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GENETICS TOPIC 4
4.1 Chromosomes, genes, alleles and mutation
State that eukaryotic chromosomes are made of DNA and proteins
Define- gene, allele, and genome
Define mutation
Explain the consequences of a base substitution mutation in relation
to the processes of transcription and translation
Chromosome Structure
During Interphase of the cell cycle chromosomes are in the form of long,
thin threads, which cannot be seen with a simple microscope
As the nucleus prepares to divide, these threads undergo coiling and become
much shorter and thicker.
Chromosomes of eukaryotic cells are associated with proteins. Each
chromosome contains a single molecule of DNA along with associated
proteins. Some of these proteins are structural and others regulate activities
of the DNA
Chromosomes, genes, and mutations
The DNA molecule consists of a
linear sequence of nucleotides. The
linear sequences of the bases
contains the genetic code in the
form of triplet bases (codons)
Gene
A gene is a heritable factor that controls a specific
characteristic. It is particular section of a DNA strand that,
when transcribed and translated forms a specific polypeptide.
http://www.youtube.com/watch?v=eOvMNOMRRm8
A triplet of bases in the DNA molecule is transcribed into a triplet of
bases in the mRNA molecule, which is then translated into a specific
amino acid.
Allele
Allele- a specific form of a gene occupying the same position as other alleles
of that gene, but differing from other alleles by small differences in its base
sequence
Genome
Genome- the whole genetic information of an organism
Mutations
The process of DNA replication is complex and mistakes sometimes occur- a
nucleotide may be left out, an extra one may be added, or the wrong one
inserted.
Gene mutation- a change in the sequence of the bases in a gene.
The insertion of an incorrect nucleotide is called a base substitution
mutation.
When DNA containing an incorrect nucleotide is transcribed and translated,
errors may occur in the polypeptide produced.
http://www.youtube.com/watch?v=5raJePXu0OQ&feature=related
This table shows the amino acids that are specified by different mRNA codons.
Most amino acids are coded for by more than one codon and so many
substitution mutations have no effect on the final polypeptide. A mutation in the
DNA triplet CCA into CCG would change the codon in the mRNA from GGU
to GGC, but would still result in the amino acid glycine
Some have serious effects
Sickle-Cell Anemia
Sickle- cell anemia results from a single base substitution.
It’s a blood disorder in which red blood cells become sickle shaped
and cannot carry oxygen properly.
Sickle-cell occurs most frequently in people with African
ancestry- about 1% suffer from the condition and between
10%-40% are carriers.
http://www.youtube.com/watch?v=9UpwV1tdxcs
Sickle-cell anemia is due to a single base substitution mutation on one of
the genes that makes hemoglobin, the oxygen-carrying pigment in red
blood cells.
Hemoglobin is made up of four subunits. Two alpha chains and two
beta chains. The beta chains are affected by sickle-cell mutation.
To form a normal beta- chain the triplet base pairing in DNA is:
-----G-----A-----G----I
I
I
-----C-----T------C-----
The C-T-C on the coding strand of the DNA is transcribed into the mRNA
triplet G-A-G, which is in turn translated into glutamic acid in the polypeptide
chain of the beta subunit
If the sickle-cell mutation occurs, the adenine base (A) is substituted for thymine
(T) on the coding strand.
-----G-----T-----G----I
I
I
-----C-----A-----C----C-A-C on the coding strand of the DNA is now transcribed into the mRNA
triplet G-U-G which is translated to give the amino acid Valine.Valine replaces
Glutamic Acid in the beta-chain.
Valine has different properties from glutamic acid and so this single change in the
amino acid sequence has very serious side effects.
The hemoglobin molecule is a different shape, it is less soluble and when in low
oxygen concentrations it deforms the red blood cell to give it the sickle shape.
Sickle cells carry less oxygen, which result in anemia. They are also rapidly
removed from circulation leading to a lack of red blood cells which can lead to
jaundice, kidney problems and enlargement of the spleen.
4.2 Meiosis
•State that meiosis is a reduction division of a diploid nucleus to form a haploid
nulcei.
•Define homologous chromosomes
•Outline the process of meiosis, including pairing of homologous chromosomes and
crossing over, followed by two divisions, which reults in four haploid cells.
•Explain non-disjunction, and how it leads to Down’s Syndrome
•State that karyotyping , chromosomes are arranged in pairs according to their size
and structure
•State that karyotyping is performed using cells collected by chorionic villus
sampling or amnioccentesis, for prenatal diagnosis of chromosome abnormalities.
•Analyze a human karyotype to determine gender and whether non-disjunction
occurred.
http://www.youtube.com/watch?v=lJzZ7p-47P8&feature=related