BIOLOGY 1.5 Marks Questions

(a) 5' ATGCATGCATGCATGC 3'
(b)

(i) To map all human genes precisely to their respective positions on chromosomes and to itentify their DNA sequence.
(ii) To store this information in databases and develop tools for data analysis.
(iii) to solve the Ethical, Legal and Social Issues (ELSI) that may arise from the project.

The three RNAs involved in protein synthesis are summarized here :
(i) mRNA (messenger RNA) : Produced during transcription. Carries the genetic instructions of a gene from the nucleus to the ribosome in the cytoplasm.
(ii) rRNA (ribosomal RNA) : Together with proteins, composes the ribosome, the organelles that are the site of protein synthesis.
(iii) tRNA (transfer RNA) : Bring the correct amino acid to the ribosome during translation.

Transcription is the process by which the information in a strand of DNA is copied into a new molecule of messenger RNA (mRNA). DNA safely and stably stores genetic material in the nuclei of cells as a reference, or template. Mean while, mRNA is comparable to a copy from a reference book because it carries the same information as DNA but is not used for long - term storage and can freely exit the nucleus. Although the mRNA contains the same information, it is not an identical copy of the DNA segment, because its sequence is complementary to the DNA template.
Transcription is carried out by an enzyme called RNA polymerase and a number of accessory proteins called transcription factors. Transcription factors can bind to specific DNA sequences called enhancer and promoter sequences in order to recruit RNA polymerase to an appropriate transcription site. Together, the transcription factors and RNA polymerase form a complex called the transcription initiation complex. This complex initiates transcription, and the RNA polymerase begins mRNA synthesis by matching complementary bases to the original DNA strand. The mRNA molecule is elongated and, once the strand is completely synthesized, transcription is terminated. The newly formed mRNA copies of the gene then serve as blueprints for protein synthesis during the process of translation.

Out of 64 codons, one initiation codon is AUG and three ending codons are UAA, UAG and UGA. These codons corresponds to the start and end of the polypeptide chain, respectively.

Important difference between Heterochromatin and Euchromatin

	Heterochromatin	Euchromatin
1.	It is a form of DNA in the chromosome that has the characterization of tight packing	It is a form of DNA in the chromosome that has the characterization of loose packing
2.	Dark stain	Light stain
3.	High density of DNA	Low density of DNA
4.	Little or no transcriptional activity here.	There is the active participation of the transcriptional process here
5.	They exist at the periphery of the nucleus. Furthermore, they exist only in eukaryotic cells.	They exist in the nucleus's inner body. Furthermore, they exist in eukaryotic as well as prokaryotic cells.
6.	Coiling is compact, regions are sticky, there is no change in the phenotype of an organism, gene expression regulation is possible, and maintenance of the cell's structural integrity.	Coiling is loose, regions are non -sticky, variation may occur due to the effect in DNA at the time of the genetic process, genetic variations are possible and genetic transcription takes place.

(i) Polymerase - I : It is the enzyme that carries in the daughter nucleotides and DNA helicase is the one that unwinds the double helix to the replication.
(ii) Polymerase - II : It is specialized repair enzyme. Like Pol I, a large number of Pol II, molecules reside in the cell (about 100). The enzyme is more processive than Pol I.

(1) The DNA model has double helical polynucleotide chains or strands which are spirally coiled clock-wise around a common central axis. The thickness of the right handed double helix is about 2 nm (20 Å).
(2) The DNA molecule assumes the shape of a spiral staircase because the nitrogenous bases of successive nucleotide monomers of opposite strands face each other and are joined by hydrogen bonds. The alternating sugar and phosphate molecules of the two strands remain towards the outsides like the long side bars or railings of a staircase. On the other hand, H-bonded bases of opposite strands form the horizontal steps (cross bars) at right angles to central axis. In this way, the two strands cannot be separated easily without completely unwinding them. This type of co coiling is called plectonic.

Difference between Nucleotide and Nucleoside :

Nucleotides	Nucleosides
1. The chemical composition of nucleotide consists of a phosphate group, a sugar and a nitrogenous base.	A nucleoside has a chemical composition that consists of a sugar and a base without the phosphate group.
2. They are one of the major causes of cancer causing agents to this very day.	They are used as agents in medicine that are primarily used against viruses and cancer causing agents.
3. Some of the major examples of nucleotides are adenosine, guanosine etc.	Some of the key examples of nucleosides are the same as nucleotides only with the addition of phosphate groups.

Southern blotting is a laboratory technique used to detect a specific DNA sequence in a blood or tissue sample. A restriction enzyme is used to cut a sample of DNA into fragments that are separated using gel electrophoresis. The DNA fragments are transferred out of the gel to the surface of a membrane.

Virus carrying reverse transcriptase enzyme is retrovirus. That enzyme catalyzes the formation of DNA from RNA. This process is called reverse transcriptase. The DNA formed in this way takes over the DNA of the host cell.

The salient feature of genetic code are as follows :
(i) The codon is a triplet. 61 codons code for 20 amino acids and 3 codons do not code for any amino acids, hence they function as stop codons.
(ii) One codon codes for only one amino acid, hence, it is unambiguous and specific.
(iii) The code is degenerate because some amino acids are coded by more than one codon.
(iv) The codon is read in mRNA in a contiguous fashion. There are no punctuations.

(a) Structure of nucleosome

(b) Histone is like a spool which needs to wind the DNA around. DNA is negatively charged and so, histone has to be positively charged for good interaction between DNA and histone. They have a highly positive charged N - terminus with many lysine and arginine residues (which are positively charged amino acids).

The process of transcription completed in following steps :
(i) The enzyme RNA polymerase binds itself to a site on DNA molecule. This is called promoter site.
(ii) As RNA polymerase moves over the DNA molecule, DNA gets uncoiled and its one strand gerves as template for the synthesis of m - RNA.
(iii) At point called termination point RNA polymerase seperates from DNA and the synthesis of m - RNA steps.