Nucleotides and Nucleic acids

• A nucleotide is a type of biological molecule and is made from a pentose sugar (ribose or deoxyribose), a base and a phosphate group
• Nucleotides are the monomers that make up DNA or RNA
• ADP and ATP are special types of nucleotide and are used to store and transport energy
• The pentose sugar in DNA is called deoxyribose
• The four possible bases are adenine, thymine, cytosine and guanine
• Adenine and guanine are a type of purine.
• Cytosine and thymine are a type of base called pyrimidine
• A purine base contains two carbon-nitrogen rings joined together
• A pyrimidine base only has one carbon-nitrogen ring, it is smaller than a purine base
• A DNA molecule contains two polynucleotide chains
• RNA contains nucleotides with a ribose sugar
• In RNA, uracil replaces thymine as a base
• An RNA molecule is made of a single polynucleotide chain
• ADP contains the base adenine, the sugar ribose and two phosphate groups
• ATP contains the base adenine, the sugar ribose and three phosphate groups
• ATP is synthesised from ADP and inorganic phosphate using the energy from an energy-releasing reaction. The ADP is phosphorylated to form ATP and a phosphate bond is formed
• Energy is stored in the phosphate bond and then ATP is broken back down into ADP and inorganic phosphate. Energy is released from the phosphate bond and used by the cell
• Nucleotides join together to form polynucleotides. The nucleotides join up between the phosphate group of one nucleotide and the sugar of another via a condensation reaction, this then forms a phosphodiester bond
• The chain of sugars and phosphates is known as the sugar-phosphate backbone
• Polynucleotides can be broken down into nucleotides again by breaking the phosphodiester bonds using hydrolysis reactions
• Two DNA polynucleotide strands join together by hydrogen bonding between the bases, each base can only join with one particular partner (complementary base pairing).
• Adenine always pairs with thymine and cytosine always pairs with guanine. 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
• DNA copies itself before cell division so that each new cell has the full amount of DNA
• 1. DNA helicase breaks hydrogen bonds between the two polynucleotide DNA strands. The helix unzips to form two single strands
• 2) Each original single strand acts as a template for a new strand. Free floating DNA nucleotides join to the exposed bases on each original template strand by complementary base pairing. A with T and C with G
• 3) The nucleotides of the new strand are joined together by the enzyme DNA polymerase, this then forms the sugar-phosphate backbone. Hydrogen bonds form between the bases on the original and new strand. These strands twist to form a double-helix
• 4) Each new DNA molecule contains one strand from the original DNA molecule and one new strand
• In semi-conservative replication half of the strands in each new DNA molecule are from the original piece of DNA
• DNA replication is really accurate to make sure genetic information is conserved each time the DNA in a cell is replicated
• Every so often a random mutation occurs. A mutation is any change to the DNA base sequence, they can alter the sequence of amino acids in a protein. This can cause an abnormal protein to be produced.
• A gene is a sequence of DNA nucleotides that code for a polypeptide, the sequence of amino acids in a polypeptide forms the primary structure of a protein
• Different proteins have a different number and order of amino acids
• Its the order of nucleotide bases in a gene that determines the order of amino acids in a particular protein
• Each amino acid is coded for by a sequence of three bases in a gene. Different sequences of bases code for different amino acids. So the sequence of bases in a section of DNA is a template thats used to make proteins during protein synthesis
• DNA molecules are found in the nucleus of the cell but the ribosomes are found in the cytoplasm. DNA is too large to move out of the nucleus so a section is copied into MRNA (transcription). The mRNA leaves the nucleus and joins with a ribosome in the cytoplasm where it can be used to synthesise a protein (translation)
• Three main types of RNA: messenger RNA(mRNA), transfer RNA (tRNA), Ribosomal RNA (rRNA)
• Messenger RNA is made in the nucleus, three adjacent bases are called a codon, it carries the genetic code from the DNA in the nucleus to the cytoplasm, where its used to make a protein during translation
• Transfer RNA is found in the cytoplasm, has an amino acid binding site at one end and a sequence of three bases at the other end called an anticodon. It carries the amino acids that are used to make proteins to the ribosomes during translation