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Summary IB Biology Genetics
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IB Biology topic 3 and topic 10 full summary notes. Everything that you need to know for these topics.
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3.1 GENESGENES
- A gene is a heritable factor that consists of a length of DNA and influences a specific
characteristic - can be passed on to offspring
- A gene codes for a polypeptide
- Each chromosome is a very long DNA molecule which has thousands of genes
- A gene occupies a specific position on one type of chromosome - gene for a particular
characteristic is always found at the same locus (position) on a particular chromosome
—> Through experiments and genetic mapping techniques, physical locations of a gene on a
chromosome can be found out
COMPARING NUMBERS OF GENES
- Species vary in the number of genes
- This is not related to the size/complexity of the organism
- Precise counts of gene numbers are not known as they are hard to determine
- Do not confuse with genome size - genome size is the total amount of DNA - most of the
genome does not code for polypeptides
ALLELES
- Various specific forms of a gene (heritable factors) are alleles
- Alleles occupy the same locus on one type of chromosome as they are alternative forms of the
same gene
- As chromosomes of eukaryotic cells occur in homologous pairs, 2 copies of each chromosome,
we have 2 copies of a gene - these could be 2 same alleles or 2 different alleles
- When the two alleles at a locus are:
—> The same = homozygous
—> Are different = heterozygous
DIFFERENCES BETWEEN ALLELES
- Alleles differ from each other by one or a few bases only
- Positions in a gene where more than one base may be present are called single nucleotide
polymorphisms - is where bases differ between alleles
—> Human genome project allowed gene sequences to be compared - can determine
evolutionary relationships
, MUTATION
- A mutation is a random change in the sequence of base pairs in a DNA molecule - can form new
alleles
- There are certain points in the cell cycle when mutations are more likely, e.g. S phase
- Can change the polypeptide coded
- Mostly are harmful or neutral
- Mutations in normal body cells are not inherited, they are removed from the population when
those cells die
- Mutations in gametes are inherited by offspring —> can cause genetic disease
- Most significant mutation is base substitution - base in DNA sequence is randomly swapped for a
different base
SICKLE CELL ANAEMIA
- Caused by a single base substitution mutation in gene Hb - this gene normally codes for alpha-
globin polypeptide in haemoglobin
—> Most humans have normal allele: Hbᴬ
—> Different polypeptide formed results in a new allele: Hb ˢ
- Thymine is replaced by adenine - DNA triplet GAG —> GTG
- mRNA is transcribed into codon GUG instead of GAG
- GUG translates into amino acid valine, instead of original amino acid glutamic acid - this occurs
on the sixth position of the polypeptide
- This produces the protein haemoglobin S instead of haemoglobin A
- Causes haemoglobin to stick together in tissues with low oxygen concentrations - bundles
formed are rigid enough to distort the red blood cells into a sickle shape
—> Limited oxygen-carrying capacity
—> Block capillaries - limiting flow of normal red blood cells
- When sickle cells return to high oxygen concentration tissues in the lung, the haemoglobin
bundles break up and the cells return back to their original shape
- Haemolgobin and plasma membrane of the cell is damaged - shortening their life
- Body can not replace red blood cells quickly - developing anaemia
- In areas with higher malaria cases, there is an increased frequency of sickle alleles
- Mutation is only inherited by offspring if it occurs in a gamete cell
GENOME
- Genome is the whole of the genetic information (DNA) of an organism - entire base sequence of
each of its DNA molecules
- Mitochondrial and chloroplast DNA is also included and also plasmids
COMPARING GENOME SIZE
- Technology advances allowed us to write the whole genome
- Virus and bacteria = very small genomes
- Prokaryotes generally have smaller genomes than eukaryotes
- Plant genomes vary widely
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