TOPIC 3: GENETICS
3.1 Genes
DNA is the genetic blueprint which codes for & determines, the characteristics of an organism
o DNA is packaged into structures called chromosomes, with each somatic cell containing
46 chromosomes (each cell contains entire genome)
Gene: A heritable factor that consists of a length of DNA and influences a specific characteristic
(codes for a certain protein)
o A gene occupies a specific position on a chromosome which is known as the locus
Allele: One specific form of a gene, differing from other alleles by one or a few bases only and
occupying the same gene locus as other alleles of the gene
o For example, the gene for eye colour has alleles that encode for different pigments
Gene Mutation: a change in the nucleotide sequence of a section of DNA coding for a specific
trait. They can be beneficial, detrimental or neutral
o Silent mutations - when the DNA change does not alter the amino acid sequence of the
polypeptide
This is possible because different codons may code for the same amino acid
o Missense mutations – when the DNA change alters a single amino acid in the
polypeptide chain
Sickle Cell Anaemia is an example of this as GAG (Glutamic Acid) GTG (Valine)
as the sixth amino acid in the haemoglobin polypeptide
o Nonsense mutations – when the DNA change creates a premature STOP codon which
shortens the polypeptide
Sickle Cell Anaemia is an example of a disorder caused by a gene mutation where a single base
was changed in the gene sequence (missense mutation)
o The sixth amino acid in the haemoglobin polypeptide is changed from GAG (Glutamic
Acid) GTG (Valine)
o This alters the structure of haemoglobin (sickle shaped) which reduces the cells ability to
carry oxygen causing the individual to feel constantly tired
Genome: the whole of the genetic information of an organism
o This includes all genes as well as non-coding DNA sequences (introns)
o This size of the genome is the total amount of DNA in 1 set of chromosomes in a species
The entire base sequence of human genes was sequenced in the Human Genome Project
o This was an international cooperative venture lasting from 1990 – 2003
o The completion of the project led to many valuable outcomes
Mapping – The number, location, size & sequence of human genes is known
Medicine – Discovery of new proteins have led to improved treatments
Screening – Allowed for the production of specific gene probes to detect
sufferers and carriers of genetic diseases
Ancestry – Comparisons with other genomes have provided insight into the
origins, evolution and migratory patterns of man
o However, ethical issues are also raised such as whether it is ethical to screen babies for
genetic disorders before birth
26
, The number of genes present in
an organism differs between
species & is NOT an indicator of
complexity
o The number of genes in
a genome is usually
predicted, therefore
final estimates can vary significantly
Databases such as the Genbank® database can be used to determine differences in the base
sequence of a gene in two species.
o The cytochrome C gene sequence is available for many different organisms and is of
interest because of its use in reclassifying organisms into three domains
o The database can show the level of similarity of a gene between two species
3.2 Chromosomes
Prokaryotes and eukaryotes have significant differences in their DNA
Prokaryotes Eukaryotes
Consist of a circular DNA molecule Contain a linear DNA molecule
Naked DNA – not associated to proteins (histones) Associated with histone proteins
Plasmids often present No plasmids
o Plasmids are smaller extra loops of DNA that Contain two or more different chromosomes that
contain only a few non-essential genes e.g. each carry different genes
could contain genes for antibiotic resistance
Consists only of a single chromosome
Eukaryotic chromosomes are linear molecules of DNA that are compacted during cell division
(mitosis or meiosis)
o Each chromosome has a constriction point called the centromere, which divides the
chromosome into two sections
The shorter section is referred to as the p arm and the longer one is the q arm
o Eukaryotic species possess multiple chromosomes that may differ in both their size and
the position of their centromere
Each chromosome will carry specific genes and the position of a particular gene
on a chromosome is called the locus
The region of a locus can be identified via 3 points – the first number refers to
the chromosome number, the second is the arm it is on (p or q) and the third
corresponds to the band location (larger number = further from centromere)
E.g. 7q31.2
The locus of a human gene and its polypeptide product can both be identified using a database
Homologous chromosomes carry the same sequence of genes but not necessarily the same
alleles of those genes
o In a normal human body cell, there are 23 pairs of homologous chromosomes
o As all humans have the same number of chromosomes this allows interbreeding
27
3.1 Genes
DNA is the genetic blueprint which codes for & determines, the characteristics of an organism
o DNA is packaged into structures called chromosomes, with each somatic cell containing
46 chromosomes (each cell contains entire genome)
Gene: A heritable factor that consists of a length of DNA and influences a specific characteristic
(codes for a certain protein)
o A gene occupies a specific position on a chromosome which is known as the locus
Allele: One specific form of a gene, differing from other alleles by one or a few bases only and
occupying the same gene locus as other alleles of the gene
o For example, the gene for eye colour has alleles that encode for different pigments
Gene Mutation: a change in the nucleotide sequence of a section of DNA coding for a specific
trait. They can be beneficial, detrimental or neutral
o Silent mutations - when the DNA change does not alter the amino acid sequence of the
polypeptide
This is possible because different codons may code for the same amino acid
o Missense mutations – when the DNA change alters a single amino acid in the
polypeptide chain
Sickle Cell Anaemia is an example of this as GAG (Glutamic Acid) GTG (Valine)
as the sixth amino acid in the haemoglobin polypeptide
o Nonsense mutations – when the DNA change creates a premature STOP codon which
shortens the polypeptide
Sickle Cell Anaemia is an example of a disorder caused by a gene mutation where a single base
was changed in the gene sequence (missense mutation)
o The sixth amino acid in the haemoglobin polypeptide is changed from GAG (Glutamic
Acid) GTG (Valine)
o This alters the structure of haemoglobin (sickle shaped) which reduces the cells ability to
carry oxygen causing the individual to feel constantly tired
Genome: the whole of the genetic information of an organism
o This includes all genes as well as non-coding DNA sequences (introns)
o This size of the genome is the total amount of DNA in 1 set of chromosomes in a species
The entire base sequence of human genes was sequenced in the Human Genome Project
o This was an international cooperative venture lasting from 1990 – 2003
o The completion of the project led to many valuable outcomes
Mapping – The number, location, size & sequence of human genes is known
Medicine – Discovery of new proteins have led to improved treatments
Screening – Allowed for the production of specific gene probes to detect
sufferers and carriers of genetic diseases
Ancestry – Comparisons with other genomes have provided insight into the
origins, evolution and migratory patterns of man
o However, ethical issues are also raised such as whether it is ethical to screen babies for
genetic disorders before birth
26
, The number of genes present in
an organism differs between
species & is NOT an indicator of
complexity
o The number of genes in
a genome is usually
predicted, therefore
final estimates can vary significantly
Databases such as the Genbank® database can be used to determine differences in the base
sequence of a gene in two species.
o The cytochrome C gene sequence is available for many different organisms and is of
interest because of its use in reclassifying organisms into three domains
o The database can show the level of similarity of a gene between two species
3.2 Chromosomes
Prokaryotes and eukaryotes have significant differences in their DNA
Prokaryotes Eukaryotes
Consist of a circular DNA molecule Contain a linear DNA molecule
Naked DNA – not associated to proteins (histones) Associated with histone proteins
Plasmids often present No plasmids
o Plasmids are smaller extra loops of DNA that Contain two or more different chromosomes that
contain only a few non-essential genes e.g. each carry different genes
could contain genes for antibiotic resistance
Consists only of a single chromosome
Eukaryotic chromosomes are linear molecules of DNA that are compacted during cell division
(mitosis or meiosis)
o Each chromosome has a constriction point called the centromere, which divides the
chromosome into two sections
The shorter section is referred to as the p arm and the longer one is the q arm
o Eukaryotic species possess multiple chromosomes that may differ in both their size and
the position of their centromere
Each chromosome will carry specific genes and the position of a particular gene
on a chromosome is called the locus
The region of a locus can be identified via 3 points – the first number refers to
the chromosome number, the second is the arm it is on (p or q) and the third
corresponds to the band location (larger number = further from centromere)
E.g. 7q31.2
The locus of a human gene and its polypeptide product can both be identified using a database
Homologous chromosomes carry the same sequence of genes but not necessarily the same
alleles of those genes
o In a normal human body cell, there are 23 pairs of homologous chromosomes
o As all humans have the same number of chromosomes this allows interbreeding
27
