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

TOPIC 4 - GENETICS 4.1 CHROMOSOMES, ALLELES, GENES & MUTATIONS 4.4 4.2 MEIOSIS 4.3 THEORETICAL GENETICS 4.4 GENETIC ENGINEERING & BIOTECHNOLOGY J WERBA – IB BIOLOGY 2

THINGS TO COVER Composition of eukaryotic chromosomes Definitions: Genes Alleles Genome Gene mutation Genetic mutations Sickle cell anaemia J WERBA – IB BIOLOGY 3

COMPARE PROKARYOTES and EUKARYOTES 2.2.4 COMPARE PROKARYOTES and EUKARYOTES J WERBA – IB BIOLOGY 4

COMPARE CHROMOSOMES: PROKARYOTIC & EUKARYOTIC 4.1.1 COMPARE CHROMOSOMES: PROKARYOTIC & EUKARYOTIC FEATURE PROKARYOTIC EUKARYOTIC Shape Circular Linear Contains DNA DNA & proteins Location Cytoplasm Nucleus J WERBA – IB BIOLOGY 5

EUKARYOTIC CHROMOSOMES Command term = STATE 4.1.1 EUKARYOTIC CHROMOSOMES Command term = STATE Eukaryotic chromosomes are composed of DNA and proteins J WERBA – IB BIOLOGY 6

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 J WERBA – IB BIOLOGY 7

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. J WERBA – IB BIOLOGY 8

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 J WERBA – IB BIOLOGY 9

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). J WERBA – IB BIOLOGY 10

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 J WERBA – IB BIOLOGY 11

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. J WERBA – IB BIOLOGY 12

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. J WERBA – IB BIOLOGY 13

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 J WERBA – IB BIOLOGY 14

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 J WERBA – IB BIOLOGY 15

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. J WERBA – IB BIOLOGY 16

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. J WERBA – IB BIOLOGY 17

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 J WERBA – IB BIOLOGY 19

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. J WERBA – IB BIOLOGY 20

Sample questions A1. D A2. C J WERBA – IB BIOLOGY 21

Sample questions Q3. Explain the causes and consequences of sickle-cell anemia. (5 marks) J WERBA – IB BIOLOGY 22

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; J WERBA – IB BIOLOGY 23