DNA, genes and protein synthesis

Question 1

What is a gene?

Answer 1

A section of DNA that contains coded information for making polypeptides and functional RNA.
The coded information is in the form of specific sequences of bases along the DNA molecule.
Genes, along with environmental factors, determine the nature and development of all organisms.
A gene is a section of DNA located at a particular position, called a locus, on a DNA molecule.

Question 2

What is the function of a gene?

Answer 2

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 3

What does a gene do?

Answer 3

It is a base sequence of DNA that codes for:
The amino acid sequence of a polypeptide.
Or a functional RNA, including ribosomal RNA and transfer RNAs.

Question 4

Why did scientists suggest the genetic code?

Answer 4

Only 20 different amino acids regularly occur in proteins.
Each amino acid must have its own code of bases on the DNA.
Only four bases 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 still inadequate.
Three bases produce 64 different codes, more than enough to satisfy the requirements of 20 amino acids.

Question 5

What is the genetic code?

Answer 5

In trying to discover how DNA bases coded for amino acids, scientists suggested that there must be a minimum of three bases that coded for each amino acid.

Question 6

What is a triplet?

Answer 6

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

Question 7

What is a degenerate code - features?

Answer 7

The genetic code is a degenerate code because most amino acids are coded for by more than one triplet.

Question 8

What is methionine?

Answer 8

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

Question 9

What are stop codes?

Answer 9

Three triplets do not code for any amino acid.
These are stop codes and mark the end of a polypeptide chain.
They act like a full stop in a sentence.

Question 10

What is the non-overlapping code?

Answer 10

The genetic code is non-overlapping.
Each base in the sequence is read only once.
Thus six bases numbered 123456 are read as triplets 123 and 456, rather than as triplets 123, 234, 345, 456.

Question 11

How is the code universal?

Answer 11

The genetic 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 12

What are the other features of the genetic code?

Answer 12

A few amino acids are coded by a single triplet.
The remaining amino acids are coded for by between two and six triplets each.
A triplet is always read in one particular direction along the DNA strand.

Question 13

What are exons?

Answer 13

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.

Question 14

What are introns?

Answer 14

Within the gene these exons are separated by further non-coding sequences called introns.

Question 15

What is DNA like in prokaryotes?

Answer 15

E.g. bacteria, DNA molecules are shorter, form a circle and are not associated with protein molecules.
They therefore do not have chromosomes.

Question 16

What is DNA like in eukaryotes?

Answer 16

The DNA molecules are longer, linear, and occur in association with proteins called histones to form chromosomes.
The mitochondria and chloroplasts also contain DNA, which like prokaryotic DNA, is short, circular and not associated with proteins.

Question 17

How are chromosomes seen?

Answer 17

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

Question 18

What are chromatids?

Answer 18

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) is highly coiled and folded.

Question 19

What is the structure of chromosomes?

Answer 19

The double helix is wound around histones to fix it into position.
This DNA-histone complex is then coiled, then looped and further coiled before being packed into the chromosome.
A lot of DNA can be condensed into a single chromosome.
It contains a single, very long molecule of DNA, with many genes along its length.
Each gene occupies a specific position along the DNA molecule.

Question 20

How many chromosomes are there?

Answer 20

The number is always the same for normal individuals of a species, but it varies from one species to another.
Humans have 46 chromosomes, potato plants have 48, and dogs have 78.
In most species there is an even number of chromosomes in the cells of adults.

Question 21

What are homologous pairs?

Answer 21

Sexually produced organisms 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 form the chromosomes provided by the mother and the other from the father.
The total number is referred to as the diploid number - 46.
It is always two chromosomes that carry the same genes, but not necessarily the same alleles of the genes.

Question 22

What is an example of a homologous pair?

Answer 22

A pair may each possess genes for tongue rolling and blood group, but one chromosome may carry the allele for non-roller and blood group A, while the other carries the allele for roller and blood group B.

Question 23

What happens during meiosis with chromosomes?

Answer 23

The halving of the number of chromosomes is done in a way which ensures that each daughter cell receives one chromosome from each homologous pair.
In this way, each cell receives one gene for each characteristic of the organism.
When these haploid cells combine, the diploid state, with paired homologous chromosomes, is restored.

Question 24

What is an allele?

Answer 24

One of a number of alternative forms of a gene.
Each gene exists in two different forms, called an allele.
Each individual inherits one allele from each of its parents.
The two alleles may be the same or different.
When they are different, each allele has a different base sequence, therefore a different amino acid sequence, so produces a different polypeptide.

Question 25

How are different alleles produced?

Answer 25

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 will lead to the production of a different polypeptide, and hence a different protein.

Question 26

What is the effect of different proteins?

Answer 26

Sometimes the different protein will not function properly or not at all.
When the protein produced is an enzyme, it may have a different shape, which may not fit the enzyme’s substrate.
So the enzyme may not function and have bad consequences for the organism.

Question 27

What is mRNA?

Answer 27

It transfers the DNA code from the nucleus to the cytoplasm by acting as a type of messenger.
It is small enough to leave the nucleus through the nuclear pores and to enter the cytoplasm, where the coded information it contains is used to determine the sequence of amino acids in the proteins which are synthesised there.

Question 28

What is a codon?

Answer 28

The sequence of three bases on mRNA that codes for a single amino acid.

Question 29

Why must DNA be transferred?

Answer 29

In eukaryotic cells DNA is largely confined to the nucleus, but the synthesis of proteins takes place in the cytoplasm.
So sections of the DNA code are transcribed onto a single-stranded molecule called ribonucleic acid (RNA).

Question 30

What is the genome?

Answer 30

The complete set of genes in a cell, including those in mitochondria or chloroplasts.

Question 31

What is the proteome?

Answer 31

The full range of proteins produced by the genome.
It can be called the complete proteome, where the proteome refers to the proteins produced by a given type of cell under a certain set of conditions.

Question 32

What is the structure of RNA?

Answer 32

A polymer made up of repeating mononucleotide sub-units.
It forms a single strand.
Each nucleotide is made up of:
Pentose sugar ribose.
One of the organic bases Adenine, Uracil, Guanine or Cytosine.
A phosphate group.
mRNA and tRNA are used in protein synthesis.

Question 33

What is the structure of mRNA?

Answer 33

It consists of thousands of mononucleotides, and is a long strand arranged in a single helix.
The bases sequence is determined by the sequence of bases on a length of DNA in transcription.
There is a great variety of types.
Once formed, mRNA leaves the nucleus via pores and enters the cytoplasm, where it associates with the ribosomes.
It then acts as a template for protein synthesis.

Question 34

How is mRNA structure linked to its function?

Answer 34

It possesses information in the form of codons, the sequence of codons determines the amino acid sequence of a specific polypeptide that will be made.

Question 35

What is tRNA?

Answer 35

It is a relatively small molecule that is made up of 80 nucleotides.
It is a single stranded chain folded into a clover-leaf shape, with one end of the chain extending beyond the other, this is where an amino acid can easily attach.
There are about 60 different types of tRNA, each of which binds to a specific amino acid.
At the opposite end is a sequence of three other organic bases, the anticodon.
The genetic code is degenerate so there must be as many tRNA molecules as coding triplets.

Question 36

Why are mRNA and tRNA suited for protein synthesis?

Answer 36

During protein synthesis, an anticodon pairs with the three complementary organic bases that make up the codon on mRNA.
The tRNA structure, with its end chains for attaching amino acids and its anticodon for complementary base pairing with the codon on the mRNA, is structurally suited to its role of lining up amino acids on the mRNA template during protein synthesis.

Question 37

What is the process of transcription?

Answer 37

Transcription is the process of making pre mRNA using the DNA as a template.
The double strands of DNA are separated by helicase, which breaks the hydrogen bonds between bases.
The bases on the template strand pair with complementary free nucleotides. Guanine pairs with cytosine and Adenine pairs with uracil.
The enzyme RNA polymerase moves along the strand and joins the nucleotides together to make a molecule of pre mRNA.
This pre mRNA then needs to be spliced to make mRNA. Splicing involves removing sections called introns, which do not code for proteins, and then joining together the ends of the remaining sections called exons, which do code for proteins.
Splicing does not occur in prokaryotic cells because they do not have introns. Once the mRNA molecules have been spliced, they leave the nucleus via a nuclear pore, and move to the ribosomes to begin translation.

Question 38

Why must pre-mRNA be spliced?

Answer 38

The DNA of a gene in eukaryotic cells is made up of sections called exons that code for proteins and sections called introns that do not.
These intervening introns would prevent the synthesis of a polypeptide in the pre-mRNA of eukaryotic cells.
Splicing of pre-mRNA produces mRNA.

Question 39

What is splicing?

Answer 39

The base sequences corresponding to the introns are removed and the functional exons are joined together.
As most prokaryotic cells do not have introns, splicing of their DNA is unnecessary.

Question 40

How does mRNA get to translation?

Answer 40

The mRNA molecules are too large to diffuse out of the nucleus and so, once they have been spliced, they leave via a nuclear pore.
Outside the nucleus, mRNA is attracted to the ribosomes to which it becomes attached, ready for translation.

Question 41

What is the process of translation?

Answer 41

The ribosome attaches to the start codon at one end of the mRNA molecule.
The tRNA with the complementary anticodon moves to the ribosome and pairs with the codon on the mRNA. This tRNA carries a specific amino acid. Another tRNA molecule will pair with the next codon on the mRNA, bringing another specific amino acid.
The two amino acids then join by a condensation reaction, forming a peptide bond, and requiring energy from the hydrolysis of ATP.
A third amino acid will then join, and the first tRNA will be released to collect another amino acid. The process continues until a stop codon is reached, and the polypeptide chain is released from the ribosome.
The polypeptide will then be folded into secondary and tertiary structure, and different chains along with non-protein groups could be linked to form quaternary structure.

Question 42

How is a protein assembled?

Answer 42

A number of polypeptides are often linked together to give a functional protein.
The protein is coiled or folded, producing its secondary structure.
The secondary structure is folded, producing its tertiary structure.
Different polypeptide chains, along with any non-protein groups, are linked to form the quaternary structure.