Biological Molecules - Nucleic Acids

Question 1

what does DNA stand for

Answer 1

deoxyribonucleic acid

Question 2

what is the function of DNA

Answer 2

to carry information i.e. the genetic code

Question 3

what must DNA be to function efficiently

Answer 3

not easily damaged so code isnt corrupted
code should easily accessed allow production of proteins hen required
allow code to be copied exactly to pass on to next generation

Question 4

how is DNA described - terms of overall molecule

Answer 4

it is a macromolecule which is a polymer- it is made of bases called nucleotides therefore it is called a polynucleotide

Question 5

what is DNA made up of

Answer 5

-a pentose sugar called deoxyribose
-a phosphate group-that makes up the backbone
-an organic or nitrogenous base = there are 4 of these
Adenine, guanine, cytosine and thymine

Question 6

how do nucleotides link up

Answer 6

phosphate to deoxyribose to base
in a condensation reaction

Question 7

what is two nucleotides linked by condensation called

Answer 7

dinucleotide

Question 8

what is multiple nucleotides linked together called

Answer 8

polynucleotide

Question 9

Who found the structure of DNA

Answer 9

watson + crick (1953)

Question 10

Structure of DNA

Answer 10

-two helical polynucleotide chains
-two chains run anti-parallel eg 5’ carbon end is opposite to 3’ on other chain
-these chains are coiled together to form a double helix
-bases on inside - sugar-phosphate on outside
-held together by hydrogen bond which are very specific
eg A-T - these are determined due to size restriction- gap fits specific base
C-G
-sequence of chains is not restricted

Question 11

how does the structure of DNA relate to its function

Answer 11

-sequence of bases are unrestricted so can carry a lot of genetic info
-highly coiled and compact so large amount of info in small space- each chain is helix and two chains double - also has histone proteins makes it more compact
-hydrogen bonds - weak individual but on mass = strong
-phosphate backbone is strong - has covalent bonds- DNA is stable
-hydrogen bonding is semi-conservative so code can be copied exactly

Question 12

what is does RNA stand for

Answer 12

ribonucleic acid

Question 13

how do the bases differ

Answer 13

-adenine
-guanine
-cytosine
-thymine is replaced by uracil

Question 14

how does the sugar differ in RNA

Answer 14

RNA lacks oxygen base so it is just a ribose

Question 15

in what type a chain does RNA form

Answer 15

a single polynucleotide chain

Question 16

how many RNA molecules are there

Answer 16

3

Question 17

what are the 3 types of RNA

Answer 17

-messenger RNA (mRNA) - formed in nucleus and length and bases vary and it involved in protein synthesis
transfer RNA (tRNA)- single chain in clover shape - many types - similar chains but 3 bases form anticodon which determines what amino acid attaches- also involved in protein synthesis
- ribosomal RNA (rRNA)- made in nucleolus and forms over half mass of ribosomes

Question 18

how does the amount of DNA and RNA differ in each cell

Answer 18

DNA- remains the same as cells have exactly the same chromosomes
RNA- vary depending on cell activity

Question 19

what was the origin of DNA structure studies

Answer 19

However, many scientific researchers at that time doubted that this newly discovered DNA molecule could carry the genetic code
They doubted this because of the relatively simple chemical composition of DNA (because DNA was only made up of simple repeating nucleotides, which themselves were only composed of three parts: a phosphate group, deoxyribose, a nitrogen-containing organic base
For example, some scientists hypothesised that genetic information must be carried by proteins, which show much higher levels of chemical complexity
For example, proteins are which are made up of 20 different amino acids whereas DNA is made up of only 4 different nucleotides
As a result, it wasn’t until the 1940s that the role of DNA in genetic inheritance began to be more fully researched and understood
By 1953, experiments had confirmed that DNA carried the genetic code
It was understood that, despite there being only 4 nucleotides, the use of the triplet code enabled much variation (the code is universal and degenerate)
The location of DNA, protected in the nucleus, enabled the security of the genetic material rather than proteins that are found in the cytoplasm and susceptible to hydrolysis
DNA is easily copied and therefore conserved throughout generations of cells and inherited between generations within families
1953 was also the year in which Watson and Crick confirmed the double-helix structure of DNA using Rosalind Franklin’s X-ray data

Question 20

what is semi-consevative DNA replication

Answer 20

Before a (parent) cell divides, it needs to copy the DNA contained within it
This is so that the two new (daughter) cells produced will both receive the full copies of the parental DNA
The DNA is copied via a process known as semi-conservative replication (semi = half)
The process is called so because in each new DNA molecule produced, one of the polynucleotide DNA strands (half of the new DNA molecule) is from the original DNA molecule being copied
The other polynucleotide DNA strand (the other half of the new DNA molecule) has to be newly created by the cell
Therefore, the new DNA molecule has conserved half of the original DNA and then used this to create a new strand
The importance of retaining one original DNA strand
It ensures there is genetic continuity between generations of cells
In other words, it ensures that the new cells produced during cell division inherit all their genes from their parent cells
This is important because cells in our body are replaced regularly and therefore we need the new cells to be able to do the same role as the old ones
Replication of DNA and cell division also occurs during growth

Question 21

what is the process of semi-conservative DNA replication

Answer 21

DNA replication occurs in preparation for mitosis, when a parent cell divides to produce two genetically identical daughter cells – as each daughter cell contains the same number of chromosomes as the parent cell, the number of DNA molecules in the parent cell must be doubled before mitosis takes place
DNA replication occurs during the S phase of the cell cycle (which occurs during interphase, when a cell is not dividing)
The enzyme helicase unwinds the DNA double helix by breaking the hydrogen bonds between the base pairs on the two antiparallel polynucleotide DNA strands to form two single polynucleotide DNA strands
Each of these single polynucleotide DNA strands acts as a template for the formation of a new strand made from free nucleotides that are attracted to the exposed DNA bases by base pairing
The new nucleotides are then joined together by DNA polymerase which catalyses condensation reactions to form a new strand
The original strand and the new strand joined together through hydrogen bonding between base pairs to form the new DNA molecule
This method of replicating DNA is known as semi-conservative replication because half of the original DNA molecule is kept (conserved) in each of the two new DNA molecules

Question 22

function of DNA polymerase

Answer 22

The bases of the free nucleoside triphosphates align with their complementary bases on each of the template DNA strands
The enzyme DNA polymerase synthesises new DNA strands from the two template strands
It does this by catalysing condensation reactions between the deoxyribose sugar and phosphate groups of adjacent nucleotides within the new strands, creating the sugar-phosphate backbone of the new DNA strands
DNA polymerase cleaves (breaks off) the two extra phosphates and uses the energy released to create the phosphodiester bonds (between adjacent nucleotides)
Hydrogen bonds then form between the complementary base pairs of the template and new DNA strands

Question 23

how is DNA synthesised

Answer 23

DNA polymerase can only build the new strand in one direction (5’ to 3’ direction)
As DNA is ‘unzipped’ from the 3’ towards the 5’ end, DNA polymerase will attach to the 3’ end of the original strand and move towards the replication fork (the point at which the DNA molecule is splitting into two template strands)
This means the DNA polymerase enzyme can synthesise the leading strand continuously
This template strand that the DNA polymerase attaches to is known as the leading strand
The other template strand created during DNA replication is known as the lagging strand
On this strand, DNA polymerase moves away from the replication fork (from the 5’ end to the 3’ end)
This means the DNA polymerase enzyme can only synthesise the lagging DNA strand in short segments (called Okazaki fragments)
A second enzyme known as DNA ligase is needed to join these lagging strand segments together to form a continuous complementary DNA strand
DNA ligase does this by catalysing the formation of phosphodiester bonds between the segments to create a continuous sugar-phosphate backbone

Question 24

what is Meselson and Stahl’s Experiment

Answer 24

Bacteria are grown in a broth containing the heavy (15N) nitrogen isotope
DNA contains nitrogen in its bases
As the bacteria replicated, they used nitrogen from the broth to make new DNA nucleotides
After some time, the culture of bacteria had DNA containing only heavy (15N) nitrogen
A sample of DNA from the 15N culture of bacteria was extracted and spun in a centrifuge
This showed that the DNA containing the heavy nitrogen settled near the bottom of the centrifuge tube
The bacteria containing only 15N DNA was then taken out of the 15N broth and added to a broth containing only the lighter 14N nitrogen. The bacteria were left for enough time for one round of DNA replication to occur before their DNA was extracted and spun in a centrifuge
If conservative DNA replication had occurred, the original template DNA molecules would only contain the heavier nitrogen and would settle at the bottom of the tube, whilst the new DNA molecules would only contain the lighter nitrogen and would settle at the top of the tube
If semi-conservative replication had occurred, all the DNA molecules would now contain both the heavy 15N and light 14N nitrogen and would therefore settle in the middle of the tube (one strand of each DNA molecule would be from the original DNA containing the heavier nitrogen and the other (new) strand would be made using only the lighter nitrogen)
Meselson and Stahl confirmed that the bacterial DNA had undergone semi-conservative replication.
The DNA from this second round of centrifugation settled in the middle of the tube, showing that each DNA molecule contained a mixture of the heavier and lighter nitrogen isotopes
If more rounds of replication were allowed to take place, the ratio of 15N:14N would go from 1:1 after the first round of replication, to 3:1 after the second and 7:1 after the third
This experiment proved Watson and Crick’s theory correct