Detailed objective-by-objective summary notes for Topic 3: Genetics for IB Biology SL/HL. Structured by Chapters (3.1 to 3.5) and categorised under each "Application", "Nature of Science" and "Understanding" statement given by IBO. Written by an IB HL Biology student who graduated with a 45/45.
IB Biology Full Course Notes + Required Drawings Units 1 - 11 & Option D
IB Human Physiology Option D - Biology HL & SL Notes for IBDP
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Unit 3: Genetics
3.1 Genes
∑ A gene is a heritable factor that consists of a length of DNA and influences a
specific characteristic
∑ A gene occupies a specific position on one type of chromosome
Genes are linked in groups that correspond to chromosomes. Position of
genes in a chromosome is called the locus (p region is short arm; q region is
longer arm of chromosome)
∑ The various specific forms of a gene are alleles
∑ Alleles differ from each other by one or a few bases only
Alleles occupy the same locus and only one allele can occupy the locus of a
gene on one chromosome.
Diploid organisms have two alleles which may be the same or different
Alleles have slight variations in base sequence since they encode similar
function
Positions in a gene where more than one base may be present are called
single nucleotide polymorphisms, abbreviated to SNPs and pronounced
‘snips’. Several snips can be present in a gene
∑ New alleles are formed by mutation
Mutations are random changes in base sequence during DNA replication due
to errors.
Mutation types: base substitution (point mutation); insertion; deletion etc.
Mutation effects:
o Silent mutation (same amino acid due to degenerate codon produced);
o Missense mutation (change in one amino acid);
o Nonsense mutation (stop codon produced, polypeptide shortened)
Mutation in cells that develop into gametes may be passed on to offspring
causing genetic diseases.
Mutations to alleles that have developed by evolution over millions of years
are unlikely to be beneficial. Most mutations are thus neutral or harmful
(diseases)
Mutations may affect rate or even ability/ type of reaction catalysed by
enzyme
Example: a few point mutations may render a person unable to taste PTC
(phenylthiocarbamide)
*reading frame of gene is not altered in point mutation
, Unit 3: Genetics
∑ The genome is the whole of the genetic information of an organism
Genome is the set of all genes, regulatory sequences, non-coding information
within organism’s DNA
o In humans genome consists of 46 molecules forming chromosomes in
addition to mitochondrial DNA
o In plant species, genome is DNA molecules in chromosomes,
mitochondrion and chloroplasts
o Prokaryote genome is much smaller, consisting of circular DNA and
plasmids
Genome will define species, and also include small subtle differences
between individuals within species
∑ The entire base sequence of human genes was sequenced in the Human
Genome Project
General facts:
o Publicly funded and a 13-year collaborative worldwide effort (1990-
2003)
o Was to have all the sequencing data freely available to everyone
Effects of the human genome project which found the base sequence of the
entire human genome is:
o Technological advancements and discoveries for cheaper genome
sequencing
o Possible to predict which base sequences are protein coding genes
(about 23,000)
o Discover that most of the genome is not transcribed. Regions include
elements that affect gene expression and highly repetitive sequences
called satellite DNA
o Allowed better understanding of evolutionary relationships
o Given rise to bioinformatics and genomics, allowing early diagnosis,
getting information about potential future illnesses etc.
, Unit 3: Genetics
ß Applications- The causes of sickle cell anemia, including base substitution
mutation, a change to the base sequence of mRNA transcribed from it and a
change to the sequence of a polypeptide in haemoglobin
Mutation of the gene that codes for alpha-globin polypeptide in haemoglobin.
The mutation is inherited if it occurs in a cell that develops to gamete cells
Mutation
o DNA: Base substitution converts 6 th codon of beta globin gene from
GAG to GTG, changing the allele from HbA to HbS.
o mRNA: 6th codon GAG to GUG
o Polypeptide: glutamic acid (polar) becomes valine (non-polar) instead
Hence, a small change in primary structure of protein can heavily affect
overall shape and structure of protein and affect its function
Effects
o Haemoglobin aggregate in tissues with low oxygen concentrations, and
bundles formed may distort red blood cells into a sickle shape
o These sickle cells damage the tissues by becoming trapped in blood
capillaries, blocking them and reducing blood flow
o When returning to high oxygen concentrations in lungs, haemoglobin
bundles break up and cells return to normal shape
o These changes occur again and again as RBC circulates, damaging
haemoglobin and plasma membrane, shortening RBC lifespan
o The body cannot replace RBC fast enough, leading to anaemia
Mild anaemia heterozygous for allele (mild symptoms, usually healthy)
Severe anaemia homozygous for HbS. allele
Benefits
o HbS. allele confers malaria resistance. This is because plasmodium
cannt live n RBCs with HbS. allele
o Hence, in malaria stricken places, heterozygous carriers are at the
advantage as they do not experience severe anaemic symptoms, while
having malarial resistance
Treatment for severe sickle cell disease: stem cell therapy
ß Applications- Comparison of the number of genes in humans
More genes does not indicate complexity. Homo sapiens have about 23,000
genes, which is less compared to most plants and some unicellular
organisms. Water fleas are an example of an organism with more genes than
humans.
o Oryza sativa (rice): 41,000 genes
o E. Coli: 3200 genes
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