Unit 2: Section 2

DNA

• DNA (deoxyribonucleic acid) contains your genetic information.
• The DNA molecules are really long and are coiled up very tightly, so a lot of genetic information can fit into a small space in the cell nucleus.
• DNA molecules have a paired structure, which makes it easier to copy itself. This is called self-replication. It's important for cell division and for passing genetic information from generation to generation.
• DNA contains genes - sections of DNA that code for a specific sequence of amino acids that forms a particular protein.
• The nucleic acid RNA (ribonucleic acid) is similar in structure to DNA. It's used to make proteins from the instructions contained within DNA.
• DNA is a polynucleotide - it's made up of lots of nucleotides joined together.
• Each nucleotide is made from a deoxyribose sugar, a phosphate group and a nitrogen containing base.
• Each nucleotide has the same sugar and phophate. The base on each nucleotide can vary.
• There are gour possible bases - adenine (A), thymine (T), cytosine (C) and guanine (G).
• A and G are purine, and C and T are pyrimidines.

• DNA nucleotides join together to form polynucleotide strands.
• The nucleotides join up between the phosphate group of one nucleotide and the sugar of another.
• Two DNA polynucleotide strands join together by hydrogen bonding between the bases.
• Each base can only join with one particular partner - this is called complementary base pairing.
• Adenine always pairs with thymine (A-T) and guanine always pairs with cytosine (G-C). A purine always pairs with a pyrimidine.
• Two hydrogen bonds form between A and T, and three hydrogen bonds form between C and G.
• Two antiparallel polynucleotide strands twist to form the DNA double-helix.

Similarites and differences between RNA and DNA

• RNA and DNA are both made from nucleotides containing sugar, nitrogen-containing bases and phosphate.
• Each nucleotide in both RNA and DNA contains one of four different bases.
• Also, the nucleotides form a polynucleotide strand that is joined up between the sugar of one nucleotide and tyhe phosphate of another.
• The sugar in RNA nucleotides is a ribose sugar (not deoxyribose).
• The nucleotides form a single polynucleotide strand (not a double one).
• Uracil (a pyrimidine) replaces thymine as a base. Uracil always pairs with adenine in RNA.

Self-replication

DNA copies itself before cell division so that each new cell has the full amount of DNA.

1. The hydrogen bonds between the two polynucleotide DNA strands break. The helix unzips to form two single strands.

2. Each original single strand acts as a template for a new strand. Free-floating DNA nucleoides join to the exposed bases on each original template strand by complementary base pairing - A with T and C with G.

3. The nucleotides on the new strand are joined together by the enzyme DNA polymerase. Hydrogen bonds form between the bases on the original and new strand.

4. Each new DNA molecule contains one strand from the original DNA molecule and one new strand.

This type of copying is called semi-conservative replication because half of the new strands of DNA are from the original piece of DNA.

DNA is copied into RNA for protein synthesis

• All the reactions and processes in living organisms need proteins.
• DNA carries the instructions to make proteins (as genes). It is found in the nucleus.
• The organelles that make proteins (ribosomes) are found in the cytoplasm. But the DNA molecules are too large to move out of the nucleus.
• Instead, sections of DNA are copied into RNA.
• The RNA leaves the nucleus and joins with a ribosome in the cytoplasm, where it can be used to synthesise a protein.
• DNA and RNA are vital for living organisms to produce proteins in order to grow and develop.