Nucleotides and nucleic acids

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  • Created by: AmyBennet
  • Created on: 17-02-17 11:53
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  • Module 2 Foundations in Biology: 2.1.3 Nucleotides and nucleic acids
    • 3.8 Nucleic acids
      • The structure of a nucleotide
        • Phosphate group, (deoxy)ribose sugar, and a nitrogen base
      • Nitrogen bases
        • Purine
          • Adenine (A)
          • Guanine (G)
          • The larger bases, which contain double carbon ring structures
        • Pyrimidine
          • Cytosine (C)
          • Thymine (T)
          • Uracil (U)
          • The smaller bases which have single cabon ring structures
        • In DNA, only A,  G, C, and T are present
        • In RNA, only A, G, C, and U are present
        • Base pairs
          • Adenine makes a base pair with thymine (2 hydrogen bonds) and uracil (2 hydrogen bonds)
          • Guanine makes a base pair with cytosine (3 hydrogen bonds)
      • Deoxyribose and ribose
        • Deoxyribose is a ribose sugar with one fewer oxygen
        • Deoxyribose is the sugar in DNA wheras ribose is he sugar in RNA
      • The synthesis and breakdown of poly-nucleotides
        • Nucleotides link together by condensation reactions to form poly-nucleotides
        • The phosphate group at the 5th carbon (5') of one nucleotide forms a covalent bond with the hydroxyl group at the 3rd carbon (3') of an adjacent nucleotide
          • These bonds are called phosphodiester bonds
    • 3.9 DNA replication and the genetic code
      • DNA structure
        • The DNA double helix is made up of two strands of nucleotides coiled into a helix
        • The two strands are held together by hydrogen bonds between the bases. Each strand has phosphate group (5') at one end and a hydroxyl group (3") at the other
          • The two strands run in opposite directions- antiparallel
        • The base pairing allows DNA to be copied. Complimentary base pairing means that there are equal amounts of A and T and equal amounts of C and G
      • The process of DNA replication
        • 1. The double helix unwinds. 2. Hydrogen bonds between the bases are broken apart to 'unzip' the DNA to form two single strands, exposing the bases.
        • 3. Hydrogen bonds form between free DNA nucleotides and exposed bases through complementary base pairing.
        • 4. Covalent bonds are formed between the phosphate of one nucleotide and the sugar of the next to seal the backbone using the enzyme DNA polymerase.
        • 5. Each new DNA molecule consists of one conserved strand plus one newly built strand. This process of DNA replication is described as semi-conservative replication.
      • Roles of enzymes in DNA replication
        • DNA helicase
          • Unwinds and separates the two DNA strands. It travels along the DNA backbone catalysing reactions that break the hydrogen bonds between complimentary base pairs
        • DNA polymerase
          • Catalyses the formation of phosphodiester bonds between the free nucleotides and the exposed bases
      • Random mutation
        • Sequences of bases aren't always matched exactly, an incorrect sequence may occur in a newly-copied strand
      • Triplet code
        • DNA carries a sequence of bases. Each sequence of three bases is called a codon which codes for an amino acid.
        • A section in DNA which contains enough codons for an entire protein is called a gene
        • The genetic code is universal- all organisms use the same code although the sequences of bases coding for each protein will be diferent
      • Degenerate code
        • There are 64 possible codons. There is one codon which codes for start and three which code for stop
          • Having stop and start codons helps to prevent codons from being read twice and limits overlapping
    • 3.10 Protein synthesis
      • Transcription
        • Occurs in nucleus, catalysed by RNA polymerase
        • DNA helix is unwound by helicase and the hydrogen bonds are broken
        • Floating RNA nucleotides form pairs with the exposed bases on the template strand (antisense strand) of DNA
        • 3 bases on the DNA are transcibed into 3 bases on the RNA
        • The mRNA molecule formed enters the cytoplasm through a nuclear pore
      • Translation
        • Occurs on the ribosomes of the rough ER
        • Beginning of the equence is marked with a start codon
        • A tRNA molecule with 3 exposed bases (anticodon) pairs with a specific codon o the mRNA
        • A specific amino acid is attached to the tRNA molecule
        • The amino acids are joined with peptide bonds to form a polypeptide
        • A stop codon signals the last amino acid in the polypeptide chain
      • RNA
        • mRNA
          • messenger RNA, made as a strand complimentary to the template strand so is therefore a copy of the coding strand of the double helix
        • rRNA
          • Ribosomal RNA, A component of rimbosomes
        • tRNA
          • Transfer RNA, carries amino acids to the ribosomes where they are bonded together to form polypeptides
      • The process of protein synthesis
        • 1. The double helix is unwound by helicase
        • 2. mRNA is made using RNA nucleotides
        • 3. The mRNA detaches from the DNA and leaves the nucleus through a nuclear pore
        • 4. The mRNA attaches to a ribosome
        • 5. The tRNA molecules in the cytoplasm are attached to a specific amino acid. The 3 bases bonded to the tRNA is the anticodon and the 3 bases bonded to the mRNA is the codon
        • 6. The tRNA with the appropriate bases binds to the mRNA strand
        • 7. The amino acids are joined together by peptide bonds in a condensation reaction to give a polypeptide
    • 3.11 ATP
      • ATP stands for adenosine triphosphate. It has a ribose sugar, a nitrogen base of adenine, and 3 phosphate groups. ADP has two phosphate groups
      • Energy is stored in bonds between phosphates
      • Instability of bonds mean it isn't a good long-term energy store
      • Rapidly formed by phosphorylation of ADP
      • Small so can easily move in, out, and within cells
      • Water soluble- energy requiring processes happen in aqueous environments
      • The bonds contain enough energy for cellular reactions but not so much that some is wasted as heat
      • Energy is released in small quantities
      • Easily regenerated

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