DNA Flashcards
nucleotide
monomer from which nucleic acids are formed
what does each nucleotide comprise ?
a pentose, a phosphate group and a purine or pyrimidine base
what are the two types of nucleic acids ?
DNA ( deoxyribonucleic acid ) and RNA ( ribonucleic acid )
Structure of nucleotides
each nucleotide consists of three substances combined together:
- a pentose
- nitrogenous base
- phosphoric acid
nitrogenous base
- a double ringed purine (either adenine or guanine) - in both DNA and RNA
- a single ringed pyrimidine (either cytosine or thymine) - DNA
(either cytosine or uracil)
polynucleotide chains
two nucleotides can be joined together by a condensation reaction catalysed by an enzyme DNA polymerase
- formation of a covalent bond, called a phosphodiester bond
how does DNA polymer occur in pairs ?
joined by hydrogen bonds between bases - creates a double helix
nucleotide + nucleotide =
dinucleotide
how do polynucleotides form ?
large numbers of nucleotides become condensed together to form huge molecules
how do backbone of the polynucleotide form ?
alternating sugar and phosphate molecules
where can hydrogen bonds only form ?
between complementary base pairs
- cytosine and guanine
- adenine and thymine
what is RNA ?
a polymer of a nucleotide formed ribose, a nitrogenous base and a phosphate group
what are the nitrogenous bases in RNA ?
adenine, guanine, cytosine and uracil
Pyrimidines
single stranded structure
- cytosine and thymine
purines
double ringed structure
- guanine and adenine
what are the three types of RNA ?
messenger RNA
transfer RNA
ribosomal RNA
messenger RNA
carries a copy of a single gene to a cells ribosomes
transfer RNA
carries individual amino acids to ribosomes during protein synthesis
ribosomal RNA
forms part of the sub units of ribosomes
what does DNA only use ?
A, T , C and G
bonding - RNA molecules
- base + sugar joins with a glycosidic bond
- the phosphate + sugar joins an ester bond
- both require condensation reaction to occur
in every DNA nucleotide:
- pentose is deoxyribose
- the base is cytosine, guanine, adenine, or thymine, but never uracil
complementary base pairing
leads to:
- stability of the DNA double helix
- transfer of genetic information from DNA to RNA
- the way amino acids are assembled into polypeptides in the cytoplasm
A joins with what
T
C joins with what
G
the bases of two strands fit together only if:
the deoxyribose molecules to which they are attached point in opposite directions
which directions do the sugar phosphate backbones ?
opposite directions - strands are antiparallel
Semi conservative replication - step 1
the double helix of the DNA molecule unwinds and the hydrogen bonds holding the strands together break - causes the bases on each strand to become exposed
catalysed by the enzyme DNA helicase
Semi conservative replication - step 2
- both exposed strands now acts as a template for the production of the new polynucleotide strand
- free nucleotides are attached to the exposed complementary bases
-DNA polymerase links the adjacent nucleotides forming phosphodiester bonds - the sugar phosphate backbone
Semi conservative replication - step 3
- DNA is replicated in segments
- DNA ligase joins the segments together - condensation reaction - forms phosphodiester bonds
Semi conservative replication - step 4
two identical DNA molecules are formed
- semi conservative replication because each strand contains 1 copy of the original DNA
Genetic code
major role: enable a cell to make specific proteins
- large DNA molecule codes for a large number of proteins
gene
a sequence of bases which code for specific amino acids in a polypeptide chain
what is a genetic code ?
a combination of three nucleotide base triplets encodes an individual amino acid
- each combination of base triplets encodes the same amino acid in all organisms
what is universal ?
triple code
With four bases, how many different combinations can be made ?
64 (4x4x4)
how many amino acids are commonly used by cells ?
20
- there is many more different and DNA base triplet combinations than are needed to encode 20 amino acids
degenerate
some amino acids are encoded by more than one base triplet
genetic code
- it has one 3 base start codon = TAC
- it has 3 different 3 base stop codons = ATT, ATC and ACT
- 3 bases code for a specific amino acid
why is the genetic code non overlapping ?
each triplet only read once and triplets don’t share any bases
