Chapter 3- 3.8- Nucleic acids Flashcards
What are nucleic acids?
Large molecules that were discovered in cell nuclei, hence the name. There are two types of nucleic acid. DNA and RNA, both have roles in the storage and transfer of genetic information and the synthesis of polypeptides (proteins). They are the basis for heredity.
What elements do nucleic acids contain?
Carbon hydrogen oxygen nitrogen phosphorus
Nucleic acid are large polymers formed from many nucleotides (monomers) linked together in a chain.
What are the three components an individual nucleotide is made up of?
1) A pentose monosaccharide (sugar), containing five carbon atoms.
2) A phosphate group, -PO(4)^2-, an inorganic molecule that is acidic and negatively charged.
3) A nitrogenous base- a complex organic molecule containing one or two carbon rings in tis structure as well as nitrogen.
Nucleotides are liked together by condensation reactions to form a polymer called a polynucleotide.
The phosphate group at the fifth carbon of the pentose sugar (5’) of one nucleotide forms a covalent bond with the hydroxyl (OH) group to 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 the individual nucleotides.
What is (DNA) Deoxyribonucleic acid?
It is a sugar with one fewer oxygen atoms than ribose (deoxyribose).
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 and purines.
What is the difference between a pyrimidine and a purine?
A pyrimidine is the smaller bases, which contain single carbon ring structures- thymine (T) and cytosine (C).
Whereas purines are the larger bases, which contain double carbon ring structures- adenine (A) and guanine (G).
What is 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 bases, much like the 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 so is said to be antiparallel.
The pairing between the bases allow DNA to be copied and transcribed- key properties required of the molecule of heredity.
What are the base pair rules?
The bases bind in a very specific way.
Adenine and thymine are both able to from two hydrogen bonds and are always joined with each other.
Cytosine and guanine form three hydrogen bonds and so also only bind to each other. This is known as complementary base paring.
These rules mean that a small pyrimidine base and a large purine base always bind together. This arrangement maintains a constant distance between the DNA “backbone”, resulting in parallel polynucleotides chains.
Complementary base pairing means that DNA always has equal amounts of adenine and thymine and equal amounts of cytosine and guanine.
This was known long before the detailed structure of DNA was determined by Watson and Crick in 1953.
It is the sequence of bases along the DNA strand that carries the information of an organism I the form of a code. In the next topic we will examine how the sequence of bases “codes” for the sequence of amino acids that are needed to make different proteins.
What is ribonucleic acid (RNA)?
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 the organisms, which is passed on from generation to generation.
However the DNA of each eukaryotic chromosome is a very long molecule, 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.
The relatively short section of the long DNA molecule corresponding to a single gene is transcribed into a similarly short messenger RNA (mRNA) molecule.
Each individual mRNA is therefore much shorter than the whole chromosome of DNA.
It is a polymer composed of many nucleotide monomers.
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 (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 DNA to make copies of particular sections of DNA.
The RNA nucleotides form polymers in the same way as DNA nucleotides- by the 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 nulceotides are released and reused.
