Nucleic Acids Flashcards
What are nucleic acids
Large molecules that were discovered in cell nuclei- there are two types of nucleic acid- DNA and RNA, and both have roles in the storage and transfer processes of genetic information and the synthesis of polypeptides (proteins). They are the basis for heredity.
What 5 elements do nucleic acids contain
Carbon, hydrogen, oxygen, nitrogen and phosphorus
What are nucleic acids made up of
They are large polymers formed from many nucleotides (monomers) linked together in a chain.
An individual nucleotide is made up of three components:
-a pentose monosaccharide (sugar) containing five carbon atoms.
-a phosphate group, an organic molecule that is acidic and negatively charged.
-a nitrogenous base-a complex organic molecule containing one or two carbon rings in its structure as well as nitrogen.
How are nucleotides linked together
By condensation reactions to form a polymer called a nucleotide. The phosphate group at the fifth carbon of the pentose sugar (5’) of one nucleotide forms a covalent bond with the hydroxyl (OH) group at the third carbon (3’) of the pentose sugar of an adjacent nucleotide. These bonds are called phosphodiester bonds. This forms a long, strong sugar-phosphate ‘backbone’ with a base attached to each sugar. The phosphodiester bonds are broken by hydrolysis, the reverse of condensation, releasing individual nucleotides.
What is deoxyribonucleic acid (DNA)
Sugar in deoxyribonucleic acid (DNA) is deoxyribose-a sugar with one fewer oxygen atoms than ribose.
How many bases are there in nucleotides
The nucleotides in DNA each have one of four different bases. This means there are four different DNA nucleotides. The four bases can be divided into two groups:
-pyrimidines-the smaller bases, which contain single carbon ring structures- thymine (T) and cytosine (C)
-purines- the larger bases, which contain double carbon ring structures- adenine (A) and guanine (G)
Complementary base pairs
The comparative sizes of pyrimidines and purines is due to the presence of either a single ring or a double ring. The complementary pair thymine and adenine form TWO hydrogen bonds and the complementary pair cytosine and guanine form THREE hydrogen bonds and purines pair with pyrimidines.
The double helix
The DNA molecule varies in length from a few nucleotides to millions of nucleotides. It is made up of two strands of polynucleotides coiled into a helix, known as the DNA double helix.
The two strands of the double helix are held together by hydrogen bonds between the bases, like rungs of a ladder. Each strand has a phosphate group (5’) at one end and a hydroxyl group (3’) at the other end. The two parallel strands are arranged so that they run in opposite directions-they are said to be antiparallel.
The pairing between the bases allows DNA to be copied and transcribed-key properties required of the molecule of heredity.
Base pairing rules
Adenine and thymine are both able to form two hydrogen bonds and always join with each other. Cytosine and guanine form three hydrogen bonds and so also only bind to each other. This is known as complementary base pairing.
These rules mean small pyrimidine bases always bind to large purine bases. This arrangement maintains a constant distance between the DNA ‘backbones’ resulting in parallel polynucleotide chains.
Complementary base pairing means that DNA always has equal amounts of adenine and thymine and equal amounts of cytosine and guanine.
It is the sequence of bases along a DNA strand that carries the genetic information of an organism in the form of a code.
Ribonucleic acid (RNA
Plays an essential role in the transfer of genetic information from DNA to the proteins that make up the enzymes and tissues of the body. DNA stores all of the genetic information needed by an organism, which is passed on from generation to generation. However the DNA of each eukaryotic chromosome is a very long molecules, comprising many hundreds of genes, and is unable to leave the nucleus in order to supply the information directly to the sites of protein synthesis.
To get around this problem the relatively short section of the long DNA molecule corresponding to a single gene is transcribed into a similar short messenger RNA (mRNA) molecules. Each individual mRNA is therefore much shorter than the whole chromosome of DNA. It is a polymer composed of many nucleotide monomers.
How are RNA nucleotides different to DNA nucleotides
Penrose sugar ribose rather than deoxyribose and the thymine base is replaced with the base uracil (U). Like thymine, uracil is a pyrimidine that forms two hydrogen bonds with adenine. Therefore the base pairing rules still apply when RNA nucleotides bind to the DNA to make copies of particular sections of DNA.
The RNA nucleotides form polymers in the same way as DNA nucleotides- by formation of phosphodiester bonds in condensation reactions. The RNA polymers formed are small enough to leave the nucleus and travel to the ribosomes, where they are central in the process of protein synthesis.
After protein synthesis the RNA molecules are degraded in the cytoplasm. The phosphodiester bonds are hydrolysed and the RNA nucleotides are released and reused.
