Understandings:
● A gene is a heritable factor that consists of a length of DNA and influences a specific characteristic. ● A gene occupies a specific position on a chromosome.
● The various specific forms of a gene are alleles.
● Alleles differ from each other by one or only a few bases.
● New alleles are formed by mutation.
● The genome is the whole of the genetic information of an organism.
● The entire base sequence of human genes was sequenced in the Human Genome Project.
NATURE OF SCIENCE
Developments in scientific research follow improvements in technology: gene sequencers are used for the sequencing of genes.
Applications and skills:
Guidance
- Genes
A gene is a heritable factor that consists of a length of DNA and influences a specific characteristic. ‘Heritable’ means passed on from parent to offspring, and ‘characteristic’ refers to genetic traits such as your hair colour or your blood type. The estimated 21 000 genes that you possess are organized into chromosomes.
- A gene is found at a particular locus on a chromosome
A gene for a specific trait occupies a corresponding place, called a locus (plural loci), on a chromosome
When geneticists map out the sequences of DNA, they carefully map the locus of each sequence. When further research reveals that a particular sequence controls a certain heritable factor, the locus of the gene is noted for further reference. For example, scientists now know that the locus of the gene controlling a protein called transducin that enables colour vision is found on chromosome 1. A mutation of this gene stops a person from being able to make the protein transducin properly, which is necessary to transmit information about colour from the eye to the brain; as a result, the person will not see in colour. This is an extremely rare genetic condition called complete achromatopsia. When we say ‘the ability to see in colour is a genetic trait’ we mean one of two things is happening with someone’s DNA: either that person has the DNA code for making colour vision possible or the person does not have it.
You will recall that you possess two copies of each gene in your body: one copy from your mother and one from your father. As a result, if you could look at the locus of the transducin gene on one of the two copies of your first chromosome, for example, you would find the same gene at the same locus on the other copy of chromosome 1. One copy would be the one your mother gave you and the other would be the copy your father gave you. Would those genes be identical? Not necessarily, because genes can come in different forms.
● A gene is a heritable factor that consists of a length of DNA and influences a specific characteristic. ● A gene occupies a specific position on a chromosome.
● The various specific forms of a gene are alleles.
● Alleles differ from each other by one or only a few bases.
● New alleles are formed by mutation.
● The genome is the whole of the genetic information of an organism.
● The entire base sequence of human genes was sequenced in the Human Genome Project.
NATURE OF SCIENCE
Developments in scientific research follow improvements in technology: gene sequencers are used for the sequencing of genes.
Applications and skills:
- Application: The causes of sickle cell anaemia, including a base substitution mutation, a change to the base sequence of mRNA transcribed from it, and a change to the sequence of a polypeptide in hemoglobin.
- Application: Comparison of the number of genes in humans with other species.
- Skill: Use of a database to determine differences in the base sequence of a gene in two species
Guidance
- Students should be able to recall one specific base substitution that causes glutamic acid to be substituted by valine as the sixth amino acid in the haemoglobin polypeptide.
- The number of genes in a species should not be referred to as genome size as this term is used for the total amount of DNA. At least one plant and one bacterium should be included in the comparison, and at least one species with more genes and one with fewer genes than a human.
- The GenBank® database can be used to search for DNA base sequences. The cytochrome c gene sequence is available for many different organisms and is of particular interest because of its use in reclassifying organisms into three domains.
- Deletions, insertions, and frame shift mutations do not need to be included.
- Genes
A gene is a heritable factor that consists of a length of DNA and influences a specific characteristic. ‘Heritable’ means passed on from parent to offspring, and ‘characteristic’ refers to genetic traits such as your hair colour or your blood type. The estimated 21 000 genes that you possess are organized into chromosomes.
- A gene is found at a particular locus on a chromosome
A gene for a specific trait occupies a corresponding place, called a locus (plural loci), on a chromosome
When geneticists map out the sequences of DNA, they carefully map the locus of each sequence. When further research reveals that a particular sequence controls a certain heritable factor, the locus of the gene is noted for further reference. For example, scientists now know that the locus of the gene controlling a protein called transducin that enables colour vision is found on chromosome 1. A mutation of this gene stops a person from being able to make the protein transducin properly, which is necessary to transmit information about colour from the eye to the brain; as a result, the person will not see in colour. This is an extremely rare genetic condition called complete achromatopsia. When we say ‘the ability to see in colour is a genetic trait’ we mean one of two things is happening with someone’s DNA: either that person has the DNA code for making colour vision possible or the person does not have it.
You will recall that you possess two copies of each gene in your body: one copy from your mother and one from your father. As a result, if you could look at the locus of the transducin gene on one of the two copies of your first chromosome, for example, you would find the same gene at the same locus on the other copy of chromosome 1. One copy would be the one your mother gave you and the other would be the copy your father gave you. Would those genes be identical? Not necessarily, because genes can come in different forms.
- One base can make a big difference
From the sections on transcription and translation of DNA, you will remember how important it is for each letter in the genetic code to be in a specific place. If, for whatever reason, one or more of the bases (A, C, G or T) is misplaced or substituted for a different base, the results can be dramatic.The difference between one version of a gene and another can mean the difference between healthy organs and organs hampered by an overproduction of mucus.
- Mutations
A mutation is a random, rare change in genetic material. One type involves a change of the sequence of bases in DNA. If DNA replication works correctly, this should not happen. But nature sometimes makes mistakes. For example, the base thymine (T) might be put in the place of adenine (A) along the DNA sequence. When this happens, the corresponding bases along the messenger RNA (mRNA) are altered during transcription. Mutated genes can have a negative effect on a person’s health. Sometimes, however, mutations can have a positive effect that is beneficial to an organism’s survival.
- Base substitution mutation
The type of mutation that results in a single letter being changed is called a base substitution mutation. The consequence of changing one base could mean that a different amino acid is placed in the growing polypeptide chain. This may have little or no effect on the organism, or it may have a major influence on the organism’s physical characteristics.
- Sickle cell disease
In humans, a mutation is sometimes found in the gene that codes for haemoglobin in red blood cells. This mutation gives a different shape to the haemoglobin molecule. The difference leads to red blood cells that look very different from the usual flattened disc with a hollow in the middle.
The mutated red blood cell, with a characteristic curved shape, made its discoverers think of a sickle (a curved knife used to cut tall plants). The condition that results from this mutation is therefore called sickle cell disease, also known as sickle cell anaemia.
The kind of mutation that causes sickle cells is a base substitution mutation. The first consequence is for the section of the haemoglobin gene’s DNA that codes for standard-shaped red blood cells, whereas the second sequence shows the mutation that leads to the sickle shape. In this case, one base is substituted for another so that the sixth codon in this sequence of haemoglobin, GAG, becomes GTG. As a result, during translation, instead of adding glutamic acid, which is the intended amino acid in the sixth position of the sequence, valine is added there instead.
Because valine has a different shape and different properties compared with glutamic acid, the shape of the resulting polypeptide chain is modified. As a result of this, the haemoglobin molecule has different properties that cause the complications associated with sickle cell disease.
From the sections on transcription and translation of DNA, you will remember how important it is for each letter in the genetic code to be in a specific place. If, for whatever reason, one or more of the bases (A, C, G or T) is misplaced or substituted for a different base, the results can be dramatic.The difference between one version of a gene and another can mean the difference between healthy organs and organs hampered by an overproduction of mucus.
- Mutations
A mutation is a random, rare change in genetic material. One type involves a change of the sequence of bases in DNA. If DNA replication works correctly, this should not happen. But nature sometimes makes mistakes. For example, the base thymine (T) might be put in the place of adenine (A) along the DNA sequence. When this happens, the corresponding bases along the messenger RNA (mRNA) are altered during transcription. Mutated genes can have a negative effect on a person’s health. Sometimes, however, mutations can have a positive effect that is beneficial to an organism’s survival.
- Base substitution mutation
The type of mutation that results in a single letter being changed is called a base substitution mutation. The consequence of changing one base could mean that a different amino acid is placed in the growing polypeptide chain. This may have little or no effect on the organism, or it may have a major influence on the organism’s physical characteristics.
- Sickle cell disease
In humans, a mutation is sometimes found in the gene that codes for haemoglobin in red blood cells. This mutation gives a different shape to the haemoglobin molecule. The difference leads to red blood cells that look very different from the usual flattened disc with a hollow in the middle.
The mutated red blood cell, with a characteristic curved shape, made its discoverers think of a sickle (a curved knife used to cut tall plants). The condition that results from this mutation is therefore called sickle cell disease, also known as sickle cell anaemia.
The kind of mutation that causes sickle cells is a base substitution mutation. The first consequence is for the section of the haemoglobin gene’s DNA that codes for standard-shaped red blood cells, whereas the second sequence shows the mutation that leads to the sickle shape. In this case, one base is substituted for another so that the sixth codon in this sequence of haemoglobin, GAG, becomes GTG. As a result, during translation, instead of adding glutamic acid, which is the intended amino acid in the sixth position of the sequence, valine is added there instead.
Because valine has a different shape and different properties compared with glutamic acid, the shape of the resulting polypeptide chain is modified. As a result of this, the haemoglobin molecule has different properties that cause the complications associated with sickle cell disease.