1. CHROMOSOMES, ALLELES, GENES & MUTATIONS
Topic 4.1
IB Biology
Miss Werba

2. TOPIC 4 - GENETICS 4.1 CHROMOSOMES, ALLELES, GENES & MUTATIONS 4.4 4.2
MEIOSIS
4.3
THEORETICAL GENETICS
4.4
GENETIC ENGINEERING & BIOTECHNOLOGY
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3. THINGS TO COVER Composition of eukaryotic chromosomes Definitions:
Genes
Alleles
Genome
Gene mutation
Genetic mutations
Sickle cell anaemia
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4. COMPARE PROKARYOTES and EUKARYOTES
2.2.4
COMPARE PROKARYOTES and EUKARYOTES
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5. COMPARE CHROMOSOMES: PROKARYOTIC & EUKARYOTIC
4.1.1
COMPARE CHROMOSOMES: PROKARYOTIC & EUKARYOTIC
FEATURE
PROKARYOTIC
EUKARYOTIC
Shape
Circular
Linear
Contains
DNA
DNA & proteins
Location
Cytoplasm
Nucleus
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6. EUKARYOTIC CHROMOSOMES Command term = STATE
4.1.1
EUKARYOTIC CHROMOSOMES Command term = STATE
Eukaryotic chromosomes are composed of DNA and proteins
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7. GENETIC DEFINITIONS Command term = DEFINE
4.1.2
GENETIC DEFINITIONS Command term = DEFINE
Gene: A segment of DNA that controls a characteristic
Genome: All of the genes carried by an organism
Allele: one specific form of a gene, occupying the same gene locus as other alleles of the gene.
Locus: Position of a gene on the chromosome
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8. GENES A gene is the basic unit of inheritance. It is a segment of DNA.
4.1.2
GENES
A gene is the basic unit of inheritance.
It is a segment of DNA.
It controls one specific characteristic.
It contains the code required to produce a protein.
The same gene will always have the same position on the same chromosome. This position is called its locus.
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9. ALLELES Alleles are different forms/versions of a gene.
4.1.2
ALLELES
Alleles are different forms/versions of a gene.
Each gene is composed of two alleles.
Alleles of a gene always occupy the same locus
These alleles may be the same or different.
Chromosome a B c D e f G A b C D E f G
Gene
Allele
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10. 4.1.2
ALLELES
The difference between alleles of the same gene may only be a few bases:
eg. AGTCACGGTACG = brown eyes allele (B)
eg. AGTACAGGTACG = blue eyes allele (b)
Your alleles are inherited from your parents.
Only identical twins have the exact same allele combinations, and there are still slight differences in their features (eg. mirroring).
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11. GENE MUTATIONS Command term = DEFINE
4.1.3
GENE MUTATIONS Command term = DEFINE
Gene mutation: A change in the nucleotide (base) sequence of a gene
These mutations can be:
Base substitutions: reading frame stays the same
one base is changed for another
Frame-shift mutations: reading frame changes
Insertions = insertion of one or more bases
Deletions = deletion of one or more bases
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12. GENE MUTATIONS Frame-shift mutations are more serious.
4.1.3
GENE MUTATIONS
Frame-shift mutations are more serious.
They result in changes to all of the codons (triplets of bases on the mRNA) following this mutation.
As the codons change, the amino acids that are coded for will be different.
The resulting protein will also be affected.
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13. SICKLE CELL ANAEMIA Haemoglobin is a protein
4.1.4
SICKLE CELL ANAEMIA
Haemoglobin is a protein
It is composed of 4 chains:
2 alpha (α) chains
2 beta (β) chains
Sickle cell anaemia is caused by a gene mutation in coding for a haemoglobin chain.
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14. 4.1.4
SICKLE CELL ANAEMIA
Gene mutation affects the beta (β) chain of the haemoglobin protein
Base substitution mutation in 6th amino acid in the beta chain
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15. SICKLE CELL ANAEMIA Changes the mRNA codon from GAG to GUG
4.1.4
SICKLE CELL ANAEMIA
Changes the mRNA codon from GAG to GUG
Amino acid changes from glutamic acid to valine
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16. SICKLE CELL ANAEMIA The resulting polypeptide (protein) is different:
4.1.4
SICKLE CELL ANAEMIA
The resulting polypeptide (protein) is different:
‘normal’ haemoglobin = HbA
‘mutant’ haemoglobin = HbS
HbS crystallises at low oxygen levels (eg. in the capillaries).
This causes the HbS blood cells to irreversibly change to a sickle shape.
The sickle-cell blood cells can block the capillaries and are less efficient at transporting oxygen.
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17. 4.1.4
SICKLE CELL ANAEMIA
Can result in acute anaemia, heart damage, kidney damage, or even death in homozygotes
The sickle-cell trait is more common among people from West-Africa and in the African-American population
Carrier-testing is advised for people in predisopsed families and amniocentesis can be performed to check for disorder in utero.
Correlation between malaria and SC anaemia – disease causing plasmodium cannot reproduce in RBCs with HbS, meaning that SC carriers have a reduced chance of contracting malaria.
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19. Sample questions
Q1. What is the difference between the alleles of a gene?
A. Their position on the chromosome
B. Their amino acid sequence
C. Their pentose sugars
D. Their base sequence
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20. Sample questions
Q2. Which of the following is the cause of sickle-cell anemia?
A. Tryptophan is replaced by leucine.
B. Leucine is replaced by valine.
C. Glutamic acid is replaced by valine.
D. Lysine is replaced by glutamic acid.
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21. Sample questions
A1. D
A2. C
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22. Sample questions
Q3. Explain the causes and consequences of sickle-cell anemia.
(5 marks)
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23. Sample questions A3. (point) mutation of gene for hemoglobin;
CTC to CAC / GAG to GTG / substitution of T/thymine with A/adenine;
mRNA copy of gene is GUG instead of GAG;
valine instead of glutamic acid;
(in homozygotes) red blood cells become sickle-shaped;
(in homozygotes) less oxygen carried;
(in homozygotes) red blood cells do not survive long / burst / block blood vessels / capillaries / circulatory problems may cause pain / organ failure / example of symptom;
heterozygotes have malaria resistance;
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