Question

HOMEWORK 8

- In your own words, describe the process of DNA replication

-In your own words, provide a detailed description of how DNA is packaged in the nucleus (10 pts).

-Identify the process and detail the steps of transcription from start to finish (In your own words) (10pts)

-RNA is derived from DNA by what process and which major enzyme? (5pts)

-What determines if a gene is functional (5 pts)

-How do mutations occur in cells? Further list 3 things that can increase the rate of mutations (10 pts)

Answer 1

Answer to above question

- Basically DNA replication means process of make multiple copies of DNA from one original DNA molecule. Process of Replication is divided into 3 parts - INITIATION ,ELONGATION ,TERMINATION .

INITIATION - DNA synthesis starts at origins ( particular points within DNA strand which are specific coding sequence ) there is formation of replication complex at these origin sites and referred as Replication Forks.

With help of enzyme DNA Helicase, which unwinds the double helix and opens each of the two strands, so that they can be used as a template for replication.

DNA can be extended by the addition of nucleotide triphosphate to the 3’- end of a chain. The double helix runs antiparallel, but DNA replication only occurs in one direction, which means growth of the two new strands is different to each other.

DNA Primase is another enzyme important in DNA replication which synthesises a small RNA primer, which acts as a ‘kick-starter’ (starting ) point for DNA Polymerase. DNA Polymerase is the main responsible enzyme for the creation and expansion of the new strands of DNA.

ELONGATION -   When DNA Polymerase has attached to the original, unzipped two strands of DNA (i.e. the template strands), it is able to start synthesising the new DNA to match the templates which is done by adding free nucleotide to 3' end which is read in 3' to 5' direction referred as Leading Strand which means that the new strand will be formed in a 5’ to 3’ direction .

Along this strand, DNA Primase synthesises RNA primer once, at the beginning, to help initiate DNA Polymerase for extending the new DNA strand in 5' to 3' direction to allow for the continued creation of RNA primers, the new synthesis is delayed and is such called the Lagging Strand.

The leading strand is one complete strand, while the lagging strand is not, It is instead made out of multiple ‘mini-strands’, known of Okazaki fragments.

Termination- Process of expanding the new DNA strands continues until there is either no more left DNA template to replicate (i.e. at the end of the chromosome), or in other words two replication forks meet and subsequently terminate.

When DNA synthesis has finished, it is important that the newly synthesised strands are bound and stabilized, two enzymes are needed to achieve this; RNAase H which removes the RNA primer that is at the beginning of each Okazaki fragment, and other one is DNA Ligase which joins two fragments together creating one complete strand.

With formation of two new strands, the DNA has been successfully replicated, and there is just another intrinsic cell systems to ‘proof-read’ the new DNA to check for any errors in replication, and for the new single strands to be stabilized.

- The answer to question is based on the fact that certain proteins tightly hold chromosomal DNA into the microscopic (small) space of the eukaryotic nucleus. These proteins are called histones, and the resulting DNA-protein complex is called chromatin. It may seem contradictory that proteins are added to DNA to make it more compact. So coiling requires work, and energy is needed for it. So, within the nucleus, histones provide the energy (mainly in the form of electrostatic interactions) to fold DNA which result, chromatin can be packaged into a much smaller volume than DNA alone.

Histones is a family (group) of small, positively charged proteins termed H1, H2A, H2B, H3, and H4 . DNA is negatively charged, due to the phosphate groups in its phosphate-sugar backbone, so histones bind with DNA very tightly.Chromosomal DNA is packaged inside microscopic nuclei with the help of histones by form complexes called nucleosomes. Each nuclesome is composed of a DNA wound 1.65 times around 8 histone proteins and in turn Nucleosomes fold(coiled) up to form a 30-nanometer chromatin fiber, which forms loops averaging 300 nanometers in length. These 300 nm fibers are compressed and folded to produce a 250 nm-wide fiber, which is tightly coiled into the chromatid of a chromosome and thus packaging of DNA into chromosome.