Biological molecules - NUCLEIC ACIDS AND DNA REPLICATION Flashcards
Why is DNA said to be degenerative, universal and non-overlapping?
- Degenerative: Amino acid can be coded for by more than one DNA triplet
- Universal: DNA triplets code for the same amino acids in [all] organisms
- Non-overlapping: DNA triplets read only once (in groups of 3)
Describe the structure of DNA (6 marks)
- Polymer of nucleotides
- Nucleotides are made of phosphate, deoxyribose sugar + base
- Nucleotides joined by phosphodiester bonds in a condensation reaction
- 4 bases: purines (2 rings) paired with pyrimidine (1 rings) [guanine + cytosine, adenine + thymine]
- Bases are joined by hydrogen bonds
- Antiparallel strands
- Coiled into a double helix
Why does DNA form a double helix?
- Stable configuration; not straining bonds
- Negative phosphates repel each other on the outside
- Bases lie flat in the centre, filling the space
What is the function of DNA?
- Instructions for making proteins and RNA’s
- Information storage (preserved for a long time without changes)
How does the structure of DNA relate to its function?
Double stranded - allows splitting and replication to make 2 identical double helixes//allows for errors to be identified during copying (decreased mutation)
Bases are inside the negative backbone - protects bases from undergoing chemical reactions ( decreased mutation)
How is RNA formed?
By a condensation reaction between 2 nucleotides, resulting in the removal of water and the formation of a phosphodiester bond.
What is the function of messenger RNA?
Copy of the code for 1 protein
- transferring genetic info to the ribosome (short term, can make more)
What is the function of ribosomal RNA?
protein synthesis and rRNA’s
What is the function of transfer RNA?
Transferring the correct amino acids to the ribsomes for protein synthesis
Describe the structure of tRNA
- Folded to form ‘clover-leaf’ shape
- Held together by hydrogen bonds
Describe the structure of mRNA
- Linear (straight chain)
- Relatively short
- Every 3 bases are referred to as codons (codes for an amino acid)
Why does DNA need to be replicated?
Cell division - every cell needs to contain a a copy of the DNA - precedes
What is the first step in DNA replication?
- 2 strands are separated by DNA helicase, which breaks the hydrogen bonds
What is the second step in DNA replication?
Free DNA nucleotides are attracted to the template strand and form hydrogen bonds.
What is the 3rd step of DNA replication?
Complementary base pairing:
Adenine with Thymine
Cytosine with Guanine
What is the fourth step of DNA replication?
DNA Polymerase catalyses the formation of phosphodiester bonds between nucleotides
Name the two scientists who proposed models of the chemical structure of DNA and of DNA replication (PPQ)
Watson and Crick
Describe the function of DNA helicase (PPQ)
Catalyses the destruction of the hydrogen bonds, separating the DNA strands.
Describe the function of DNA polymerase (PPQ)
Catalyses the formation of phosphodiester bonds between the nucleotides.
Contrast the structures of ATP and a nucleotide found in DNA to give two differences (PPQ)
- ATP only contains adenine, whereas a nucleotide in DNA could adenine, thymine, cytosine and guanine.
- ATP contains 3 phosphate groups, whereas a nucleotide in DNA only has 1 phosphate group.
Give 3 ways in which prokaryotic DNA is different from eukaryotic DNA (PPQ)
- Prokaryotic DNA is circular, eukaryotic is linear
- Prokaryotic DNA is coding (exons), whereas eukaryotic DNA is non coding (introns)
- Eukaryotic DNA is bound to histone proteins within the cell
Which part of the DNA molecule contains nitrogen? (PPQ)
The nitrogenous base
Which scientists found evidence for semi-conservative replication? (PPQ)
Meselson and Stahl
Describe how, after the parent DNA strands separate, the second strand of DNA is formed (PPQ)
- Semi conservative replication
- Complementary pairing
- Hydrogen bonding of nucleotides
- Due to DNA polymerase
Give two differences between the structure of DNA and the structure of RNA (PPQ)
- DNA contains thymine, whereas RNA contains uracil
- DNA has a deoxyribose sugar, whereas RNA has a ribose sugar
What is the role of the single stranded DNA fragments in semi conservative replication? (PPQ)
- Act as the template strand
- Determines order of nucleotides
What is the role of the DNA nucleotides in semi conservative replication? (PPQ)
- Forms complementary base pairs
A polypeptide has 51 amino acids in its primary structure.
What is the minimum number of DNA bases required to code for the amino acids in this polypeptide?
153
- 51 amino acids = 51 codons
- In each codon there are 3 bases
- Hence 3 x 51
