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Gene

A short piece of DNA which is responsible for the inheritance of a particular characteristic. It codes for the production of a specific protein. Genes occupy a fixed position, called a locus, on a particular DNA molecule.

Bases

Bases, sometimes called nitrogenous bases, are the parts of the DNA molecule that join the two helix strands. They are like rungs on a ladder. There are four bases: adenine (A), thymine (T), guanine (G) and cytosine (C). Each base can only join with one other base; i.e. they join together in pairs: A with T and G with C.

Pentose sugar

A sugar with five carbon atoms. This can be either deoxyribose or ribose.

Genes and DNA

Genome

Genetic information is stored in DNA, which is found in the nucleus of eukaryotic cells. The complete amount of genetic information present in an organism is called the genome. Understanding the human genome can be seen as the key to understanding the mystery of human life itself. Click here for more information on the Human Genome Project.

Chromosomes

Nitrogenous bases

Four nitrogen-containing molecules, sometimes just called bases, adenine, A, Thymine, T, Guanine, G and Cytosine, C, which form the 'rungs' of the DNA molecule 'ladder'.

Genetic code

Sequence of bases in a DNA molecule which codes for the sequence of amino acids in a protein. Each group of three bases, codes for an amino acid.

Amino acids

The basic building blocks of proteins. There are twenty amino acids used, in different combinations, to make every protein required by the human body.

Recessive

The description of an allele which is overridden by an equivalent dominant allele.

Polymer

A large molecule which is made up of repeated individual units called monomers.

Nucleotide

Monomer unit of the nucleic acids DNA and RNA. Each nucleotide is made up of three parts: a pentose sugar, a phosphate group and a nitrogenous base.

Micrometres

1000nm

Homozygous

The description of an individual who has two identical alleles for one particular gene.

Heterozygous

The description of an individual who has two different alleles for one particular gene.

Dominant

The description of an allele which overrides an equivalent recessive allele.

Alleles

Alternative forms of the same gene.

Most cells in the human body have a nucleus containing 23 pairs of chromosomes. Chromosomes are made from very long molecules of DNA. Each molecule is in a ladder-like shape, with the ‘rungs’ being made from a pair of nitrogenous bases, known as A, T, G or C, and the sides made from repeated pentose sugars and phosphate groups. A group of one base, one pentose molecule and one phosphate together is known as a nucleotide and DNA can be regarded as a polymer of these nucleotides.

The bases are always paired A with T and G with C and it is the exact sequence of these base pairs in the DNA molecule that maps out the genetic code of the individual.

The DNA molecule is twisted into a double helix and coiled so tightly, that the total length of the human genome, about 1.8 m, fits inside a nucleus of diameter about 6 micrometres.

Model of a small section of a DNA helix
Model of a small section of a DNA helix

A small section of a chromosome that controls particular characteristics, is called a gene. Each chromosome carries hundreds of genes; scientists have identified 20,000-25,000 human genes. Only some of the full set of genes is used in any one cell – some genes are ‘switched off’.

Genes and alleles

The chromosomes in a pair carry the same genes at the same positions. One of each pair has come from the mother and the other from the father, so the genes are also in pairs and both code for the same characteristic. These alternative forms of one gene are called alleles or variants.

An individual may have two alleles the same or two different alleles for any one gene. If both alleles are the same they are known as homozygous alleles and if they are different they are known as heterozygous. When alleles are heterozygous it may be that one of the alleles is dominant and the other recessive. A dominant allele always shows its related characteristic in the individual.

A gene is a piece of DNA which has the code for a particular protein. The order of the bases in the gene determines the order in which amino acids are put together to make the protein. Proteins have special functions in the body so it is very important that the correct proteins are made.

Exam style questions

1.

Define ‘genome’:
Total amount of genetic material in a cell
Total amount of protein in a cell
The process by which genetic material is passed from one generation to the next
Total amount of protein and genetic material in a cell
Total amount of genetic material in a cell
Total amount of protein in a cell
(The correct answer is:
'Total amount of genetic material in a cell')
The process by which genetic material is passed from one generation to the next
(The correct answer is:
'Total amount of genetic material in a cell')
Total amount of protein and genetic material in a cell
(The correct answer is:
'Total amount of genetic material in a cell')
null
(The correct answer is:
'Total amount of genetic material in a cell')

2. Select the correct option:

a. How many chromosomes are there in the majority of human cells?
23
3,000
4,600
46
23
(The correct answer is:
'46')
3,000
(The correct answer is:
'46')
4,600
(The correct answer is:
'46')
46
null
(The correct answer is:
'46')
b. What is the approximate number of genes in the human genome?
200-250
20,000-25,000
46
32
200-250
(The correct answer is:
'20,000-25,000')
20,000-25,000
46
(The correct answer is:
'20,000-25,000')
32
(The correct answer is:
'20,000-25,000')
null
(The correct answer is:
'20,000-25,000')
c. How do the bases (A, G, C and T) pair up in the DNA molecule?
A-C and G-T
A-T and G-C
A-G and C-T
A-C and G-T
(The correct answer is:
'A-T and G-C')
A-T and G-C
A-G and C-T
(The correct answer is:
'A-T and G-C')
null
(The correct answer is:
'A-T and G-C')
null
(The correct answer is:
'A-T and G-C')
d. What three molecules make up a nucleotide?
a nucleus, a protein and a gene
a pentose sugar, a phosphate and a nitrogenous base
an allele, a polymer and an amino acid
a nucleus, a protein and a gene
(The correct answer is:
'a pentose sugar, a phosphate and a nitrogenous base')
a pentose sugar, a phosphate and a nitrogenous base
an allele, a polymer and an amino acid
(The correct answer is:
'a pentose sugar, a phosphate and a nitrogenous base')
null
(The correct answer is:
'a pentose sugar, a phosphate and a nitrogenous base')
null
(The correct answer is:
'a pentose sugar, a phosphate and a nitrogenous base')
e. What is meant by an allele being described as dominant?
the allele will not show its characteristic in the individual
the allele is bigger
the allele will show its characteristic in the individual
the allele will not show its characteristic in the individual
(The correct answer is:
'the allele will show its characteristic in the individual')
the allele is bigger
(The correct answer is:
'the allele will show its characteristic in the individual')
the allele will show its characteristic in the individual
null
(The correct answer is:
'the allele will show its characteristic in the individual')
null
(The correct answer is:
'the allele will show its characteristic in the individual')
f. What is the term used when both alleles are the same?
Heterozygous
Homozygous
Recessive
Heterozygous
(The correct answer is:
'Homozygous')
Homozygous
Recessive
(The correct answer is:
'Homozygous')
null
(The correct answer is:
'Homozygous')
null
(The correct answer is:
'Homozygous')
g. What are the smaller units of which proteins are made?
Amino acids
Citric acids
Alleles
Molecules
Amino acids
Citric acids
(The correct answer is:
'Amino acids')
Alleles
(The correct answer is:
'Amino acids')
Molecules
(The correct answer is:
'Amino acids')
null
(The correct answer is:
'Amino acids')