Question

Genes are specific sequences of nucleotides that code for mRNA which may be translated into a protein. The nucleotide is the building block of nucleic acids, RNA and DNA. A nucleotide is composed of a pentose sugar, attached to one of five nitrogenous bases, guanine (G), cytosine (C), thymine (T), or adenine (A) and phosphate groups. The bases are covalently bonded through an ester bond between the hydoxyl group of one ribose and the phosphate group of another nucleotide. All cells contain both DNA and RNA, in contrast to viruses which are acellular and have either RNA or DNA. DNA is double stranded and the two stranded of nucleotides are held together by hydrogen bonds between the nitrogenous bases.

1. What is the central dogma of biology using DNA, RNA and proteins?

DNA is associated are packed with proteins to form chromosomes, the typical structure of chromosomes is different for prokaryotes and eukaryotes. Prokaryotes have supercoiled circular DNA, which is the structure of DNA in mitochondria. The nucleoid region of the prokaryotic cell contains its genome. Eukaryotic cells have chormosomes in the nucleus and are organized into nucleosomes which form chromatin fibers.  

2. Compare and contrast a plasmid in a prokaryotic cell with the chromosomes of a prokaryotic cell. Use at least three significant features,for example what kind of information is stored in each type of DNA molecule.

DNA replication is semiconservative, so each newly strand of DNA, which is double stranded, remains associated with the parental strand that had served as a template. DNA replication requires five types of enzymes to complete, in contrast to RNA is capable of self replication. Examples of enzymes needed for DNA replication are:

helicase which unwinds and/or unzips DNA, so it breaks the hydrogen bonds that hold the two strands together.  

DNA polymerase which may be DNA pol I, II or III each has a different function but all synthesize the new strand of DNA by adding the nucleotide ONLY to the 3'hydroxyl group.

Primase to synthesize the RNA primers for DNA pol to begin joining the nucleotides which are base paired with their complementary base on the parental strand. Remember the leading strand requires only one RNA primer because DNA pol can join the nucleotides continuously and on the lagging strand multiple RNA primers are needed because DNA pol joins the nucleotides of the new strand. The discontinuous synthesis on the lagging strand in which the parental strand ends with a 5'phosphate group creates Okasaki fragments.

DNA ligase which connects the Okasaki fragments to form a continuous DNA strand.

3. Succintly explain the difference between the leading and lagging strand on the DNA replication diagram. How does the direction in which DNA pol connect nucleotides lead to the differences?

4. Name and explain three functions of methylation which occurs after DNA replication.

Gene function involves several steps in a process that is regulated by interruption or enhancement of the steps. The transfer of information begins with transcription in which RNA pol links the bases of RNA that that paired with complementary bases in ssDNA, the gene. Transcription begins at the region of DNA called the promoter for that gene. Transcription ends at the terminator.  

5. How does the initiation and termination of transcription differ between prokaryotes and eukaryotes?

6. How soon after the mRNA is synthesized is it ready for translation in the ribosomes? Is there a difference in this process between eukaryotes and prokaryotes, if so describe that difference. k

Answer 1

1. Central dogma of life

DNA ---> DNA = Replication

DNA ---> RNA = Transcription

RNA ---> DNA = Reverse transcription

RNA ---> Protein = Translation

DNA --------> RNA --------> Protein

2. Prokaryotes:

i. Closed circular double-stranded DNA

ii. Naked (Absence of histone proteins)

iii. Found in the cytoplasm

Eukaryotes:

i. Linear double-stranded DNA

ii. Found in a complex with histones (Chromatin)

iii. Found in the nucleus

3. Leading strand is continuously synthesized in the 5'-3' direction. The lagging strand is synthesized in discrete fragments known as Okazaki fragments in the 5'-3' direction.

Leading strand gets extended in the same direction as the replication fork.

Lagging strand gets extended in the opposite direction as the replication fork.

See image

4. Functions of DNA methylation:

i. It marks the parental DNA

ii. It protects from cleavage of enzymes

iii. It helps in gene regulation.