chapter 3 p5 Flashcards
(36 cards)
Nucleic acids are
large molecules that were discovered in cell nuclei - hence their name.
There are two types of nucleic acid - DNA and RNA, and both have roles in the storage and transfer of genetic information and the synthesis of polypeptides (proteins).
They are the basis for heredity.
Nucleic acids contain the elements
carbon, hydrogen, oxygen, nitrogen and phosphorus.
They are large polymers formed from many nucleotides (the 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, -PO,, an inorganic 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.
Nucleotides are linked 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 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 the individual nucleotides.
Deoxyribonucleic acid (DNA):
the sugar in deoxyribonucleic acid (DNA) is deoxyribose - a sugar with one fewer oxygen atoms than ribose, as shown in Figure 3.
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:
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).
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, much like the rungs of a ladder.
- Each strand has a phosphate group (5’) at one end and a hydroxy! 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:
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- 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 that a small pyrimidine base always binds to a larger purine base.
Base pairing rules:
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- 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.
- 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 a DNA strand that carries the genetic information of an organism in the form of a code.
Ribonucleic acid (RNA): p1
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 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.
Ribonucleic acid (RNA): p2
To get around this problem, 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 (Figure 3) and the thymine base is replaced with the base uracil (U) (see Figure 9).
Ribonucleic acid (RNA): p3
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 nucleotides are released and reused.
DNA extraction:
DNA can be extracted from plant material using the following procedure: p1
Grind sample in a mortar and pestle - this breaks down the cell walls.
Mix sample with detergent - this breaks down the cell membrane, releasing the cell contents into solution As it disrupts membrane structure; phospholipids form suspension in aqueous solution
Add salt - this breaks the hydrogen bonds between the DNA and water molecules.
Add protease enzyme - this will break down the proteins associated with the DNA in the nuclei.
DNA extraction:
DNA can be extracted from plant material using the following procedure: p2
Add a layer of alcohol (ethanol) on top of the sample - alcohol causes the DNA to precipitate out of solution.
The DNA will be seen as white strands forming between the layer of sample and layer of alcohol (Figure 9). The DNA can be picked up by spooling it onto a glass rod.
The temperature needs to be kept low throughout this procedure to reduce activity of enzymes (1); reduce breakdown of DNA (1)
DNA replication process
Cells divide to produce more cells needed for growth or repair of tissues.
The two daughter cells produced as a result of cell division are genetically identical to the parent cell and to each other. In other words, they contain DNA with a base sequence identical to the original parent cell.
When a cell prepares to divide, the two strands of DNA double helix separate and each strand serves as a template for the creation of a new double-stranded DNA molecule.
The complementary base pairing rules ensure that the two new strands are identical to the original.
This process is called DNA replication.
Semi-conservative replication:
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For DNA to replicate, the double helix structure has to unwind and then separate into two strands, so the hydrogen bonds holding the complementary bases together must be broken (Figure 1c).
Free DNA nucleotides will then pair with their complementary bases, which have been exposed as the strands separate.
Hydrogen bonds are formed between them.
Semi-conservative replication:
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Finally, the new nucleotides join to their adjacent nucleotides with phosphodiester bonds (Figure 1d).
In this way, two new molecules of DNA are produced. Each one consists of one old strand of DNA and one new strand.
This is known as semi-conservative (meaning half the same) replication.
Roles of enzymes in replication: p1
DNA replication is controlled by enzymes, a class of proteins that act as catalysts for biochemical reactions.
Enzymes are only able to carry out their function by recognising and attaching to specific molecules or particular parts of the molecules.
Before replication can occur, the unwinding and separating of the two strands of the DNA double helix is carried out by the enzyme DNA helicase.
Roles of enzymes in replication: p2
It travels along the DNA backbone, catalysing reactions that break the hydrogen bonds between complementary base pairs as it reaches them.
This can be thought of as the strand ‘unzipping’.
Free nucleotides pair with the newly exposed bases on the template strands during the ‘unzipping’ process.
A second enzyme, DNA polymerase catalyses the formation of phosphodiester bonds between these nucleotides.
