3.9 DNA Replication

Question 1

What does replicating semi conservative mean

Answer 1

Thst each new molecule of DNA will contain half of it a new strand and half of it an old strand

Question 2

Advantage of semi conservative replication ?

Come back to this

Answer 2

• this ensures that the least amount of mistakes are made as possible, atleast one half will be completely correct

Question 3

How Is DNA replicated ?

Role of enzymes ligase and helicase for now

Answer 3

1) DNA Heilfasten is added, catalysing reactions that break the hydrogen bonds between the complimentary bases as pairs . Thus the double helix “unwinds”
2) Both strands act as templates and free nucleotides line up to the exposed COMPLIMENTARY bases
3) DNA polymerase then catalyses reactions that allow phospodiester bonds to be made in the new nucleotides in new chain , creating the SUGAR PHOSPHATE BACKBONE . Finally hydrogen bonds between complimentary bases are made and now you have two identical strands,

These will be semi conservative

Question 4

What is continuous vs discontinuous replication , lagging vs leading and how does polymerase and ligase come in to this?

Okazaki ?

Why doesnt it just split apart all the way and then do it!

Answer 4

• polymerase can only work in the direction of 3’ end to 5’. It attaches to the 3’ end first
• helicase will unzip the DNA for the shortest amount of time to avoid damage (so instead of unzipping all and letting polymerase do in both directions it does it bit by bit)
• as DNA unzips, the 3 to 5’ end it can just bind and rep,irate as more envious no problem. This is CONTINOUS REPLICATION, and this strand is the LEADING STRAND

2) in the other strand, as a bit is opened, polymerase attached to 3’ and goes to 5’, and then as more opens, it has to go back and reattach to another 3’ and go back.
- it does this in reasonable sections , and these detachments are called OKAZAKI FRAGMENTS

• these fragments are then joined by DNA LIGASE, creating final few phosphodiester bonds…
  This is DISCONINTOUS REPLICATION + LAGGING STRAND!

Question 5

What is a mutation

Answer 5

A random error / change in the DNA base sequence

Question 6

What is the genetic code ?

What are the features of it

Answer 6

The sequence of base triplets that code for amino acids, present in al, organisms as they all made from proteins.

(Proteins made form amino acids thus genetic code codes for the amino acids)

2) - degneratre, universal, non overlapping

Question 7

What does it being universal mean

Answer 7

Each codon base triplet codes for an amino acid

But for it is universal, all organisms in the world will all have the same three bases CODE for the same amino acid, also all have only same 4 bases etc

Question 8

What does it being degenerate mean p

Why is this good

Answer 8

As there are 4 bases the combinations of amino acids= 4^3=64.
- however we only need 20 amino acids to survive including start and stop amino acids

• AS A RESULT , amino acids can be coded from through more than one combination of codons base triplets
• as a result if there is a mutation it is likely that same amino acid will be coded for ( especially in third base, and this way damage from mutations is REDUCED

Question 9

How is the genetic code non overlapping?

Answer 9

There is a start and a stop codon that codes for an amino acid. This helps it be read in from and codon by codon, such thst there is non overlap

Eg it’s read from base 1, not 2 or three giving complete different combinations

This makes the code non overlapping

Question 10

What happens if a mutation may occur ?

Answer 10

Mutation changes the base sequence

• could code for a different amino acid
• as a result the wrong sequence of amino acids occur so now primary structure is changed , after secondary teriwtwry and perhaps quaternary the 3d structure will not be the same
• if it is not the same then it may not serve right functions

Question 11

What actually codes for the amino acids and everything

What is the template

What directions

Answer 11

This is the sense strand , that runs from 5’ to 3’

Anitsense strand 3’ to 5’ is the template that mRNA uses to become exactly identical to DNA Z

Question 12

Thus why and how is the sense strand and DNA PROTECTED

2 ways)(meh don’t need to know thst mich

Answer 12

The sense strand must be protected or all info lost
It is done by
- being in a double helix so bases are never exposed, second strand don’t code for anything but atLeast protects and acts as a templtwte
- the fact that the double helix is too big to leave the nucleus means it is locked in from potential dangers and thus is safe too

Question 13

Why can’t dna leave the nucleus

Answer 13

Too big

- surrounded by double membrane called nuclear en Envelope to protect

Question 14

How does transcription take place

What enzymes involved
What special about RNA have to remember

Answer 14

1) DNA helicase is uses to break hydrogen bonds around a gene
2) Free RNA nucleotides LINE UP AGAINST THE ANTISENSE exposed (3’to 5’, template) strand complimentarily!
- here there is no thymine, but uracil is still complimentary!
3) RNA POLYMERASE THEN catalyses reactions to give phosphodiester bonds between the RNA nucloetides to make Sugar phosphate backbone , also pushes it for next codon
4) after it hits stop codon, completed strand is now mRNA molecule and detaches from DNA template to leave the nucleus from a nuclear pore
5) DNA rexips

Question 15

MRNA will then travel to the ribosome

Describe structure of the ribosome .

Answer 15

• ribosomes made out of two subunits one small and one large
• they made from equal amount of protein and RNA in the form of ribosomal RNA rRNA

Question 16

What is the purpose of rRNA in the ribosome?

2

Answer 16

• important for maintaining structural stability of the protein synthesis sequence (rRNA holds mRNA in place on smal, subunit)
• PLAYS A BICOCHEMICAL ROLE IN CATALYSING reactions of PEPTIDE BONDS in the protein

Question 17

After mRNA transcribed where does it go

Answer 17

It goes to the ribosome, on the small subunit, which holds mRNA and reads it / decodes into amino acid in translation

Question 18

What is tRNA and how does it help (transfer )

Answer 18

• tRNA is a form of RJA, a single strand this is folded so that it has three bases called the anticodon on one end of the molecule
• and it’s COMPLIMENTARY AMINO ACID attached to the other side

Clover shaped

Question 19

So how does translation process

Hod doe tRNA and enzyme do it, when does it end etc

Answer 19

1) mRNA on small subunit of ribosome, where it is decided from the start codon so non overlaps
2) mRNA is read, and based on three bases / codon, tRNA molecule will bind its anticodon to the codon that based on complimentary rules, BRINGING AN AMINO ACID BASED ON THE CODON
- temporary hydrogen binds made between bases
3) another tRNA brings another amino acid on next codon, maximum two can happen at once
4) then peptidyl TRANSFERASE enzyme ( an rRNA component of ribosome ) catalyses reaction to form a PEPTIDE BOND between them.
5) now tRNA can detach and bring others as ribosome reads the mRNA
6) repeats until it hits STOP codon

Question 20

What happens when translation is done

2
What can happen with mRNA and what eventually happens

Answer 20

• now the primary structure of the protein is made, the polypeptide is released and it folds to form secondary tertiary etc
• it may be packaged into vesicles to go to Golgi to be modified and etc

2) - multiple same proteins can be translated at same time with same mRNA strand
- eventually mRNA breaks down and rna nuckeltides go back to nucleus to be reused