2.3- How DNA replicates

Question 1

What does DNA polymerase mean?

Answer 1

Enzyme that catalyses formation of DNA from activated deoxyribose nucleotides, using single stranded DNA as a template.

Question 2

What does Helicase mean?

Answer 2

Enzyme that catalyses the breaking of hydrogen bonds between the nitrogenous pairs of bases in a DNA molecule.

Question 3

What does semi conservative replication mean?

Answer 3

How DNA replicates, resulting in 2 new molecules, each of which contains one old strand and one new strand. One old strand is conserved in each new molecule.

Question 4

What does all the DNA in an organism do?

Answer 4

All the DNA within a cell (the genome) and within every cell of an organism, carriers the coded instructions to make and maintain that organism. Every time a cell divides the DNA has to be copied so that each new daughter cell receives the full set of instructions. Each molecule of DNA replicates. This replication takes place during interphase before the cell actually divides. In eukaryotes this results in each chromosome ( a chromosome being one molecule of DNA) having an identical copy of itself. At first they are joined together at the centromere, forming two sister chromatids.

The DNA within mitochondria and chloroplasts also replicates each time theses organelles divide, which is just before the cell divides

Question 5

How is semi- conservative replication played out?

Answer 5

To make a new copy of itself, each DNA molecule:

• unwinds- the double helix is untwisted a bit at a time, catalysed by a gyrase enzyme
• unzips- hydrogen bonds between the nucleotide bases are broken. This is catalysed by DNA helicase and results in tow single strands of DNA with exposed nucleotide bases

Next:

• Free phosphorylated nucleotides present in the nucleoplasm within the nucleus are bonded to the exposed bases, following complementary base-pairing rule.
• The enzyme DNA polymerase catalyses the addition of the new nucleotide bases in the 5’ and 3’ direction, to the single strands of DNA; it uses each single strand of unzipped DNA as a template .
• The leading strand is synthesised continuously whereas the lagging strand is in fragments (discontinuous) that are later joined, catalysed by ligase enzymes
• Hydrolysis of the activated nucleotides, to releases the extra phosphate groups, supplies the energy to make phosphodieseter bonds between the sugar residue of one nucleotide and the phosphate group of the next nucleotide.

The product of the replication is two DNA molecules, identical to each other and to the parent molecule. Each of theses molecules contains one old strand and one new strand.

The loops of DNA, in prokaryotes and inside mitochondria and chloroplasts also replicate semi-conservatively. A bubble sprouts from the loop and this unwinds and unzips and then complementary nucleotides join to the exposed nucleotides. Eventually the whole loop is copied.

Question 6

What are mutations?

Answer 6

During DNA replication errors may occur and the wrong nucleotide may be inserted. This is estimated to occur in 1 in 10^8 base pairs. This could change the genetic code and is an example of a point mutation. During the replication process there are enzymes that can proofread and edit out such incorrect nucleotide, reducing the rate that mutations are produced.
However many genes have such changes to their nucleotide sequence. Different versions of a particular gene are called alleles or gene variation.

Not all mutations are harmful. Some appear to give neither advantages or disadvantages (if you can roll your tongue or not) and some can be advantageous, for example a white fur coat on animals during winter when its snowing.