3.9 DNA Replication & the Genetic Code

In what stage of the cell cycle does DNA replication occur?

interphase

Why must DNA be copied when cells divide?

so that each daughter cell is genetically identical

Why must the copying of DNA be accurate? Why might it not be accurate?

To conserve the genetic information. A mutation can cause differences in copied DNA

What is a mutation?

a random and spontaneous change in the formation of DNA base sequences

What is the term given to the process by which DNA replicates?

semi-conservative replication

What does the term semi-conservative replication mean?

for every 2 molecules of DNA produced/formed, each molecule contains 1 parent strand and one new strand

What enzyme unzips the DNA molecule before replication?

DNA helicase

In what part of the cell are free, activated nucleotides found?

nucleoplasm

What enzyme joins adjacent nucleotides in the new strand of DNA?

DNA polymerase

What is the term describing the part of DNA where free nucleotides joins once it has been unzipped?

replication fork

Describe how DNA replicates by semi-conservative replication

The DNA helix unwinds and the enzyme DNA helicase unzips the molecule by breaking the hydrogen bonds between the CBP at the replication fork.

Free, activated nucleotides form CBP with each parent template strand - A→T by 2 H bonds, C→G by 3 H bonds, always purine to pyrimidine

DNA polymerase joins adjacent nucleotides on each new strand by phosphodiester bonds in condensation reactions (releasing water molecules) to create a sugar phosphate backbone

2 genetically identical DNA molecules are formed, each containing one parent strand that acted as a template strand for a new, complementary strand

Where does the energy to form the phosphodiester bonds come from?

the extra phosphate groups of the free activated nucleotides, which are hydrolysed off

How are free nucleotides activated in the nucleoplasm?

with extra phosphate groups

True or false? DNA in prokaryotes also replicates semi-conservatively

true

Describe how DNA replicates in prokaryotes, and inside mitochondria and chloroplasts

A bubble sprouts from the loops of DNA, unwinding and unzipping it so complementary nucleotides can join the exposed area until the entire loop is copied

This image shows semi-conservative replication. Which letter represents the replication fork?

B

What were the 3 theories of how DNA self-replicated called?

conservative, semi-conservative, dispersive

What was the theory of conservative DNA replication?

original molecules act as templates and new molecules are made

What was the theory of dispersive DNA replication?

original molecules break into nucleotides and each one joins to a complementary nucleotide, which join into strands

Using the diagram, briefly describe the Meselson and Stahl experiment that provided evidence of semi-conservative replication

• nucleotides contain nitrogen, so free nucleotides in heavy nitrogen isotopes contain N15, and the ones formed in light nitrogen contain N14

• bacteria was grown in a heavy (N15) nitrogen medium

• it was transferred to a medium of light nitrogen (N14) and after a single replication the DNA has a medium density, being made from both N15 (parental strand) and N14 (new strand)

• the next replication lead to some DNA only containing N14 isotopes and some contain N15-N14, forming 2 bands when spun in a centrifuge

• for future replications, the band at N15-N14 will get thinner, while the band at N14 gets thicker

Fill in the blanks: DNA contains the ___ ___ which codes for the synthesis of all ____

genetic code, polypeptides

What is a gene?

the sequence of DNA that codes for the synthesis of a specific polypeptide

Why is the genetic code contained in the nucleus?

so it can be conserved/protected

Fill in the blanks: polypeptides are chains of ___ ___ joined by ____ bonds

amino acids, peptide

How many different amino acids are there?

20

What is a genome?

the entire sequence of DNA in an organism

What is the position of a gene of a chromosome called?

locus (loci)

What is found in between genes of DNA?

non-coding DNA

Give 5 words/phrases to describe the genetic code

triplet code, degenerate, widespread (but not universal), contains punctuation, is non-overlapping

Why is the genetic code a triplet code?

the sequence of DNA nucleotide bases found within a gene is determined by a triplet code, with each sequence of three bases, a codon, coding for 1 amino acid

How many different codons are there if 4 DNA bases are grouped into triplets?

64 (4³)

Why is the genetic code degenerate?

there are 64 different codons that code for only 20 amino acids, so multiple codons can code for the same amino acid

Why is it important that the genetic code is degenerate?

it helps to limit the number of mutations that have an serious effect

Fill in the blanks: the genetic code is (w)____ but not (u)_____

widespread, universal

Why is the genetic code not completely universal?

a few organisms have different nitrogenous bases

How is the genetic code said to have punctuation?

it has a universal start codon (AUG) and stop codons

True of false? the start codon of the genetic code also codes for an amino acid, but the stop codons don’t

true

What is meant by the term gene expression?

the sequence of bases expressed as an amino acid through the processes of transcription and translation