Nucleotides and Nucleic acids Flashcards
What is a nucleotide ?
- DNA and RNA are nucleic acids: polymers that are made up of many repeating units (monomers) called nucleotides
- Each nucleotide is formed from:
- A pentose sugar (a sugar with 5 carbon atoms)
- A nitrogen-containing organic base
- A phosphate group
DNA nucleotide
- DNA nucleotides - to store genetic information
- The components of a DNA nucleotide are:
- A deoxyribose sugar with hydrogen at the 2’ position
- A phosphate group
- One of four nitrogenous bases - adenine (A), cytosine(C), guanine(G) or thymine(T)
RNA nucleotide
- The components of an RNA nucleotide are:
- A ribose sugar with a hydroxyl (OH) group at the 2’ position
- A phosphate group
- One of four nitrogenous bases - adenine (A), cytosine(C), guanine(G) or uracil (U)
Why is RNA more susceptible to hydrolysis ?
- The presence of the 2’ hydroxyl group makes RNA more susceptible to hydrolysis
- This is why DNA is the storage molecule and RNA is the transport molecule with a shorter molecular lifespan
What are purines and pyridymines ?
Purines & pyrimidines
The nitrogenous base molecules that are found in the nucleotides of DNA (A, T, C, G) and RNA (A, U, C, G) occur in two structural forms: purines and pyrimidines
The bases adenine and guanine are purines – they have a double ring structure
The bases cytosine, thymine and uracil are pyrimidines – they have a single ring structure
What are the differences between DNA and RNA ?
-DNA has the bases of A,C,TG whereas RNA has A,C,U,G
- RNA has a ribose , DNA has a deoxyribose sugar
What is a phosphodiester bond ?
- DNA and RNA are polymers (polynucleotides), meaning that they are made up of many nucleotides joined together in long chains
- Separate nucleotides are joined together via condensation reactions
- These condensation reactions occur between the phosphate group of one nucleotide and the pentose sugar of the next nucleotide
- A condensation reaction between two nucleotides forms a phosphodiester bond
- It is called a phosphodiester bond because it consists of a phosphate group and two ester bonds
- The chain of alternating phosphate groups and pentose sugars produced as a result of many phosphodiester bonds is known as the sugar-phosphate backbone (of the DNA or RNA molecule)
- As the synthesis of polynucleotides requires the formation of phosphodiester bonds, the same is true for the reverse process: the breakdown of polynucleotides requires the breakage of phosphodiester bonds
What is ATP?
- Adenosine triphosphate (ATP) is the energy-carrying molecule that provides the energy to drive many processes inside living cells
- ATP is another type of nucleic acid and hence it is structurally very similar to the nucleotides that make up DNA and RNA
It is a phosphorylated nucleotide
DNA structure
- DNA molecules are made up of two polynucleotide strands lying side by side, running in opposite directions – the strands are said to be antiparallel
- Each DNA polynucleotide strand is made up of alternating deoxyribose sugars and phosphate groups bonded together to form the sugar-phosphate backbone. These bonds are covalent bonds known as phosphodiester bonds
The phosphodiester bonds link the 5-carbon of one deoxyribose sugar molecule to the phosphate group from the same nucleotide, which is itself linked by another phosphodiester bond to the 3-carbon of the deoxyribose sugar molecule of the next nucleotide in the strand - Each DNA polynucleotide strand is said to have a 3’ end and a 5’ end (these numbers relate to which carbon on the pentose sugar could be bonded with another nucleotide)
As the strands run in opposite directions (they are antiparallel), one is known as the 5’ to 3’ strand and the other is known as the 3’ to 5’ strand
Explain the hydrogen bonds
-The two antiparallel DNA polynucleotide strands that make up the DNA molecule are held together by hydrogen bonds between the nitrogenous bases
-These hydrogen bonds always occur between the same pairs of bases:
-The purine adenine (A) always pairs with the pyrimidine thymine (T) – two hydrogen bonds are formed between these bases
-The purine guanine (G) always pairs with the pyrimidine cytosine (C) – three hydrogen bonds are formed between these bases
-This is process is known as complementary base pairing and the pairs are known as complementary base pairs
What is the DNA structure ?
Double helix
What type of replication takes place (DNA)?
Semi - conservative replication
What is meant by the term ‘ semi- conservative replication’ ?
- The process is called this because in each new DNA molecule produced, one of the polynucleotide DNA strands (half of the new DNA molecule) is from the original DNA molecule being copied
- The other polynucleotide DNA strand (the other half of the new DNA molecule) has to be newly created by the cell
- Therefore, the new DNA molecule has conserved half of the original DNA and then used this to create a new strand
What is the importance of retaining one original DNA strand ?
- Retaining one original DNA strand ensures there is genetic continuity (i.e. genetic information is conserved) between generations of cells
- In other words, it ensures that the new cells produced during cell division inherit all their genes from their parent cells
- This is important because cells in our body are replaced regularly and therefore we need the new cells to be able to do the same role as the old ones
- Replication of DNA and cell division also occurs during growth
What stage of the cell cycle does DNA replication take place ?
DNA replication occurs during the S phase of the cell cycle (which occurs during interphase, when a cell is not dividing)
What happens in DNA replication ?
-The enzyme DNA helicase unwinds the DNA double helix by breaking the hydrogen bonds between the base pairs on the two antiparallel polynucleotide DNA strands to form two single polynucleotide DNA strands. —The enzyme DNA gyrase unwinds the DNA
- The double helix structure unzips and unwinds
- There are free DNA nucleotides in the nucleus.
- The bases of the free floating nucleotides complimentary base pair with nucleotides on DNA template A-T,C-G (The Adenine - Thymine base pair is held together by 2 hydrogen bonds while the Guanine - Cytosine base pair is held together by 3 hydrogen bonds. )
- The new nucleotides are then joined together by the enzyme DNA polymerase which catalyses condensation reactions to form a new strand
- The original strand and the new strand join together through hydrogen bonding between base pairs to form the new DNA molecule
- This method of replicating DNA is known as semi-conservative replication because half of the original DNA molecule is kept (conserved) in each of the two new DNA molecules
What is DNA polymerase ?
-The enzyme DNA polymerase synthesises new DNA strands from the two template strands
-It does this by catalysing condensation reactions between the deoxyribose sugar and phosphate groups of adjacent nucleotides within the new strands, creating the sugar-phosphate backbone of the new DNA strands
-DNA polymerase cleaves (breaks off) the two extra phosphates and uses the energy released to create the phosphodiester bonds (between adjacent nucleotides)
What is a mutation ?
- Although the process is astonishingly accurate considering it is happening constantly in cells and at a considerable speed, occasional mistakes occur in the form of:
- Bases being inserted into the complementary strand in the wrong order
- An extra base being inserted by accident
- A base being left out by accident
- These mistakes in the process of semi-conservative replication of DNA result in the occurrence of random, spontaneous mutations (i.e. errors in the genetic code)
What is a gene ?
- A gene is a sequence of nucleotides that forms part of a DNA molecule (one DNA molecule contains many genes)
Do genes determine protein structure?
- The genes in DNA molecules, therefore, control protein structure (and as a result, protein function) as they determine the exact sequence in which the amino acids join together when proteins are synthesised in a cell
What is a triplet code ?
-The sequence of DNA nucleotide bases found within a gene is determined by a triplet (three-letter) code.
-Each sequence of three bases (i.e. each triplet of bases) in a gene codes for one amino acid
- These triplets codes for different amino acids – there are 20 different amino acids that cells use to make up different proteins.
For example:
-CAG codes for the amino acid valine
-TTC codes for the amino acid lysine
- Some of these triplets of bases code for start (TAC – methionine) and stop signals
- These start and stop signals tell the cell where individual genes start and stop
- As a result, the cell reads the DNA correctly and produces the correct sequences of amino acids (and therefore the correct protein molecules) that it requires to function properly
What are the three important things about the genetic code ?
- non-overlapping
- degenerate
- universal
What is meant by non-overlapping ?
-Each base is only read once in which codon it is part of (don’t overlap)
What is meant by degenerate ?
-Multiple codons can code for the same amino acids
-The degenerate nature of the genetic code can limit the effect of mutations
