dna replication and the genetic code Flashcards
define DNA polymerase
enzymes that catalyse the formation of DNA from activated deoxyribose nucleotides, using single stranded DNA as a template
define helicase
enzyme that catalyses the breakdown of hydrogen bonds between the nitrogenous pairs of bases in a DNA molecule
define semi conservative replication
how DNA replicates resulting in two new molecules, each of which contains the old strand and one new strand. one old strand is conserved in each molecule
why do cells divide
to produce more cells needed for growth and repair of tissues
what is produced when cells divide
two daughter cells that contain DNA with a base sequence identical to the parent cell
what happens when a cell prepares to divide
the two strands of DNA in the double helix separate, each strand serving as a template for the creation of a new double stranded DNA molecule
what does the complementary base pairing ensure
the two new strands are identical to the original
what does the DNA within a cell contain
all the coded instructions for that cell
when does DNA replication take place
during interphase
what happens in semi conservative replication
two new molecules are produced. one consists of an old strand and the other consists of a new strand
what controls DNA replication
enzymes
when only can enzymes carry out their function
by recognising and attaching to specific molecules or parts of it
what did scientists know about DNA
they knew it was a self replicating molecule but didnt know how the molecule made copies of itself
what were the three theories about dna replication
the replication was
- conservative = the original molecules acts as a template and a new molecule is made
- dispersive = the original molecule breaks up into nucleotides, each one joins to a complementary nucleotide and new ones join up again
- semi conservative = the new molecule consists of one original strand and one newly formed strand
what did Meselson and Stahl do
they grew bacteria in e.coli in N15 (heavy) medium. they transferred some bacteria to N14 (light). bacterial growth continued
what happened in Meselson and Stahls experiment
- before bacteria reproduce, all parental DNA is heavy
- after one replication, the DNA was hybrid (mix of n14&n15) which shows it does not replicate conservatively otherwise it would have been one heavy, one light
- after another division, the DNA was extracted and centrifuged. ot produced two more bands of DNA, one hybrid, one light showing DNA replication is semi conservative not dispersive
what is the first two things that have to happen before DNA can replicate
the DNA has to unwind and unzip itself
how does DNA unwind
the double helix untwists, a bit at a time catalaysed by a gyrase enzyme
how does DNA unzip
hydrogen bonds between the nucleotide bases are broken. this is catalysed by DNA helicase resulting in two single strands of DNA with exposed nucleotide bases
how does DNA helicase travel
along the DNA backbone catalysing reactions that break the hydrogen bonds between complementary base pairing
what happens after DNA unzips
free activated nucleotides in the nucleus pair with the newly exposed bases on the template strand. involves hydrogen bonding and follows complementary base pairing rules
what does DNA polymerase do
catalyses the formation of phosphodiester bonds between these nucleotides.
how are phosphodiester bonds
formed between the phosphate group of one nucleotide anbd the sugar group of the next to make the backbone
how does DNA polymerase work
works 3’ to 5’ on the template strand allowing hydrogen bonds to form between complementary bases
