Genetic Information and Variation - DNA, Genes and Chromosomes

Question 1

What is a gene?

Answer 1

A gene is a section of DNA that contains the coded information for making polypeptides and functional RNA. This coded information is in the form of a specific sequence of bases along the DNA molecule.

Polypeptides make up proteins and so genes determine the proteins of an organism. Enzymes are proteins. As enzymes control chemical reactions, they are responsible for an organism’s development and activities.

Question 2

What do genes determine?

Answer 2

Genes, along with environmental factors, determine the nature and development of all organisms.

Question 3

What is a locus?

Answer 3

A gene is a section of DNA located at a particular position, called a locus, on a DNA molecule.

Question 4

What does the gene code for?

Answer 4

The gene is a base sequence of DNA that codes for:

• the amino acid sequence of a polypeptide
• or a functional RNA, including ribosomal DNA and transfer RNAs

Question 5

What is the primary structure of proteins?

Answer 5

The sequence of amino acids coded for by DNA is the primary structure of a protein. It is the primary structure that gives rise to the tertiary structure and hence the shape of the proteins. So DNA codes indirectly for the shape of proteins, including enzymes.

Question 6

What is DNA made up of and what does it code for?

Answer 6

It is made up of nucleotides, but CODES for amino acids.

Question 7

Why must there be a minimum of three bases that code for each amino acid?

Answer 7

• only 20 different amino acids regularly occur in proteins
• each amino acid must have its own code of bases on the DNA
• only four different bases (adenine, guanine, cytosine and thymine) are present in DNA
• if each base coded for a different amino acid, only four different amino acids could be coded for
• using a pair of bases, 16(4^2) different codes are possible, which is inadequate
• three bases produce 64(4^3) different codes, more than enough to satisfy the requirements of 20 amino acids

Question 8

What is a triplet?

Answer 8

As the code has three bases for each amino acid, each one is called a triplet (codon). As there are 64 possible triplets and only 20 amino acids, it follows that some amino acids are coded for by more than one triplet.

Question 9

How are a few amino acids coded for?

Answer 9

A few amino acids are coded for by only a single triplet.

Question 10

How are the remaining amino acids coded for?

Answer 10

The remaining amino acids are coded for by between two and six triplets each.

Question 11

What is a degenerate code?

Answer 11

Genetic code is degenerate meaning that more than one triplet codes for the same amino acid, this reduces the number of mutations which are mistakes in the base sequence such as base deletion, insertion or substitution.

A change in the base sequence of DNA alters the amino acid sequence and the protein therefore it can have various effects. Some mutations are harmful such as the mutation which leads to production of sticky mucus and causes cystic fibrosis or sickle anaemia in which a mutated form of haemoglobin distorts the shape of red blood cells.

Question 12

How is a triplet read?

Answer 12

A triplet is always read in one particular direction along the DNA strand.

Question 13

What is always at the start of a DNA sequence?

Answer 13

The start of a DNA sequence that codes for a polypeptide is always the same triplet. This codes for the amino acid methionine. If the first methionine molecule does not form part of the final polypeptide, it is later removed.

Question 14

Do all triplets code for an amino acid?

Answer 14

No, three triplets do not code for any amino acid. These are called ‘stop codes’ and mark the end of a polypeptide chain. They act in much the same way as a full stop at the end of a sentence.

Question 15

What does non-overlapping mean?

Answer 15

The code is non-overlapping, meaning that each base in the sequence is read only once.

Question 16

Is the code universal?

Answer 16

The code is universal, with a few minor exceptions each triplet codes for the same amino acid in all organisms. This is indirect evidence for evolution.

Question 17

What are exons and introns?

Answer 17

Much of the DNA in eukaryotes does not code for polypeptides. For example, between genes there are non-coding sequences made up of multiple repeats of base sequences. Even within genes, only certain sequences code for amino acids. These coding sequences are called exons. Within the gene these exons are separated by further non-coding sequences called introns. Some genes code for ribosomal RNA and transfer RNAs.

Question 18

What is DNA like in prokaryotic cells?

Answer 18

In prokaryotic cells, such as bacteria, the DNA molecules are shorter, form a circle and are not associated with protein molecules. Prokaryotic cells therefore do not have chromosomes.

Question 19

What is DNA like in eukaryotic cells?

Answer 19

In eukaryotic cells, the DNA molecules are longer, form a line (are linear) rather than a circle and occur in association with proteins called histones to form structures called chromosomes. The mitochondria and chloroplasts of eukaryotic cells also contain DNA which, like the DNA of prokaryotic cells, is short, circular and not associated with proteins.

Question 20

What is the structure of chromosomes?

Answer 20

Chromosomes are only visible as distinct structures when a cell is dividing. For the rest of the time they are widely dispersed throughout the nucleus. When they first become visible at the start of cell division, chromosomes appear as two threads, joined at a single point (centromere).

Each thread is called a chromatid because DNA has already replicated to give two identical DNA molecules. The DNA in chromosomes is held by histones. The considerable length of DNA found in each cell (around 2m in every human cell) is highly coiled and folded.

DNA is loosely wrapped around histones, but becomes tighter during cell division in order to condense and take up less space.

Question 21

How is DNA condensed into a single chromosome?

Answer 21

DNA is a double helix, which is wound around histones (together called chromatins) to fix it in position. This DNA-histone complex is then coiled. The coil in turn is looped and further coiled before being packed into the chromosome. In this way a lot of DNA is condensed into a single chromosome.

Question 22

How many DNA molecules are in a chromosome?

Answer 22

A chromosome contains a single molecule of DNA, although this is very long. This single DNA molecule has many genes along its length. Each gene occupies a specific position (locus) along the DNA molecule.

Question 23

Is the number of chromosomes in every organism the same?

Answer 23

Although the number of chromosomes is always the same for normal individuals of a species, it varies from one species to another. In most species, there is an even number of chromosomes in the cells of adults.

Question 24

What are homologous chromosomes?

Answer 24

Sexually produced organisms, such as humans, are the result of the fusion of a sperm and an egg, each of which contributes one complete set of chromosomes to the offspring. Therefore, one of each pair is derived from the chromosomes provided by the mother in the egg (maternal chromosomes) and the other is derived from the chromosomes provided by the father in the sperm (paternal chromosomes). These are known as homologous pairs and the total number is referred to as the diploid number. In humans this is 46.

Question 25

What is a homologous pair?

Answer 25

A homologous pair is always two chromosomes that carry the same genes but not necessarily the same alleles of the genes.

Question 26

What is an allele?

Answer 26

An allele is one of a number of alternative forms of a gene. We have seen that genes are sections of DNA that contain coded information in the form of specific sequences of bases. Each gene exists in two, occasionally more, different forms. Each of these forms is called an allele.

Each individual inherits one allele from each of its parents. These two alleles may be the same or they may be different. When they are different, each allele has a different base sequence, therefore a different amino acid sequence, so produces a different polypeptide.

Question 27

What does a change in the base sequence cause?

Answer 27

Any changes in the base sequence of a gene produces a new allele of that gene (=mutation) and results in a different sequence of amino acids being coded for. This different amino acid sequence will lead to the production of a different polypeptide, and hence a different protein.

Sometimes this different protein may not function properly or may not function at all. When the protein produced is an enzyme, it may have a different shape. The new shape may not fit the enzyme’s substrate. As a result, the enzyme may not function and this can have serious consequences for the organism.

Question 28

What is the difference between genes and alleles?

Answer 28

A gene refers to a particular characteristic. Genes can exist in two or more different forms called alleles.

Question 29

What is the structure of DNA molecules in mitochondria and chloroplasts?

Answer 29

The mitochondria and chloroplasts of eukaryotic cells also contain DNA which, like the DNA of prokaryotes, is short, circular and not associated with protein.