2.1.3 - Nucleotides and Nucleic Acids Flashcards
What is a nucleotide?
The monomer from which nucleic acids are made
What is the structure of a nucleotide?
- Pentose sugar
- Nitrogen containing base
- Phosphate group
What are purines?
2 carbon ring structures
Which bases are purines?
- Adenine
- Guanine
What are pyrimidines?
1 carbon ring structures
Which bases are pyrimidines?
- Cytosine
- Thymine
- Uracil
What is the structure of a DNA nucleotide?
- Deoxyribose sugar
- Phosphate group
- Organic base (A,T,C or G)
What is the structure of a RNA nucleotide?
- Ribose sugar
- Phosphate group
- Organic base (A,U,C or G)
What does a condensation reaction between two nucleotides form?
A phosphodiester bond to form a polynucleotide
What does a hydrolysis reaction between two nucleotides do?
It breaks the phosphodiester bond between them
What is the structure of ATP?
- Ribose sugar
- Adenine
- 3 phosphate groups
What is the structure of ADP?
- Ribose sugar
- Adenine
- 2 phosphate groups
What is the structure of DNA?
- Double stranded molecule that twists into a double helix
- Two sugar phosphate backbones held in place by pairs of complementary bases (A to T and C to G) joined by hydrogen bonds
- The two DNA strands are anti parallel running in opposite directions
How is the structure of RNA different to DNA?
It is shorter and is a single stranded molecule
How is DNA purified?
- Blend the DNA source from fruit or vegetable into a ‘soup’ and then add salt to remove DNA bound proteins and ice cold water
- Pass through a mesh and collect the liquid
- Add detergent to help disrupt the cell membrane and mix
- Add protease enzymes to hydrolyse the proteins
- Pour solution into a test tube and add ice cold ethanol at a 45° angle to cause the DNA to precipitate
- The DNA appears as white strands
Why is DNA replication described as semi-conservative?
In replication one strand is conserved and one new strand is created
How does semi-conservative DNA replication occur?
- DNA helicase unwinds the double helix and separates the two strands by breaking the hydrogen bonds between the complementary base pairs
- New DNA nucleotides align opposite to exposed complementary bases on the DNA template strand and hydrogen bonds form between them
- DNA polymerase catalyses the condensation reaction that joins the adjacent nucleotides together forming a phosphodiester bond and forming new strands
- The daughter DNA molecules each rewind into a double helix
Why is replication important?
It conserves genetic information with accuracy but copying errors can occur randomly and spontaneously resulting in a change to the DNA base sequence known as a mutation
What is the genetic code?
It is carried as a sequence of three DNA bases called a triplet or codon which code for a specific amino acid or code for STOP during transcription
What are the 3 key features of the genetic code?
- Universal - same triplet of bases code for the same amino acid in all organisms
- Non-overlapping - each base in a gene is only part of one triplet of bases that codes for one amino acid and each codon is read as a discrete unit
- Degenerate - amino acids are coded for by more than one triplet code
What is a gene?
A sequence of triplets specifying the order of amino acids of a polypeptide or protein
What is protein synthesis?
The mechanism by which a DNA template is transcribed into a messenger RNA (mRNA) molecule in the nucleus. The mRNA is then translated into an amino acid sequence in association with transfer RNA (tRNA) on ribosomes in the cytoplasm
What are introns?
Sequences of bases in a gene that do not code for amino acids and therefore polypeptide chain. These get spliced out of mRNA molecules after transcription
What are exons?
Sequences of bases in a gene that code for a sequence of amino acids
What is the process of transcription?
- DNA helicase breaks hydrogen bonds between bases causing DNA to unzip
- Free mRNA nucleotides align opposite exposed complementary DNA bases
- The enzyme RNA polymerase joins together the adjacent RNA nucleotides forming phosphodiester bonds to create a new mRNA polymer chain
- Once one gene is copied the mRNA is modified and then leaves the nucleus through the nuclear envelope pores
What is the process of translation?
- mRNA moves from the nucleus through a nuclear pore to the cytoplasm and the start codon attaches to a ribosome
- A tRNA with a complementary anticodon carrying a specific amino acid moves to the ribosome and pairs with the first mRNA codon
- The ribosome moves along the mRNA to the next codon and again pairs up with a complementary tRNA to bring the two amino acid carrying tRNAs together
- Energy released from ATP is used to form a peptide bond between the amino acids
- The ribosome moves to the third mRNA codon releasing the first tRNA and pairing up a third
- When the ribosome reaches a stop codon the polypeptide is complete and the mRNA and tRNAs are released from the ribosome
