AS Biology Chapter 2.1 Nucleic Acids

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Chapter 2.1 ­ Nucleic Acids ­ The molecules of life
Within every cell is a set of instructions for the assembling of new cells, both to form offspring and
to produce identical cells for growth.
Nucleic acids are the information molecules of the cell. They carry information needed to form new
cells, in the chromosomes of the nucleus of the cell.
Nucleic acids take the form of a code in the molecules of DNA. Parts of the code are copied into one
form of RNA, then to another form and finally used to make proteins that build up the cell and
control its actions.
RNA and DNA are polymers. Their single monomer units are called nucleotides or mononucleotides.
Each mononucleotide contains 5-carbon, or pentose sugars, a nitrogen containing base and
phosphoric acid.
There are two types of nitrogen containing bases found in nucleic acids. The purine bases have two
nitrogen containing rings while the pyrimidine bases only have one.
DNA contains a combination of 4 different bases with equal numbers of pyrimidine's and purines.
The purines are adenine (A) and guanine (G), and the pyrimidines are cytosine (C) and thymine (T).
RNA contains the purines adenine (A) and guanine (G) and the pyrimidines cytosine (C) and uracil (U)
Inorganic phosphate ions (PO³¯) are present in the cytoplasm of every cell. It makes the
mononucleotides, and hence the nucleotides, acidic.
The sugar, the base and the phosphate group are joined together by condensation reactions (2
water molecules are removed) to form the nucleotide.
Mononucleotides undergo condensation reactions to form polynucleotides. The sugar of one
nucleotide bonds with the phosphate group of the next nucleotide, so the polynucleotides always
have a hydroxyl group at one end and a phosphate group at the other end.
RNA molecules form single polynucleotide strands which may be folded into complex shapes or
remain as long thread-like molecules.
A DNA molecule is made up of 2 nucleotides strands twisted around each other. A purine always
pairs with a pyrimidine so in DNA, adenine pairs with thymine and cytosine pairs with guanine. This
forms a double helix.
The two strands of the double helix are held together by hydrogen bonds between the
complimentary base pairs. There are ten pairs for each complete twist of the helix.
When the DNA replicates, the two strands of the DNA molecule `unzip' along the line of hydrogen
bonds and unravels. The exposed bases attract free DNA nucleotides and new hydrogen bonds are
formed between matching base pairs. Two new strands of DNA, identical with the original, are

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Meselson and Stahl's experiment supports the theory of semiconservative replication. They grew
several generations of the gut bacteria in a medium where there only source of nitrogen was the
radioactive isotope ¹N. The bacteria used ¹N to make cell chemicals, including proteins and DNA.
The bacteria was then moved to a medium containing normal ¹N, as their only source of nitrogen.…read more

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Chromosomal mutations ­ changes in the position of genes within the chromosomes.
Whole-chromosome mutations ­ An entire chromosome is either lost during meiosis or duplicated
in one cell by errors in the process.
Exposure to mutagens such as X-rays, ionising radiation and certain chemicals, increases the rate at
which mutations occur.
Reactions that build up new chemicals are known as anabolic reactions.
Reactions that break substances down are known as catabolic reactions.
Both these processes are known as a metabolism.…read more

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Between 0-40°C, the Q for any reaction is 2. This shows that every 10°C, the rate of the reaction
doubles. At 40°C the enzymes start to lose their tertiary and quaternary structures so they become
Different enzymes work best at different pH levels. Changes in pH affect the formation of the
hydrogen bonds and sulphur bridges that hold together the enzyme.…read more


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