A: Understand the structure and function of nucleic acids to describe gene expression and the process of protein synthesis.
B: Explore how the process of cell division in eukaryotic cells contributes to genetic variation.
Grade Distinction.
A: Understand the structure and function of nucleic acids to describe gene expression and the
process of protein synthesis.
Nucleotide structure, main features and functional role of DNA and RNA
Nucleotides are organic molecules that are made up of a pentose, a phosphate group and a
nitrogenous base. There are 5 different nitrogenous bases, which can be divided into two main
groups; purines and pyrimidines. Purines have a two ring structure and consist of adenine (A) and
guanine (G). Pyrimidines have a one ring structure and consist of cytosine (C), thymine (T) and uracil
(U). Nucleic acids are large biomolecules that store and express genomic information, two main
examples of nucleic acids are deoxyribonucleic acid or DNA and ribonucleic acid or RNA. DNA and
RNA are made up of slightly different nitrogenous bases as DNA contains adenine, thymine, guanine
and cytosine, whereas RNA contains adenine, uracil, guanine and cytosine. This is because RNA has a
short lifespan in comparison to DNA and uracil is less stable and therefore much easier to break
down in comparison to thymine.
DNA is a polymer that is made up of two strands which form a double helix. The DNA is contained in
structures called chromosomes that are found in the nucleus of a cell. The DNA molecule consists of
two sugar-phosphate backbone chains where the sugar is a 5-carbon sugar called deoxyribose. The
two chains are described as anti-parallel as they run in opposite directions. The backbone chains are
joined together by nitrogenous bases, and the bases are joined together by hydrogen bonds. DNA is
replicated in a process called semi-conservative replication. DNA replication takes place in every cell
before cell division takes place as the cell must have twice the normal amount of chromosomes
before it divides. As the DNA is copied, nucleotides band together and a new backbone strand forms
and two identical DNA molecules are formed. The genetic material is identical to each other and
identical to the parent molecule. This means that each new DNA molecule contains one old strand
and one new strand. This process of DNA replication is catalysed by an enzyme called DNA
polymerase.
RNA has a very similar structure to DNA but there is no helix structure as there is only one sugar-
phosphate backbone chain. There are several different types of RNA as they are each involved
differently in protein synthesis. Messenger RNA (mRNA) carries the genetic code from the DNA in the
nucleus to the ribosomes in the cytoplasm. Once in the ribosome the codon sequence of the genetic
code is read. The amino acids are assembled according to this sequence to then form a protein.
Transfer RNA (tRNA) carries amino acids from the cytoplasm if the cell to the ribosomes. The tRNA
attaches itself to the amino acid in a process called amino acid activation, which requires ATP and a
specific enzyme. Each amino acid has a specific tRNA molecule, therefore meaning there would be 20
different tRNA molecules for 20 different amino acids. At a special region there is a triplet of bases
called an anticodon which is complementary to one of the codons on mRNA. Ribosomal RNA (rRNA)
is a type of non-coding RNA which forms part of the protein synthesising organelles called
ribosomes. rRNA is also exported to the cytoplasm to aid in translating the information in mRNA into
proteins. Silencing or short interfering RNA (siRNA) is another type of non-coding RNA and is used to
specifically target particular mRNA for degradation. siRNA is important as it is required to regulate
the expression of genes.
How genetic code allows proteins to be synthesised with minimal errors
The encoded information in genetic material is not corrupted because the nitrogenous bases, that
are held together by hydrogen bonds, are protected by the sugar-phosphate backbones. The cells
also have mechanisms in place for repairing damaged DNA. There are 5 major pathways for which
damaged DNA can be repaired; base excision repair (BER), nucleotide excision repair (NER),
mismatch repair (MMR), homologous recombination (HR) and non-homologous end joining (NHEJ).
Each one of these repair methods are active throughout the different stages of the cell cycle, and
therefore allowing cells to repair any DNA damage.
Triplet codes are a specific set of three consecutive nucleotides that are a part of the genetic code.
They are used to produce amino acids and proteins because each set of 3 nucleotides code for a
specific amino acid in a polypeptide chain. A sequence of three consecutive nucleotides that are
found in a DNA or RNA molecule is called a codon. Some specific codons, called start or stop codons,
are able to signal the start and end of translation and therefore the start or end of a polypeptide
chain. Along with this, a non-overlapping code means that the same letter cannot be used for two
different codons, and therefore no single nucleotide can be a part of more than one codon.
Anticodons are also a sequence of 3 nucleotides but are found on tRNA molecules and are used to
base pair with the codon on a strand of mRNA during translation. Anticodons make sure that the
correct amino acid is added onto the polypeptide chain and therefore ensures that the proteins are
being made correctly. Both codons and anticodons allow proteins to be synthesized with minimal
errors by controlling the beginning and end of proteins along with the specific amino acids which are
making up the proteins. A degenerate code means that the genetic code has a higher number of
sense codons than amino acids, meaning that some amino acids are coded by more than one codon.
For example; UUA, UUG, CUU, CUC and CUG all code for the amino acid Leucine. This means that if a
mutation occurs in a codon, the mutation would change the codon to another codon which may
code for the same amino acid. Having a degenerate code in living organisms prevents mutation from
affecting the production of proteins.
Protein synthesis a process which is required for cells to make proteins. There are two main stages of
protein synthesis and those stages are transcription and translation. Transcription takes place in the
nucleus of eukaryotic cells. DNA material is required to produce proteins but the DNA itself cannot
leave the nucleus. Therefore, the instructions for making the proteins are transcribed from the DNA
to the messenger RNA (mRNA). RNA polymerase is an enzyme which catalyses this reaction as it is
responsible for transferring the genetic information found in DNA to RNA. Transcription occurs as the
DNA to be transcribed unwinds and the hydrogen bonds found between the base pairs break apart.
One of these two DNA strands is used as a template, from which a complementary mRNA strand can
be produced. The genetic code isn’t copied directly as the resulting strand of mRNA contains uracil
instead of thymine. Temporary hydrogen bonds are formed by RNA nucleotides which have lined up
along the template strand of DNA. Along with this, ligase enzymes catalyse the joining of sugar
phosphate groups in adjacent nucleotides. These new bonds that are formed are called
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