OCR A Level Biology- Nucleic Acids

Which elements do nucleotides contain?

Carbon, hydrogen, nitrogen, oxygen and phosphorous

What are the two types of nucleic acid?

DNA and RNA

What is a nucleic acid?

A large monomer made up of many nucleotide monomers linked in a chain

What are the 3 components of DNA and RNA?

1. Phosphoryl group
2. Pentose monosaccharide (Deoxyribose in DNA, Ribose in RNA)
3. An organic, nitrogenous base (ATGC in DNA, AUGC in RNA)

How are the components in a nucleic acid monomer joined?

By strong covalent bonds formed via condensation reaction

What is the difference between ribose and deoxyribose?

Deoxyribose has 1 less oxygen (4, not 5)

What part of a nucleic acid is acidic?

The phosphoryl group, which is also negatively charged (PO4²⁻)

How is a polynucleotide formed?

When the phosphoryl group (5th carbon of sugar, aka 5') of one nucleotide and OH group of the 3rd carbon of the adjacent nucleotide's pentose monosaccharide attach via a condensation reaction

What is the bond between 2 nucleotides called?

Phosphopdiester bond

What is the part of a nucleic acid other than the bases called?

Sugar-phosphate backbone

What way does each strand of DNA run?

From 3' to 5'

What does it mean that the strands in DNA are antiparallel?

The sugar-phosphate backbone of each strand run in opposite directions

What bonds attract the base pairs of DNA to each other?

Hydrogen bonds

How many hydrogen bonds are there between A and T/U bases?

2

How many hydrogen bonds are there between G and C bases?

3

Which bases are purine?

Adenine and Guanine

What are the full names of A,T,G,C and U?

Adenine, thymine, guanine, cytosine, uracil

What is the difference between pyrimidine and purine molecules?

Purines have double carbon ring structures, pyrimidines have single carbon ring structures

Which bases are pyrimidine?

Thymine, Cytosine and Uracil

What is the difference between the bases of DNA and RNA?

DNA has Thymine, RNA has Uracil

How do purine and pyrimidine molecules fit together?

In a complementary fashion

What does complementary base pairing in DNA ensure about the relative amounts of ATGC?

There is exactly the same amount of Adenine and Thymine, and likewise for Guanine and Cytosine

Which enzyme catalyses the unwinding and separation of DNA's two strands?

DNA Helicase

How does the DNA molecule unwind?

DNA Helicase travels along the DNA backbone, catalysing reactions which break the hydrogen bonds between complementary base pairs as it reaches them.

What happens after the DNA strand has been 'unzipped' in DNA replication?

Free deoxyribose nucleotides present in the nucleus form hydrogen bonds with their complementary bases.

What does DNA Polymerase do?

Causes a phosphodiester bond to form between unjoined deoxyribose nucleotides which have been attracted to the unzipped strand of DNA

What direction is the only one DNA polymerase works in?

From the 3' end to the 5' end of the DNA strand

Why does DNA have to replicate each template strand in opposite directions?

Because DNA polymerase only runs along a strand in one direction, but DNA only unwinds and unzips in one direction

Which strand of DNA is the leading strand, and can therefore undergo continuous replication?

The strand which is unzipped from the 3' end, as DNA polymerase can replicate it continuously as the strands unzip

What happens to the strand unzipped from the 5' end in DNA replication?

DNA has to wait until a section has been unzipped, then work back along the strand. This results in DNA being produced in segments called Okizaki Fragements.

What joins Okizaki Fragments together?

DNA Ligase

What is the name of the strand unzipped from the 5' end, and what type of replication does it undergo?

Called the lagging strand and undergoes discontinuous replication

How does a mutation occur during DNA replication?

Because sequences of bases aren't always matched exactly, and an incorrect sequence can occur in a copied strand. These errors are random and spontaneous

What is semi-conservative replication of DNA?

The idea that each new DNA molecule contains one parent strand (from the original DNA molecule) and one new strand made up of nucleotides not from the original molecule

What experiment provided evidence for semi-conservative replication?

The Meselson-Stahl experiment

Provide a brief overview of the Meselson-Stahl experiment.

1. Bacteria were initially grown in heavy nitrogen (¹⁵N), so all bases in their DNA contained only heavy nitrogen, leaving a band of heavy DNA near the bottom of a test tube
2. Then, bacteria were grown in normal (¹⁴N) nitrogen. The 1st generation of DNA all had a medium weight, as one strand was heavy and one light, so there was a band of DNA near the middle of the test tube.
3. Another generation of bacteria was grown in ¹⁴N nitrogen, producing DNA which was 1/2 lightweight (2 light strands) and 1/2 midweight (1 light 1 heavy), meaning that 2 bands formed in the test tube (1 at the top and 1 in the middle)

What is a gene?

A section of DNA used to encode a protein

What do genes control?

All metabolic processes

What is a gene locus?

The position each gene occupies on a chromosome

What is the proper name for a triplet of three bases?

Codon

What sort of code is the genetic code?

A triplet code with 64 different combinations and a degenerative code

What does it mean that the genetic code is degenerative?

That multiple codons can code for one amino acid

What is the importance of each gene in DNA containing a 'START' codon?

It ensures that codons are read 'in frame' i.e. in order so that the correct order of amino acids is produced

What is special about the ATG codon?

At the very start of a gene, it is a START codon- afterwards, it just codes for methionine

How many STOP codons are there?

3

What do STOP codons do?

End the chain of amino acids, ensuring that no new amino acids are added to the protein

Why is the genetic code a triplet code?

There are around 20 amino acids regularly used by organisms. If it were a single or double code, it would only have 4 or 16 possibilities, not enough to satisfy the needs of organisms

What is transcription of a gene?

The process by which the DNA sequence in a gene is transcribed onto a piece of messenger RNA

Why is gene transcription used in eukaryotes?

Because the full chromosomal DNA molecule is too large to leave the nucleus via a nuclear pore

What is the first step in gene transcription?

RNA nucleotides in the nucleolus are activated by adding two phosphoryl groups (i.e. adenine to ATP)

What is the second step in gene transcription?

DNA helicase is used to unwind the gene to be copied and break the hydrogen bonds between complementary bases (this can occur in the middle of a DNA strand)

What is the third step in gene transcription?

The activated RNA nucleotides bind using hydrogen bonds to their complementary bases on the template strand

What catalyses the joining of adjacent nucleotides on the RNA strand during gene transcription?

RNA Polymerase

What bonds are formed between adjacent RNA nucleotides by RNA polymerase during gene transcription?

Phosphodiester bonds

What action provides the energy for the joining of adjacent RNA nucleotides during gene transcription?

The removal of the two phosphoryl groups from the nucleotides

What are the 3 types of RNA?

1. Messenger RNA
2. Ribosomal RNA
3. Transfer RNA

What is the fourth step in gene transcription?

The mRNA strand is released from the DNA and passes out of the nucleus via a nuclear pore, before binding to a ribosome

Which strand of DNA is the one that is replicated during gene transcription?

The sense/coding strand, which runs from 5' to 3'

What is another name for the template strand?

Antisense strand

What is an exon?

Part of a gene which codes for a protein

What is an intron?

Part of a gene which doesn't code for a protein

What does pre-RNA contain in terms of exons and introns?

A mixture of both

What is splicing (of a pre-mRNA molecule to a mature mRNA molecule)?

The removal of introns from the pre-mRNA

How can the same piece of pre-mRNA lead to the production of different proteins?

Because exons can be rearranged or even left out during splicing

What type of mRNA binds to a ribosome?

Mature mRNA

When does gene splicing occur?

Once the pre-mRNA molecule has reached the ribosome

What is DNA translation?

The assembly of polypeptides at ribosomes, dictated by the sequence of codons on the single-stranded mRNA

How do mRNA molecules fit into a ribosome?

Along a groove between the large and small sub units

Where is transfer RNA made?

The nucleus

What is an anticodon on a tRNA molecule?

3 unpaired nucleotides (at the opposite end to the amino acid) which are complementary to the codon being bound to

What shape does a tRNA molecule have?

A hairpin

What is found on a tRNA molecule at the opposite end to the anticodon?

Three exposed bases for an amino acid to bind onto

How many tRNA molecules are there and why?

61, as there is no tRNA molecule for the 3 STOP codons

What is the first step in gene translation?

mRNA molecule binds onto the small subunit of the ribosome by its first 2 codons

What is the second step in gene translation?

A tRNA molecule with the anticodon to the START codon carries methionine into the ribosome and attaches to the mRNA, using ATP to form hydrogen bonds with the codon

What is the maximum amount of codons which can be translated by any one ribosome at once?

Two

What is the third step in gene translation?

Another tRNA molecule brings a different amino acid and binds to the second exposed codon on the mRNA

What is the fourth step in gene translation?

A peptide bond forms between the two amino acids, catalysed by rRNA and the enzyme peptidyl transferase in the small ribosomal subunit

What is the fifth step in gene translation?

The ribosome moves along the mRNA, causing the first tRNA to leave (leaving their amino acid behind), translating codons one by one until a stop codon is reached

Why does gene translation stop once a STOP codon is reached?

There are no tRNA molecules for the 3 STOP codons, so the polypeptide chain is broken

How can protein synthesis be sped up?

Multiple ribosomes can be bound onto the same piece of mRNA