nucleic acids + DNA replication Flashcards
DNA
deoxyribonucleic acid - used to store genetic information - all the instruction an organism needs to grow and develop from a fertilised egg to a fully grown adult
RNA
ribonucleic acid - one of its main functions is to transfer genetic information from the DNA to the ribosomes - ribosomes are the body’s protein factories - they read the RNA to make polypeptides in a process called translation
ribosomes are made from RNA and proteins
what are DNA and RNA made of
polymers of nucleotides
what is a nucleotide
a type of biological molecule which is made from pentose sugar, a nitrogen-containing organic base and a phosphate group
structure of DNA
the pentose sugar in a DNA nucleotide is called deoxyribose
each DNA nucleotide has the same sugar and a phosphate group
the base on each nucleotide may vary
the four possible bases are adenine, thymine, cytocine and guanine
structure of RNA
RNA contains nucleotides with a ribose sugar
also has a phosphate group and one of four different bases
adenine, cytocine and guanine are the same as DNA
has uracil instead of thymine
how are nucleotides joined together
through a condensation reaction between the phosphate group of one nucleotide and the sugar of another
this forms a phosphodiester bon
the chain of sugars and phosphates is known as the sugar-phosphate backbone
DNA and double helix structure
two DNA polynucleotide strands join together by hydrogen bonding between the bases
each base can only join with one particular partner - complementary base pairing
two antiparallel polynucleotide strands twist to form the DNA double-helix
what binds with adenine
thymine (uracil in RNA)
what binds with cytocine
guanine
how many hydrogen bonds
two between A and T
three between C and G
how does DNA replicate
semi-conservative replication
the enzyme DNA helicase breaks the hydrogen bonds between bases on the two polynucleotide DNA strands. This makes the helix unwind to form two single strands
Each original single strand acts as a template for a new strand. Complementary base pairing means that free-floating DNA nucleotides are attracted to their complementary exposed bases on each original template
Condensation reactions join the nucleotides of the new strands together - catalysed by the enzyme DNA polymerase. Hydrogen bonds form between the bases on the original and new strands
Each new DNA molecule contains one strand from the original DNA molecule and one new strand
how does DNA polymerase move
in opposite ways along antiparallel DNA strands
each end of a DNA strand is slightly different in its structure
One end is called the 3’ (3 prime) end and one end is called the 5’ end
In a DNA helix, the strands run in opposite directions - they’re antiparallel
The active site of DNA polymerase is only complementary to the 3’ end of the newly formed DNA strand - so the enzymes can only add nucleotides to the new strand at the 3’ end causing the polymerase working on one of the template strands to move in the opposite direction to the DNA polymerase working on the other template strand
who found evidence for semi-conservative replication
Meselson and Stahl
evidence for semi-conservative replication
- two samples of bacteria were grown - one in a nutrient broth containing light nitrogen one and one in a nutrient broth containing heavy nitrogen - as the bacteria reproduced, they took up nitrogen from the both to help form the nucleotides, so the nitrogen gradually because part of the bacteria’s DNA
- a ample of DNA was taken from each batch of bacteria, and spun in a centrifuge - the DNA from the heavy nitrogen bacteria settled lower down in the centrifuge tube than the DNA from the light nitrogen - because its heavier
- the bacteria grown in the heavy nitrogen broth were taken out and put in a broth containing only light nitrogen - the bacteria were left for one round of DNA replication and then another DNA sample was taken out and spun in the centrifuge
if conservative - the heavy DNA would still be together
if semi-conservative - new bacteria would contain one strand of heavy nitrogen and one strand of light nitrogen
The DNA settled in the middle, showing that the DNA molecules contained a mixture of heavy and light nitrogen - the bacterial DNA had replicated semi-conservatively in the light nitrogen
how is atp an immediate source of energy in a cell
plant and animal cells release energy from glucose - this process is called respiration
a cell can’t get its energy directly from glucose
so, in respiration, the energy released from glucose is used to make ATP (adenosine triphosphate)
once made, ATP diffuses to the part of the cell that needs energy
the energy in ATP is stored in high energy bonds between the phosphate groups
it’s released via hydrolysis reactions
structure of ATP
made from the nucleotide base, adenine, combined with a ribose sugar and three phosphate groups
it’s known as a nucleotide derivative because it’s a modified form of a nucleotide
hydrolysis of ATP
when energy is needed by a cell, ATP is broken down into ADP (adenine diphosphate) and Pi (inorganic phosphate)
this is a hydrolysis reaction - a phosphate bond is broken and energy is release - the reaction is catalysed by the enzyme ATP hydrolase
ATP hydrolysis can be ‘coupled’ to other energy-requiring reactions in the cell so energy can be used directly to make the coupled reactions occur, rather than being lost as heat
inorganic phosphate
can be added to another compound (phosphorylation), which often makes the compound more reaction
ATP re-synthesis
by a condensation reaction between ADP and Pi
this happens during both respiration and photosynthesis, and is catalysed by the enzyme ATP synthase
how can a genetic code be degenerate
there are more possible combinations of triplets than there are amino acids
this means that some amino acids are coded for by more than one base triplet
