2.1.3 Nucleotides and nucleic acids Flashcards
1
Q
What are nucleic acids?
A
- Large molecules that were discovered in the cell nuclei
- There are 2 types of nucleic acid: DNA and RNA
2
Q
Structure of DNA
A
- The nucleotides in DNA have 4 different bases, meaning there are 4 different DNA nucleotides. These can be divided into 2 groups: Pyrimidines (the smaller bases which contain single carbon ring structures thymine and cytosine) and Purines (the larger bases which contain double ring structures adenine and guanine)
3
Q
The double helix
A
- Made up of 2 strands of polynucleotides coiled into a helix (double helix).
- The 2 strands of double helix are held together by hydrogen bonds between the bases.
- Each strand has a phosphate group (5’) at one end and a hydroxyl group (3’) at the other end.
- These 2 parallel strands are arranged so that they run in opposite directions, they are antiparallel.
4
Q
Base pairing rules
A
- Adenine and Thymine are both able to form 2 hydrogen bonds together.
- Cytosine and Guanine form 3 hydrogen bonds.
- This is known as complementary base pairing.
- These rules mean that a small pyrimidine base will always bind to a larger purine base. This arrangement maintains a constant distance between the DNA backbones, resulting in parallel nucleotide chains.
5
Q
Nucleotides and nucleic acids
A
- Contain the elements carbon, hydrogen, oxygen, nitrogen and phosphorus (CHONP).
- Nucleic acids are large polymers formed from nucleotides linked together in a chain.
- An individual nucleotide is made up of a pentose monosaccharide (deoxyribose sugar) containing 5 carbon atoms, a phosphate group, a nitrogenous base.
- Nucleotides are joined together in condensation reactions to form polynucleotides.
- The phosphate group at the fifth carbon of the pentose sugar of one nucleotide forms a covalent bond with the hydroxyl group (OH) at the third carbon of the pentose sugar of an adjacent nucleotide. These bonds are called phosphodiester bonds. This forms a long sugar phosphate backbone with a base attatched to each sugar.
- Phosphodiester bonds are broken by hydrolysis.
6
Q
Ribonucleic acid (RNA)
A
- RNA plays an essential role in the transfer of genetic information from DNA to the proteins that make up the enzymes and tissues in the body.
- DNA is too long of a molecule to leave the cell so the relatively short section of a long DNA molecule corresponding to a single gene is transcribed into mRNA.
- RNA nucleotides are different to DNA nucleotides as the pentose sugar is ribose rather than deoxyribose and the thymine base is replaced with the base uracil.
- Uracil is a pyrimidine that forms 2 hydrogen bonds with adenine.
7
Q
Semi - conservative replication
A
- For DNA to replicate the double helix structure has to unwind and seperate into 2 strands, so the hydrogen bonds between the base pairs must be broken.
- Free DNA nucleotides will then pair with their complementary bases, which have been exposed as the strands are seperate. Hydrogen bonds are formed between them.
- Finally the new nucleotides join to their adjacent nucleotides with phosphodiester bonds.
8
Q
Roles of enzymes in replication
A
- DNA replication is controlled by enzymes.
- The seperating of the 2 strands in the double helix is carried out by the enzymes helicase. It travels along the backbone, catalysing reactions that break the hydrogen bonds between complementary base pairs as it reaches them.
- Free nucleotides pair with the newely exposed bases on the template strands during the unzipping process. The enzyme DNA polymerase catalyses the formation of phosphodiester bonds between these nucleotides.
9
Q
The properties of the genetic code
A
- The genetic code is degenerate. There are 64 codons but only 20 amino acids. So most amino acids have more than 1 codon coding for them. So the genetic code is degenerate because for most amino acids there is more than 1 codon coding for it.
- The genetic code is non overlapping. Each codon is read seperately from the codon before it and the codon after it. Codons are discrete units. This feature is important as it does not restrict the aa sequence of the protein.
- The genetic code is universal so the same specific codons code for the same amino acids in all living organisms.
10
Q
Mutations
A
- A mutation is a change in the base sequence of the DNA.
- Mutations occur randomly, either during DNA replication or from environmental causes.
- Most mutations are harmless as they occur in non coding DNA. But some can be deleterious as they occur in a gene and change the base sequence so the wrong amino acid is coded in the protein.
11
Q
Transcription
A
- First stage of protein synthesis
- The synthesis of RNA from DNA
- DNA cannot leave the nucleus as it is too large.
- Proteins are synthesized on ribosomes in the cyptoplasm therefore a copy of the gene must be made which can leave the nucleus. This is called mRNA.
- The triplet code on the DNA is transcribed to the mRNA as codons.
- Only 1 of 2 strands contains the code for the protein to be synthesised. This is the sense strand and it runs from 5’ to 3’. The other strand is complementary to the sense strand and does not code for a protein, this is the antisense strand and it acts as a template strand during transcription so that the complementary RNA strand formed carries the same base sequence as the sense strand.
- Free RNA nucleotides will base pair with complementary bases exposed on the antisense strand when the DNA unzips. The thymine base is replaced with uracil.
- Phosphodiester bonds are formed between RNA nucleotides by DNA polymerase.
12
Q
ribosomal RNA (rRNA)
A
- This and ribosomal protein make up the ribosome.
- RNA is important in maintaining the structural stability of the protein synthesis sequence and plays a biochemical role in catalysing the reaction.
- After leaving the nucleus the mRNA binds to a specific site on the small subunit of the ribosome. The ribosome holds the mRNA in position while it is decoded or translated into a sequence of amino acids.
13
Q
transfer RNA (tRNA)
A
- Necessary for the translation of mRNA.
- It is composed of a strand of RNA folded in such a way that 3 bases (anticodon) will bind to the complementary codon on mRNA.
- The tRNA molecules carry an amino acid corresponding to that codon.
- When tRNA anticodons bind to the codons the amino acids are brought together in the correct sequence to form the primary structure of the protein coded for by the mRNA.
14
Q
Translation
A
- mRNA attaches to a ribosome and transfer RNA collects amino acids from the
cytoplasm and carries them to the ribosome. tRNA is a single stranded molecule with a binding site at one end thus it can only carry one type of amino acid, and a triplet of bases at the other - tRNA attaches itself to mRNA by complementary base pairing – two molecules attach to mRNA at a time
- The amino acids attached to two tRNA molecules join by a peptide bond and then
tRNA molecules detach themselves from the amino acids, leaving them behind - This process is repeated thus leading to the formation of a polypeptide chain until a
stop codon is reached on mRNA and ends the process of protein synthesis
15
Q
Universal energy currency
A
- Cells require energy for 3 main types of activity: Synthesis, transport and movement.
- In molecules ATP (adenosine triphosphate) are able to supply this energy.
- ATP is a nucleotide.
16
Q
Structure of ATP
A
- Composed of a nitrogenous base, a pentose sugar and 3 phosphate groups.
17
Q
How ATP releases energy
A
- A small amount of energy is needed to break the bond holding the last group in ATP, however a large amount of energy is released when the liberated phosphate undergoes other reactions involving bond formation.
- ATP is hydrolysed into ADP (adenosine diphosphate) and a phosphate ion releasing energy.
- The instability of phosphate bonds in ATP means that it is not a good long term energy store.
- Fats and carbohydrates are a better energy store, the energy released in the breakdown of these molecules is used to create ATP, this occurs by reattatching a phosphate molecule to the ADP in a process called phosphorylation (a condensation reaction).
18
Q
Properties of ATP
A
- Small so moves easily into, out of and within cells.
- Water soluble = energy requiring processes happen in aqueous environemnts.
- Contains bonds between phosphates with intermediate energy so large enough to be used for cellular reactions but not large enough that energy is wasted as heat.
- Releases enerfy in small quantities.
- Easily regenerated.
19
Q
What does the enzyme RNA polymerase synthesise?
A
mRNA from a DNA template
20
Q
What would be the result of analysing part of a DNA molecule?
A
As DNA is made of nucleotides and in each nucleotide there is one phosphate, one pentose sugar and one nitrogenous base these will all be in equal proportion. As DNA is double stranded and there is complementary base pairing between the two strands there will be equal proportions of adenine and thymine, as well as equal proportions of cytosine and guanine.
