Genes and Chromosomes
I. Genes
● A gene is a heritable factor that consists of a length of DNA and influences a specific
characteristic.
How can we link genes, chromosomes and DNA together?
● Gene consists of a length of DNA.
● Chromosomes carry many genes.
● So, chromosomes have much longer lengths of DNA.
Does complexity in structure, physiology and behaviour determine the number of genes a
species has?
● No.
Location of Genes
What do you call the specific position a gene occupies?
● Locus of the gene
How many positions can a gene occupy?
● Each gene occupies a specific position on one type of chromosome.
● Genes are linked in groups and each group corresponds to one of the types of
chromosome in a species.
Alleles
What are alleles?
● Various specific forms of a gene
● Different heritable factors
What similarities do alleles have?
● Have the same locus
Can 2 alleles be present in one organism?
● Yes.
● Most plant and animal cells have 2 copies of each chromosome.
How do alleles differ from each other?
● Slight variations in the base sequence
● One or a few bases differ
Differences in Base Sequence of Gene
What can the results of comparisons of gene sequences be used for?
● Determine evolutionary relationships
● Can lead to identification of conserved sequences and allows species to be chosen for
exploring the function of that sequence.
,Mutations
How are new alleles formed from other alleles?
● Gene mutation
What are mutations?
● Random changes in base sequence
What is the most significant type of mutation?
● Base substitution
● One base in the sequence is replaced by a different base.
Are random changes to an allele likely to be beneficial?
● No.
● Almost all mutations are neutral or harmful.
● Some mutations are lethal and cause the death of the cell in which the mutation occurs.
Can mutations be eliminated?
● Mutations in body cells are eliminated when the individual dies.
● Mutations in cells that develop into gametes can be passed onto offspring and cause
genetic disease.
Sickle Cell Anaemia
Caused by base substitution mutation
What is the cause of sickle cell anaemia?
● It is caused by a mutation of the gene that codes for alpha-globin polypeptide in
haemoglobin.
● Base substitution mutation that converts the 6th codon of the gene from GAG to GTG.
● People without the disease have the allele HbA.
● People with the dease have the allele HbS.
What happens when the HbS allele is transcribed?
● The mRNA produced has GUG as its sixth codon instead of GAG
What happens when this mRNA is transcribed?
● The sixth amino acid in the polypeptide is valine instead of glutamic acid.
What are the physical effects of the mutation?
● Haemoglobin molecules stick together in tissues with low oxygen concentration.
● Bundles of haemoglobin molecules are rigid enough to distort the RBCs into a sickle
shape.
● Sickle cells cause damage to tissues by becoming trapped in blood capillaries, blocking
them and reducing blood flow.
● Both the haemoglobin and plasma membrane become damaged.
● The lifespan of a RBC can be shortened to as little as 4 days.
● The body cannot replace RBCs at a rapid encough rate and anaemia develops.
What happens when sickle cells return to high oxygen conditions in the lung?
● The haemoglobin bundles break up and the cells return to their normal shape.
How can the mutation be inherited?
● Only inherited by offspring if it occurs in a cell of the ovary or testis that develops into an
egg or sperm.
,Where is the HbS allele commonly found in?
● Places with high levels of malaria
● The ideal situation is to have one copy so that both normal haemoglobin and the mutant
form can be made.
Genome
How can you define a genome?
● The whole of the genetic information of an organism contained in DNA.
What constitutes a living organism’s genome?
● The entire base sequence of each of its DNA molecules.
What is the genome in humans?
1. 46 molecules that form the chromosomes in the nucleus and
2. DNA molecule in the mitochondrion
What is the genome in plants?
1. DNA molecules of chromosomes in the nucleus and
2. DNA molecules in the mitochondrion and the chloroplast
What is the genome in prokaryotes?
1. DNA in the circular chromosome and
2. Any plasmids that are present
Human Genome Project
What was the aim of the Human Genome Project?
● To find the base sequence of the entire human genome
What discoveries have been made?
● Most of the genome is not transcribed
● Highly repetitive sequences called satellite DNA
How has knowledge of the entire base sequence been used?
● As a ruch mine of data that researchers can use
● Can predict which base sequences are protein-coding genes
● Identify base sequences that give us genetic unity
● Identify single nucleotide polymorphisms which contribute to human diversity
● Determine base sequences of other species and allow comparisons to reveal previously
unknown aspects of evolutionary history
II. Chromosomes
Bacterial Chromosomes
How would you describe bacterial chromosomes?
● Prokaryotes have one chromosome consisting of a circular DNA molecule.
● The DNA is naked as it is not associated with proteins.
, How many copies of each gene are present?
● Usually one because only one chromosome is present.
● Two identical copies are present briefly after the chromosome has been replicated for
cell division.
What are plasmids?
● Plasmids are small, circular, naked extra DNA molecules that are commonly found in
prokaryotes but not in eukaryotes.
What do plasmids contain?
● They contain a few genes that may be useful to the cell but not those needed for its
basic life processes, e.g. genes for antibiotic resistance that are only beneficial in the
presence of antibiotics.
Are plasmids replicated at the same time or rate as chromosomes of a prokaryotic cell?
● No.
● Hence there may be multiple copies of plasmids and plasmids maynot be passed to
both cells formed by cell division.
What role do plasmids play in gene transfer?
● Copies of plasmids can be transferred to another cell, allowing spread through the
population.
● Plasmids can cross the species barrier when a plasmid that is released when a
prokaryotic cell dies is absorbed by a cell of a different species.
● Natural and artificial methods of gene transfer.
Audiography
Used to establish the legh of DNA molecules in chromosomes.
What was autoradiography used for by biologists?
● To discover where specific substances were located in cells or tissue
What was the technique used to produce images of DNA molecules from E. coli?
1. Cells were grown for 2 generations in a culture medium containing tri/ti/ated the/mid/ine.
2. Thymidine consists of thymine linked to deoxyribose. It is used by E. coli to make
nucleotides for DNA replication.
3. Tritiated thymidine contains tritium, a radioactive isotope of hydrogen.
4. E. coli produced radioactively labelled DNA via replication.
5. The cells were placed onto a dialysis membrane.
6. Their cell walls were digested using lysozyme, an enzyme.
7. The cells were gently burst to release their DNA onto the surface of the membrane.
8. A thin film of photographic emulsion was applied to the surface of the membrane and left
in darkness for 2 months.
9. During that time, some of the tritium atoms in the DNA decayed and emitted high energy
electrons.
10. The electrons reacted with the film.
11. At the end of the 2 months, the film was developed and examined with a microscope.
12. The dark grains were caused by the decaying of the tritium atoms and indicated the
position of the DNA.
I. Genes
● A gene is a heritable factor that consists of a length of DNA and influences a specific
characteristic.
How can we link genes, chromosomes and DNA together?
● Gene consists of a length of DNA.
● Chromosomes carry many genes.
● So, chromosomes have much longer lengths of DNA.
Does complexity in structure, physiology and behaviour determine the number of genes a
species has?
● No.
Location of Genes
What do you call the specific position a gene occupies?
● Locus of the gene
How many positions can a gene occupy?
● Each gene occupies a specific position on one type of chromosome.
● Genes are linked in groups and each group corresponds to one of the types of
chromosome in a species.
Alleles
What are alleles?
● Various specific forms of a gene
● Different heritable factors
What similarities do alleles have?
● Have the same locus
Can 2 alleles be present in one organism?
● Yes.
● Most plant and animal cells have 2 copies of each chromosome.
How do alleles differ from each other?
● Slight variations in the base sequence
● One or a few bases differ
Differences in Base Sequence of Gene
What can the results of comparisons of gene sequences be used for?
● Determine evolutionary relationships
● Can lead to identification of conserved sequences and allows species to be chosen for
exploring the function of that sequence.
,Mutations
How are new alleles formed from other alleles?
● Gene mutation
What are mutations?
● Random changes in base sequence
What is the most significant type of mutation?
● Base substitution
● One base in the sequence is replaced by a different base.
Are random changes to an allele likely to be beneficial?
● No.
● Almost all mutations are neutral or harmful.
● Some mutations are lethal and cause the death of the cell in which the mutation occurs.
Can mutations be eliminated?
● Mutations in body cells are eliminated when the individual dies.
● Mutations in cells that develop into gametes can be passed onto offspring and cause
genetic disease.
Sickle Cell Anaemia
Caused by base substitution mutation
What is the cause of sickle cell anaemia?
● It is caused by a mutation of the gene that codes for alpha-globin polypeptide in
haemoglobin.
● Base substitution mutation that converts the 6th codon of the gene from GAG to GTG.
● People without the disease have the allele HbA.
● People with the dease have the allele HbS.
What happens when the HbS allele is transcribed?
● The mRNA produced has GUG as its sixth codon instead of GAG
What happens when this mRNA is transcribed?
● The sixth amino acid in the polypeptide is valine instead of glutamic acid.
What are the physical effects of the mutation?
● Haemoglobin molecules stick together in tissues with low oxygen concentration.
● Bundles of haemoglobin molecules are rigid enough to distort the RBCs into a sickle
shape.
● Sickle cells cause damage to tissues by becoming trapped in blood capillaries, blocking
them and reducing blood flow.
● Both the haemoglobin and plasma membrane become damaged.
● The lifespan of a RBC can be shortened to as little as 4 days.
● The body cannot replace RBCs at a rapid encough rate and anaemia develops.
What happens when sickle cells return to high oxygen conditions in the lung?
● The haemoglobin bundles break up and the cells return to their normal shape.
How can the mutation be inherited?
● Only inherited by offspring if it occurs in a cell of the ovary or testis that develops into an
egg or sperm.
,Where is the HbS allele commonly found in?
● Places with high levels of malaria
● The ideal situation is to have one copy so that both normal haemoglobin and the mutant
form can be made.
Genome
How can you define a genome?
● The whole of the genetic information of an organism contained in DNA.
What constitutes a living organism’s genome?
● The entire base sequence of each of its DNA molecules.
What is the genome in humans?
1. 46 molecules that form the chromosomes in the nucleus and
2. DNA molecule in the mitochondrion
What is the genome in plants?
1. DNA molecules of chromosomes in the nucleus and
2. DNA molecules in the mitochondrion and the chloroplast
What is the genome in prokaryotes?
1. DNA in the circular chromosome and
2. Any plasmids that are present
Human Genome Project
What was the aim of the Human Genome Project?
● To find the base sequence of the entire human genome
What discoveries have been made?
● Most of the genome is not transcribed
● Highly repetitive sequences called satellite DNA
How has knowledge of the entire base sequence been used?
● As a ruch mine of data that researchers can use
● Can predict which base sequences are protein-coding genes
● Identify base sequences that give us genetic unity
● Identify single nucleotide polymorphisms which contribute to human diversity
● Determine base sequences of other species and allow comparisons to reveal previously
unknown aspects of evolutionary history
II. Chromosomes
Bacterial Chromosomes
How would you describe bacterial chromosomes?
● Prokaryotes have one chromosome consisting of a circular DNA molecule.
● The DNA is naked as it is not associated with proteins.
, How many copies of each gene are present?
● Usually one because only one chromosome is present.
● Two identical copies are present briefly after the chromosome has been replicated for
cell division.
What are plasmids?
● Plasmids are small, circular, naked extra DNA molecules that are commonly found in
prokaryotes but not in eukaryotes.
What do plasmids contain?
● They contain a few genes that may be useful to the cell but not those needed for its
basic life processes, e.g. genes for antibiotic resistance that are only beneficial in the
presence of antibiotics.
Are plasmids replicated at the same time or rate as chromosomes of a prokaryotic cell?
● No.
● Hence there may be multiple copies of plasmids and plasmids maynot be passed to
both cells formed by cell division.
What role do plasmids play in gene transfer?
● Copies of plasmids can be transferred to another cell, allowing spread through the
population.
● Plasmids can cross the species barrier when a plasmid that is released when a
prokaryotic cell dies is absorbed by a cell of a different species.
● Natural and artificial methods of gene transfer.
Audiography
Used to establish the legh of DNA molecules in chromosomes.
What was autoradiography used for by biologists?
● To discover where specific substances were located in cells or tissue
What was the technique used to produce images of DNA molecules from E. coli?
1. Cells were grown for 2 generations in a culture medium containing tri/ti/ated the/mid/ine.
2. Thymidine consists of thymine linked to deoxyribose. It is used by E. coli to make
nucleotides for DNA replication.
3. Tritiated thymidine contains tritium, a radioactive isotope of hydrogen.
4. E. coli produced radioactively labelled DNA via replication.
5. The cells were placed onto a dialysis membrane.
6. Their cell walls were digested using lysozyme, an enzyme.
7. The cells were gently burst to release their DNA onto the surface of the membrane.
8. A thin film of photographic emulsion was applied to the surface of the membrane and left
in darkness for 2 months.
9. During that time, some of the tritium atoms in the DNA decayed and emitted high energy
electrons.
10. The electrons reacted with the film.
11. At the end of the 2 months, the film was developed and examined with a microscope.
12. The dark grains were caused by the decaying of the tritium atoms and indicated the
position of the DNA.
