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Summary document of Chapter 5 Evolution and biodiversity IB Biology
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Genetics3.1 genes
Essential idea: Genes contain the information of
Every living organism inherits a blueprint for life from an individual, and determine its
its parents. characteristics.
Genes and genetic information is inherited from parents, but the combination of genes inherited
by each offspring will be different. In sexual reproduction, each parent can only pass on 50%
of their genes as the other 50% comes from the second parent.
The number of genes doesn’t necessarily depend on the amount of DNA because not all DNA
codes for proteins. Complexity does not only depend on the number of genes but on non-coding
regions of DNA which contain introns, and are regulatory regions (depending on how regulated
our genes’ transcription is). We have fewer genes than plants but more regulatory regions.
By coiling DNA, we can store a much larger quantity of gene information. It is stored in the
nucleus of the cell.
Vocabulary
The genome: the complete DNA ‘blueprint’ (info needed to construct a house) for an organism
Genes: a heritable factor that consists of a part of DNA and influences a specific
characteristic.
Mutations: changes in the sequence of DNA
Alleles: different forms of the same gene (same locus of the gene in homologous chromosomes).
These only differ on a few bases and that change already produces a whole different protein
(f. ex. Brown instead of blue eyes). The DNA sequence tends to be very similar, differing only
by a few bases.
Þ Example: Mother: blue eyes (chromosome 11 = blue);
Father: brown eyes (chromosome 11 = brown)
, We can have 46 types of alleles but you can only carry 2 (or 1 if you inherited the
same allele from both your parents).
Blueprint: genes that we carry that we inherit from our parents
Chromosomes: made of DNA, the place where genes are located
Gene locus: specific location of genes on a chromosome. All individuals of a species carry the
same genes at the same loci on the chromosomes
Genes are heritable factors that control/influence specific characteristics (the environment
affect them too), consisting of a length of DNA occupying a particular position on a
chromosome.
Þ Several genes can influence 1 characteristic
Þ 1 gene can influence several characteristics
Chromosomes
Chromosomes are structural units that are made up of DNA and proteins.
Organisms store their genes on long stretches of DNA called chromosomes.
By coiling the DNA around the proteins many times, a large amount of
information can be stored inside the nucleus. A single chromosome may have
hundreds or thousands of genes, each in a specific locus.
All members of a species have the same number of chromosomes, each with the same genes in
the same gene loci. The number of chromosomes varies within species.
The different, specific forms of a gene are called alleles, and they often vary from each other
by only one or a few bases.
• Having a pair of each kind of chromosome is Hierarchy of genetic information:
called the diploid state; having one copy of each • Genome: consists of the entire
chromosome is the haploid state. Humans are genetic material of an organism.
• Chromosomes: long DNA molecules,
diploid and inherit one copy of each chromosome
which contain many genes.
maternally and the other paternally. As a result,
• Genes: short stretches of DNA,
humans have two copies of every gene.
which usually code for one
• You are homozygous for a gene if you have two
characteristic.
copies of the same allele; you are heterozygous • Allele: a specific form of a gene.
if you have two different alleles.
• The alleles you have for a gene is called your genotype.
• The trait you have as a result of your alleles is called your phenotype.
,The human genome project
It was a 13-year project (1990-2003) to sequence (know
the order of the letters) all the genes from the human genome.
Exam tip
You should be able to compare the
It was possible thanks to the existence of PCR and other
number of genes in humans with:
technological advancements. • A named species with more
Key advances in technology: genes than humans
• Biotechnology techniques such as PCR are used to • A named species with fewer
genes than humans
prepare samples: the DNA needs to be copied to prepare
• A named bacterial species
sufficiently large pure samples to sequence
• A named plant species
• Computers automate the sequencing process
• Fluorescent labeling techniques enable all four nucleotides
to be analyzed together
• Lasers are used to fluoresce the dye markers
• Digital camera technology reads the dye markers
• Computers are used to assemble the base sequence
Knowing which alleles a patient has for key genes tells
doctors which medicines are likely to be effective. It is
possible that affordable genetic sequencing will begin an era
of ‘personalized medicine’.
Gene mutations
When DNA is replicated, sometimes errors are
made. The daughter cells then contain DNA that
differs from the DNA in the parental cell. These
changes in DNA sequence are called mutations. All
new alleles are created by mutation. However, once
a new allele exists it is copied and passed on in the
same manner as the original sequence.
Mutations can cause bases to be deleted from or added to the DNA sequence; or cause a
different base to be placed, known as base-substitution mutation.
Gene mutations are permanent changes in the sequence of DNA. Some give you different
characteristics (like 2-color eyes), and they can result in no change (silent mutation, when
there is a different codon but it produces the same amino acid) or result in an evolutionary
, advantage. Most of the time polymerase corrects these errors but f. ex. radiation or some
chemicals, known as mutagens (like nicotine) make polymerase able to correct them less.
Ultimately, this can result in cancer (if the mutagens cause tumorigenesis, then they are also
known as carcinogens).
• One-point mutation: it happens when only one amino acid changes.
o Missense mutation: different base, different aminoacid.
o Silent mutation: different base, same amino acid.
o Nonsense mutation: different base = stop codon. Different polypeptide, different
structure.
These mutations can be inherited (arise in your parent sexual cells, so all cells will have the
mutation) or arise in one of your cells.
Evidence suggests that all life on Earth shares a common ancestor. From that original species,
every difference between each living thing on Earth has arisen by mutation.
Sickle cell anemia
A single base substitution leads to the production of valine instead of
glutamine (glutamic acid). This leads to the production of abnormal red
blood cells. The gene involved is a stretch of DNA on chromosome 11 called
HBB. It codes for the beta subunit of hemoglobin, a polypeptide 146 amino
The DNA strand that is not acids long. The standard Hb A allele
transcribed is the sense strand and reads GAG at the 6th triplet of the
has the same sequence as the mRNA
sense of the DNA strand. The Hb S
molecule (except T/U).
The transcribed strand is known as allele reads GTG in the same location. Otherwise, the alleles
the antisense strand and is are identical.
complementary to the mRNA molecule.
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