Biology 3 Marks Question

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A gene is the functional unit of inheritance which are located on the DNA. The DNA sequence coding for t-RNA or r-RNA molecule also define a gene. A cistron is a segment of DNA coding for a polypeptide, the structural gene in a transcription unit could be said as monocistronic (mostly in eukaryotes) or polycistronic (mostly in bacteria or prokaryotes). In eukaryotes, the monocistronic structural genes have interrupted coding sequences—the genes in eukaryotes are split. The coding sequences or expressed sequences are defined as exons. Exons are said to be those sequence that appear in mature or processed RNA. The exons are interrupted by introns. Introns or intervening sequences do not appear in mature or processed RNA.
Before processing, the primary transcript of RNA contains both exons and introns and are known as heterogenous nuclear RNA/hn-RNA. The split-gene arrangement further complicates the definition of a gene in terms of a DNA segment. Inheritance of a character is also affected by promoter and regulatory sequences of a structural gene. Hence, sometime the regulatory sequences are loosely defined as regulatory genes, even though these sequences do not code for any RNA or protein.

The formation of a poly-peptide chain from mRNA is known as translation. The process of translation takes place on ribosome. The sequence of three nitrogen bases present in 5'-3' direction on m-RNA is called genetic code. This code determines the sequence of amino acids in a poly-peptide chain. Ribosome joins the m-RNA at 5' and when it moves towards 3', messages of DNA present as codon in m-RNA are translated.
The process of translation is completed in following steps :
1. Activation of Amino acids
2. Charging of t-RNA
3. Initiation
4. Elongation of polypeptide chain
5. Termination of polypeptide chain

Heteronuclear RNA : In eukaryotes, the RNA formed after transcription of DNA is called hnRNA. It is also called high molecular weight RNA or pre-nuclear RNA or DNA-like RNA. This primary messenger RNA is made up of two types of parts. One of these is called intron. There is no code in it. The second part is called exon which carries the genetic code. Of these, introns are removed by the process of RNA splicing. A nuclear organ called spliceosome helps in this process. After this m-RNA is formed, which takes part in the action of translation.
Small nuclear RNA : Small nuclear RNA (snRNA) is one of the small RNA localized in the nucleus of eukaryotic cell. they are complexed with many proteins to form RNA-protein complexes, termed as small nuclear ribonucleoproteins (snRNPs), in the cell nucleus. they are also called snurps. Small nuclear RNAs (snRNAs) are critical components of the spliceosome that catalyze the splicing of RNA.

The enzymes participating in DNA replication are as follows :
(i) Helicase enzymes : Open the double helix of DNA.
(ii) S.S.B. proteins keep the position stable by associating with single stranded binding proteins.
(iii) Topoisomerase enzymes : Reduce the coiling tension.
(iv) RNA polymerase or primase enzyme (R.N.A. primer) : make small part of RNA to process DNA synthesis.
(v) DNA polymerase III enzyme : work to advance the primer in 5'-3' direction.
(vi) DNA Polymerase I : Fills the spaces found between the DNA.
(vii) Ligase enzymes : Join the DNA fragments.
(viii) Endonuclease and exonuclease enzymes : work to connect the correct nucleotides.

Many nucleotides join together to form a chain of polynucleotides and form the structure of DNA and RNA. Each nucleotide has three components : nitrogenous base, pentose sugar (ribose in RNA and de-oxyribose in DNA) and a phosphate group. There are two types of nitrogenous bases—purines (adenine and guanine) and pyrimidines (cytosine, uracil and thymine). Cytosine is found in both DNA and RNA, whereas thymine is found in DNA. Uracil is found in RNA in place of thymine.
Nitrogenous base by binding to pentose sugar, forms nucleotides, such as - adenosine or deoxyadenosine, guanosine or deoxy guanosine, cytidine or deoxycytidine and uridine and deoxy thymidine. When the phosphate group gets attached to the nucleoside 5' hydroxyl group by phosphoester bond, then the corresponding nucleotide is formed. It happens that two nucleotides are joined by 3'-5' phosphoester bond to form a dinucleotides. In this way, several nucleotides join together to form a polynucleotide chain.