DNA Flashcards
What is a nucleotide?
The monomer that joins together to form the polymer of DNA (nucleic acid)
Describe/draw the structure of a nucleotide
Phosphate (circle) bonded to deoxyribose sugar (pentagon), bonded to nitrogenous base (rectangle)
How are nucleotides formed?
H from phosphate and OH from sugar bond in condensation reaction, form ester bond; OH from sugar and H from base bond in condensation reaction, forms glycosidic bond
What are the four bases and how do these pair up?
Adenine + thymine, guanine + cytosine
Describe basic DNA structure
- two phosphate backbone strands held together by bases
How are the bases bonded
A + T = 2 hydrogen bonds
G + C = 3 hydrogen bonds
How do nucleotides join together to form polynucleotides?
Condensation reactions, phosphate group of one and carbon of the pentose sugar of the other react forming a phosphodiester bond
Describe the first stage of protein synthesis
Transcription
- takes place in the nucleus
- enzyme DNA helicase attaches to DNA, breaking hydrogen bonds between paired bases and DNA unwinds
- the template (antisense) strand sequence is transcribed to make an mRNA molecules with same sequence as coding strand
- complementary RNA nucleotides align themselves in position and phosphodiester bonds form to produce mRNA, catalysed by RNA polymerase
- when complete, DNA zips up and mRNA leaves through a pore in the nuclear envelope
Describe the second stage of protein synthesis
Translation
- takes place in ribosome
- mRNA attaches to ribosome’s smaller subunit, so two mRNA codons face the two binding sites in the larger one
- tRNA has anticodons on one end - complementary to mRNA codon for an amino acid
- free amino acids in the cytoplasm attach to the correct tRNA molecules, which hydrogen bond to the correct codon
- peptide bond forms between two amino acids, and the mRNA moves along to reveal a new codon
-translation continues until a stop codon/chain terminator is reached and polypeptide detaches from the ribosome
What is the nature of genetic code?
- triplet codes -> 3 bases code for one amino acid
- non overlapping -> triplet code is adjacent; each base read only once and based do not overlap
- degenerate -> several triplet codes can code for the same amino acid
What are differences between DNA and RNA?
- RNA has a ribose sugar whilst DNA has a deoxyribose sugar
- RNA is single stranded whilst DNA is double stranded
- in RNA uracil replaces thymine
Which bases are purines and pyrimidines?
A + G = purines, have double ring structure
C, U + T = pyrimidines, have single ring structure
Describe the process of DNA replication
1 entire DNA double helix unwinds from one end + hydrogen bonds between bases break
2 free DNA nucleotides line up along each DNA strand and hydrogen bonds form between complimentary bases
3 the enzyme DNA polymerase links adjacent nucleotides with phosphodiester bonds to form new strands
4 this forms two new identical DNA molecules
What are the three theories of DNA replication?
fragmentary replication – all DNA strands are a mixture of original parent DNA nucleotides and new nucleotides
conservative replication – one DNA molecule has 2 patent strands and the other has 2 new strands
semi conservative replication – each DNA molecule contains 1 strand from the parent and 1 new strand
Describe which experiment was conducted to confirm a theory of DNA replication
Mesleton & Stahl
- used heavy and light strands of DNA to distinguish between old and new
- grew E. coli in a medium containing only the heavy isotope nitrogen
- then moved this bacteria into a medium with only normal nitrogen, meaning new nucleotides were ´light’ and original were ‘heavy’
- replicated DNA twice to see results
