Nucleotides and nucleic acids
- 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)
- Purine
- 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
- The structure of a nucleotide
- 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
- DNA helicase
- 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
- There are 64 possible codons. There is one codon which codes for start and three which code for stop
- DNA structure
- 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
- mRNA
- 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
- Transcription
- 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
- 3.8 Nucleic acids
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