A revision document with short summaries, schemes, pictures and example questions made using several IB Biology Textbooks, class notes and online resources.
Genes
A gene is a heritable factor that consists of a length of DNA and influences a specific
characteristic.
The position of a gene on a chromosome is called the locus.
Alleles
Alleles are the alternate forms of a gene that code for the different variations of a specific trait.
They occupy the same position on one type of chromosome - they have the same locus.
Alleles differ from each other by one or a few bases only.
New alleles are formed by gene mutation. Mutations are random changes; the most significant
type of mutation is a base substitution.
,Sickle cell anemia
most common genetic disease in the world;
due to a mutation of the gene that codes for the alpha-globin polypeptide in hemoglobin;
base substitution converts the sixth codon of a gene from GAG to GTG;
when this allele is transcribed, the mRNA has GUG instead of GAG (as its sixth codon);
as a result the sixth amino acid in the polypeptide is valine instead of glutamic acid;
this causes hemoglobin molecules to stick together in tissues with low oxygen concentration
and hence the red blood cells attain sickle shape (they can cause damage by becoming
trapped in blood capillaries, blocking them, etc.);
when sickle cells return to high oxygen concentration (in the lungs), they return to the normal
shape.
Example question: Explain the causes of sickle-cell anaemia [8]
caused by gene mutation;
(sickle-cell anemia) due to a base substitution (mutation);
changes the code on the DNA;
which leads to a change in transcription / change in mRNA;
DNA changes from CTC to CAC/GAG to GTG / mRNA changes from GAG to GUG;
which (in turn) leads to a change in translation / change in polypeptide chain/protein;
(the tRNA) adds the wrong amino acid to the polypeptide chain;
glutamic acid replaced by valine;
produces abnormal hemoglobin;
causing abnormal red blood cell/erythrocyte shape / sickle shape;
which lowers the ability to transport oxygen;
sickle-cell allele is codominant;
homozygote/ Hb^S Hb^S have sickle cell anemia/is lethal / heterozygote/Hb^S Hb^A has
the sickle trait/is carrier (and is more resistant to malaria)
, Genome
The genome is the whole of the genetic information of an organism.
It includes all genes and non-coding sequences
Example question: Explain the consequences of altering a DNA base in the
genome of an organism [8]
altering a base (in DNA) is a mutation;
only has an effect if a base is in a gene;
when mRNA is produced by transcription one mRNA base is different;
one codon in mRNA is different;
one amino acid is different in the polypeptide;
polypeptide produced by translation of mRNA;
some base changes do not change the amino acid coded for;
structure of polypeptide/protein may be altered;
usually the polypeptide/protein does not function as well;
example given:
disease: sickle cell anemia;
mutation: GAG to GTG;
consequence in translation: glutamic acid to valine;
consequence for protein: hemoglobin altered so sickle cell formed;
consequence for individual: less oxygen can be carried;
The Human Genome Project
Aim was to find the base sequence of the entire human genome.
The project drove rapid improvements in base sequencing techniques and the whole
genome was sequenced earlier than expected.
Example question: Outline outcomes of the human genome project [4]
complete human DNA/chromosomes sequenced;
identification of all human genes / find position/map (all) human genes;
find/discover protein structures/functions;
find evidence for evolutionary relationships/human origins/ancestors;
find mutations/base substitutions/single nucleotide polymorphisms;
find genes causing/increasing chance of/develop test for/screen for diseases;
develop new drugs (based on base sequences) / new gene therapies;
tailor medication to individual genetic variation / pharmacogenomics;
promote international cooperation/global endeavours;
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