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
What were the results of the experiment testing theories of DNA replication?
• After one round of DNA replication
- a single band of medium density
- no heavy DNA so conservative could be eliminated
• After a second round
- one band was medium and the other was light
- prescience of both medium and light eliminated fragmentary replication
How does light, medium and heavy DNA arrange in a test tube?
Heavy (heavy isotope of nitrogen) sinks to the bottom, Medium (containing both heavy and light nucleotides) in the middle, Light (normal nitrogen) at the top
Define monohybrid inheritance
Inheritance of characteristics controlled by a single gene
Define incomplete dominance
When both alleles are expressed in the phenotype; occurs when heterozygous for a trait
Define gene
Specific sequence of DNA bases coding for a particular protein or RNA
Define allele
Alternative form of a gene occurring at same locus on homologous chromosomes
