BIOLOGY 3 Marks Question

1. Used in forensic science to identify suspects.
2. Used to find out history of an organism.
3. Used to find out paternity and family relations.

• Two strands running opposite to each other, wherein bases will always pair with their counterpart-A with T and G with C (specific pairing).
• If H bonds break and bases of one strand lie exposed, unpaired, they will easily pair up with free nucleotides as well. This type of arrangement in DNA molecule led to the hypothesis that DNA replication is semi-conservative. where the two strands separate and act as a template for the synthesis of a new complementary strand.

Universal -Codon and its corresponding amino acid are the same in all organisms.

Example:- more-Bacteria to human UUU codes for phenylalanine (phe).

Degenerate - Some amino acids are by more than one codon.

Example:- UUU and UUC code for phenylalanine (phe).

1. Centrifugation in a CsCl density gradient.
2.  

1. To develop new and improved medicines.
2. To predict and prevent diseases.
3. To ensure that the diagnosis is accurate.

1. It is attached at the 3' end of RNA.
2. UUU and UUC.
3. Aminoacyl-tRNA synthetase.

1. Molecule 'X' is a repressor protein. When an inducer combines with it, it is inactivated.
2. 'z' gene.
3. Transcription of gene stops.

1. Substrate (lactose) is not available.
2. Energy source (glucose) is available to the cells.

1. a to a' is 5' → 3'. No more amino acid will be added.
2. TCA; anticodon is UCA.
3.  

1. The untranslated regions are required for efficient translation process.
2. They are present before the initiation codon at the 5'-end and after the stop/ termination codon, at the 3'-end.

1. Both the strands of parent DNA function as template strands.
2. On the template strand with 3' → 5' polarity, the new strand is synthesised as a continuous stretch as the DNA polymerase can carry out polymerisation of the nucleotides only in 5' → 3' direction; this is called continuous synthesis and the strand is called leading strand.
3. On the other template strand with 5 → 3' polarity, the new strand is synthesised from the point of replication fork, also in 5' → 3' direction, but in short stretches; they are later joined by DNA-ligases to form a strand, called lagging strand.

1. Restriction endonuclease.
2. Agarose gel.
3. Nitrocellulose/ nylon/ synthetic membrane.
4. Variable Number Tandem Repeats (VNTR).
5. Hybridisation with VNTR probe.
6. Autoradiography.

1. At the replication site, unwinding of double stranded DNA takes place by DNA gyrase and helicase.
2. ssBPs (single-stranded binding proteins) bind to the separated strands to avoid recoiling or to provide stability.
3. Since DNA polymerase can synthesise DNA only in 5′ → 3′ direction, DNA synthesis occurs discontinuously on the lagging strand.
4. These small fragments of DNA are called Okazaki fragments.
5. The enzyme primase adds primers after every fragment is formed.
6. These Okazaki fragments are then joined by DNA ligase.

1. Since eukaryotes have split gene arrangement, the hRNA has both coding sequences (exons) and non-coding sequences (introns) and is non-functional; so it has to undergo splicing, the process, in which introns are removed and exons are joined.
2. It has to undergo two other processes, namely capping and tailing to become functional.
3. In capping, an unusual nucleotide, called methyl guanosine triposphate residues are added at the 5' end of hnRNA.
4. In-tailing, about 200-300 adenylate residues are added at the 3' end, in a template-independent manner.
5. These changes take place in the nucleus of the cell.

(DNA dependent) RNA polymerase, binds to the promoter, at 5’ end, associates transiently with initiation factor/sigma factor, using nucleoside triphosphates as substrate, and energy initiates transcription.

1. They do not code for any proteins,
2. They form large part of the human genome,
3. They show high degree of polymorphism/Specific to each individual.

1. DNA replication occurs in small replication forks. It does not occur in its entire length in one time as DNA is a very large molecule and only that part of DNA opens up which is being replicated. The opening of the whole DNA molecule would be an energetically more expensive process.
2. The main enzyme involved in DNA replication is the DNA-dependent DNA polymerase. This enzyme catalyzes the polymerization of deoxynucleotides along the 5′ → 3′ direction, and hence, replication is continuous along the 3′ → 5′ strand (leading strand) and discontinuous along the template, i.e., the 5′ → 3′ direction (lagging strand).
3. Okazaki fragments are short DNA segments on the lagging strand, formed in the 5’ – 3’ direction, starting from RNA primers. A separate RNA primer is needed for the synthesis of each Okazaki fragment. These discontinuously synthesized fragments are later joined by the enzyme DNA ligase.
4. Ori stands for Origin of replication. This site has the highly conserved sequence of DNA among various species. The replication of DNA starts here because this site attracts some proteins which help in the opening and unwinding of DNA and this leads to the initiation of replication.

The function of DNA Ligase is to join the two nucleotides. During the DNA replication process, it joins the Okazaki fragments of the daughter DNA to form the complete DNA molecule on the lagging strand.

1. In the mammalian cells (or eukaryotes) there is a set of positively-charged basic proteins, called histones.
2. Histones are organised to form a unit of eight molecules, called histone octamer.
3. The negatively charged DNA is wrapped around the positively charged histone octamer to form a structure, called nucleosome.
4. A typical nucleosome contains 200 bp of DNA helix.
5. The nucleosomes constitute the repeating units of chromatin, which appear as beads-on-string structure under an electron microscope.
6. These are further packaged to form the chromatin fibres, which condense to form chromosomes.
7. The packaging of chromatin at higher levels requires additional set of proteins called non-histone chromosomal (NHC) proteins.

1. Aminoacylation of RNA.
2. mRNA codon-AUG, RNA anticodon-UAC.
3. Enzyme-aminoacyl RNA synthetase.

1. The two strands of a DNA are said to be complementary to each other, i.e., a purine of one strand always pair with a pyrimidine.

1. Adenine (a purine) pairs with thymine (a pyrimidine) by forming two hydrogen bonds.
2. Guanine (a pyrimidine) by forming three hydrogen bonds.

2. The two chains of a DNA shows antiparallel polarity, which means that if one strand has 5' 3' polarity, the other strand has 3' → 5' polarity.

1. The regulatory gene in lac operon (also called i gene or inhibitor gene) codes for a protein, called repressor; the repressor is synthesised all the time constitutively in the cell.
2. The repressor has high affinity to the operator; it binds to the operator region and prevents the RNA polymerase from transcribing the structural genes of the operon.
3. It is called negative regulation because the operon is switched off and transcription is prevented.

1. DNA-dependent DNA polymerase is the enzyme.
2. Two characteristic features of this enzyme include:

1. It catalyses the polymerisation of nucleotides in 5' → 3' direction only.
2. It polymerises approximately 2000bp per second, with high degree of accuracy.

3. Origin of replication is the region on E.coli DNA, where this enzyme can initiate replication.

1. a to d' is 5 → 3'.

No more amino acid will be added.

1. TCA; anticodon is UCA.
2. The untranslated regions are required for efficient translation process.

They are present before the initiation codon at the 5' end and after the stop/termination codon, at the 3' end.

1. AUG GAG UAC UUC GUG UGA
2. It will code for five amino acids.

The last codon, UGA is a termination codon. that does not code for any amino acid.

1. TACAGATAT
2. UACAGAUAU
3.  

1. The regions of a chromatin, which are stained light, are the regions where chromatin is loosely packed; they are called euchromatin.
2. The regions of a chromatin, which are stained dark, are the regions where chromatin is tightly packed; they are called heterochromatin.
3. Euchromatinis transcriptionally more active, while heterochromatin is inactive.

1. The operator' region is adjacent to the promoter region.
2. The operator regulates the accessibility of promoter region to the RNA polymerase for transcription.
3. The operator binds to the repressor protein coded by the regulatory gene and prevents the RNA polymerase from binding to the promoter; the operon is switched "off".
4. When an inducer binds to the repressor and inactivates it, it cannot bind to the operator; this allows RNA polymerase access to the promoter and transcription continues, i.e., the operon is switched 'on'.

• Purification of biochemicals like Proteins, RNA & DNA from S cells. (heat killed)
• Presence of Protein & RNA in medium did not affect transformation.
• DNA alone from S Bacteria caused R Bacteria to transform.
• Digestion with DNAase did inhibit transformation,

Conclusion: DNA is the transforming chemical/biochemical.

1. X is N-glycosidic linkage; it forms a nucleoside.
2. Y is phosphoester linkage; it forms a nucleotide.
3. Z is a phosphodiester linkage; it forms a dinucleotide.

1. The properties that can be correlated:

1. UAA does not code for any amino acid; it is a termination codon.
2. Genetic code is specific and unambiguous, i.e. one codon codes for a particular amino acid only.
3. Each codon is a triplet.
4. Genetic code is degenerate, as one amino acid is coded by more than codon, e.g. UUU and UUC code for phenylalanine.
5. Genetic code is read in a contiguous manner without any punctuation.

2. The property that cannot be correlated:

AUG has a dual function; it is initiation codon as well as codes for methionine.

1. A-5', B-3
2.  

1. The figure represents the continuous and discontinuous synthesis of DNA strands at the replication fork, during replication of DNA.
2. Replication fork is the Y-shaped structure formed with small opening of DNA double helix.
3. DNA polymerase catalyses polymerisation of nucleotides only in 5' → 3' direction.
4. Both the strands of parental) DNA act as templates for the synthesis of new strands.
5. On the template strand with 3' → 5' polarity, the new strand is synthesised as a continuous stretch; it is called continuous synthesis.
6. On the other strand with 5' → 3' polarity, DNA is synthesised as short stretches; it is called discontinuous synthesis.
7. Later the short stretches of DNA are joined by DNA-ligases into a continuous strand.

B. DNA polymerase catalyses the polymerisation of nucleotides, Ligase joins the fragments of discontinuous synthesis.

 Meselson and Stahl

1. E.coli grown in medium containing ¹⁵NH₄CL (¹⁵N - heavy Nitrogen) for many generations to ensure that all DNA in the bacteria were heavy,
2. Heavy E.coli transferred to a medium with normal ¹⁴NH₄CL,(after 20 minutes) DNA of generation I extracted to measure their densities, they were of intermediate density,
3. After 40 minutes DNA of II generation were extracted and tested for their densities, they were of equal amounts of (hybrid) intermediate DNA, and light DNA/¹⁴NH₄Cl.

1. At the 3' end.
2. UUU or UUC.
3. Aminoacyl tRNA synthetase.

1. Replication of DNA occurs in small replication forks, because DNA is such a long molecule that the separation of the two strands along its entirelength requires a very high amount of energy.
2. DNA polymerase can catalyse the polymerization of nucleotides only in 5' → 3' direction.

1. So on the template strand with 3' → 5' polarity, DNA replication is continuous.
2. On the template strand with 5' → 3' polarity, DNA synthesis occurs in short stretches as the opening of replication fork continues; later these short stretches are joined by the action of DNA ligases .

2. Replication of DNA does not initiate randomly, and DNA polymerases on their own cannot initiate replication.

1. So, there is a specific sequence on DNA, called origin of replication; DNA polymerase bindsto it and continues the process.

2. On one hand, it reads the code.

On the other hand, it binds to specific amino acid.

a - AUG.

b - UAA/UAG/UGA.

AUG code for Methionine.

UAA/UAG/UGA - Stop codon/Nonsense codon/Does not code for any amino acid.

Charged tRNA are brought closer together on mRNA in the ribosomes, and Ribosomes acts as a catalyst (ribozyme) forming peptide bond.

1. VNTR stands for “Variable Number of Tandem Repeats”.

The VNTR belongs to a class of satellite DNA referred to as mini-satellite. A small DNA sequence is arranged tandemly in many copy numbers. The copy number varies from chromosome to chromosome in an individual. The numbers of repeat show very high degree of polymorphism. As a result th size of VNTR varies in size from 0.1 to 20kb. Consequently, after hybridization with VNTR probe, the autoradiogram gives many bands of differing sizes. These bands give characteristic pattern for an individual DNA which is used to identify individuals.

2. Since DNA from every tissue (such as blood, hair-follicle, skin, bone, saliva, sperm etc.), from an individual show the same degree of polymorphism, they become very useful identification tool in forensic applications to identify criminals. Further, as the polymorphisms are inheritable from parents to children, DNA fingerprinting is the basic of paternity testing, in case of disputes.

1. Since 'RNA on one hand binds to a specific amino acid and on the other hand reads the codon of the amino acid bound to it through its anticodon, it is called an 'adapter'.
2.  

It actually looks like an inverted L.

2. The RNA transcribed is 5' AUGCAUGCAUAG 3'.