
– PKU (phenylketonuria) - metabolic disorder
K. Chamberlain 2008
Genes and chromosomes
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Humans have 20,000 - 40,000 genes, which are
found in chromosomes
Chromosome = structure formed of a very long piece
of DNA and proteins tightly wound together
Chromosomes contain the double helix strands of
DNA
Human chromosome 1 has 3000 genes, whereas
chromosome 7 has 1400 genes.
Each species has its own number of chromosomes.
– Humans - 46 chromosomes
– Mice - 40 chromosomes
– Ferns - 1200 chromosomes
K. Chamberlain 2008
Genes and chromosomes
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A karyotype is a picture that allows us to see
chromosomes arranged in pairs and by
number
 A karyotype detects whether the individual’s
sex and some genetic disorders
– Female
– Male
– Trisomy 21 (Down Syndrome)
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The karyotype shows 22 pairs of autosomes
(non-sex chromosomes) and 2 sex
chromosomes K. Chamberlain 2008
K. Chamberlain 2008
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Genes and chromosomes
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Each chromosome in a pair has
the same genes in the same
location. A gene location is
called the loci or a locus
When the cells contain pairs of
chromosomes the species is
considered to be diploid
– One copy of each chromosome
is haploid (Bacteria)
– Three copies is triploid (Wheat)
– Four copies is tetraploid (Gray
Treefrog Hyla versicolor)
K. Chamberlain 2008
Part Two
Genes and proteins
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Genes control the production of proteins,
which are important for the regulation of
growth, structure and function of cells
– Haemoglobin - used to carry O2 and CO2 in red
blood cells
– Pepsin - a human digestive enzyme
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Amino acids form the building blocks of
protein
– 20 different amino acids exist
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Proteins are made up of long chains of amino
acids called polypeptides
K. Chamberlain 2008
DNA and gene expression
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Each strand of DNA is made
up of many units called
nucleotides
Nucleotides consist of a
sugar group, a phosphate
group and one of four bases,
arranged in different orders
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Adenine (A)
Cytosine (C)
Guanine (G)
Thyamine (T)
The bases pair up with their
complementary base on the
other strand of DNA
– A pairs with T
– C pairs with G
K. Chamberlain 2008
From gene to protein
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The first step in reading genetic code is called
transcription and takes place in the nucleus
 Specific enzymes cause the DNA to unwind
and strands to separate, producing RNA
(ribonucleic acid).
 The order of bases in the DNA is represented
by a complementary order of bases in the
RNA, except thymine (T) is replaced by Uracil
(U)
K. Chamberlain 2008
From gene to protein
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The RNA molecule (known as messenger RNA or
mRNA) travels out of the nucleus into the cell’s
cytoplasm and is joined by ribosomes (proteinbuilding units)
The ribosomes move along the mRNA and ‘read’ the
codon (a group of 3-bases) which acts as instruction
to add a particular amino acid or to stop growing.
This process is called translation
– Codon to stop translation - UAA, UAG or UGA
– Codon to start translation - AUG
– Codon to add amino acid,
phenylalanine - UUU
K. Chamberlain 2008
Mutation
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Occurs when mispairing of bases
occurs and the wrong nucleotide is
inserted
 This may mean the wrong amino acid
is inserted into a protein, part of a
polypetide is missing, no functioning
protein is made at all, or a whole gene
is duplicated
 Can lead to cell death, impair
important functions, or change the
phenotype of a cell
 A mutation could affect the colour of
skin or hair in an animal. In humans, it
could also result in a genetic disease
such as removing the ability to digest
some foods. Ie/ Lactose intolerance,
Gluten intolerance (Celiac)
K. Chamberlain 2008
Mutations
Mutation
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Mutation also causes trisomy-21 (Down’s Syndrome)
Sometimes a mutation may have no effect at all, other times it
may be of benefit to the individual, but this is rare
Interestingly, the Human Genome Project reveled we all have
mutations in our DNA sequence which do not affect the
phenotype!!
Occurs at a very low rate: about 1 in 1mil bases.
UV radiation and some chemicals can increase the rate
– These agents are linked to cancer - cancer arises when there is a
change in the genes that control normal growth
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A mutation is only passed along if it occurs during meiosis
(when gametes are formed), which is how new genetic variation
arises
If the mutation generates an advantage for the individual over
other individuals of the same species, then it may increase in
frequency over generations
K. Chamberlain 2008
Which mouse is the mutation? Why?
K. Chamberlain 2008
Sickle-cell anaemia
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Alleles are different versions of
a gene, containing differences
in their DNA sequence
 If a DNA differs by only one
gene, it can have dramatic
effects.
 For example, sickle-cell
anaemia. There is only one
allele different in it’s DNA
sequence, but it’s enough to
change the structure of the
haemoglobin, giving the RBC a
sickle shape and causing them
to clump and block small
vessels
K. Chamberlain 2008
Normal RBC
Sickle-cells
Part Three
Theory of evolution
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Charles Darwin noticed different finch species were similar in
colour and size but had variations in their beaks that made them
suited to the food sources they had available
He proposed that all species were the descendents of one
shared, ancestral species over generations the different finches
had evolved to suit their different environments and feeding
habits
Alfred Wallace also reached this conclusion
Evolution is the study of change in inherited characteristics
within a group of organisms
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Natural Selection
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Is the suggested means by
which evolution occurs
 Variation occurs between
different individuals in a species
 Anything that gives an individual
a competitive edge will increase
its chance of surviving and
reproducing
– Also known as the survival of
the fittest
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Variation can be inherited as
offspring generally resemble
their parents
 Over generations the successful
characteristics become more
common
 Natural Selection has the effect
that more favourable DNA
codes are kept and less K. Chamberlain 2008
favourable ones are lost
Speciation
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The formation of a new species when a population is divided in
some way
Over generations each separate population accumulates small
genetic changes due to mutation and selection
Over time they will be unable to breed because they are
genetically isolated.
– Called reproductive isolation
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Examples
– Eastern and Northern Rosella
– Eastern Blue Groper and the Western Blue Groper
K. Chamberlain 2008