CSIR NET Life Sciences Question Paper: June 2015: with Answer Key and Explanations: Part I

Structure of Eukaryotic Cilia (Image Source Wikipedia)

(26).  The key determinant of the plane of cytokinesis in mammalian cells is the position of:

a.       Chromosome
b.      Central spindle
c.       Centrioles
d.      Pre-prophase band

(27).  Beating of cilia is regulated by:

a.       Actin
b.      Myosin
c.       Cofilin
d.      Nexin

(28).  Cystic fibrosis transmembrane conductance regulator (CFTR) is known to control the transport of which ion?

a.       Ca 2+
b.      Mg2+
c.       HCO3-
d.      Cl-

(29).  In, bacteria, heat-shock response is primarily controlled by :

a.       Sigma S (σs)
b.      Sigma 32 (σ32)
c.       Sigma E (σE)
d.      Sigma 70 (σ70)

(30).  In type II splicing:

a.       A G-OH from outside makes a nucleophilic attack on 5’P of first base of intron
b.      A free 2’O of an internal adenosine makes a nucleophilic attack on 5’P of first base of intron
c.       A 3’O of an internal adenosine makes a nucleophilic attack on 5’P of first base of intron
d.      The hydrolysis of last base of exon is carried out by U2/U4/U6

(31).  Given below are some statements about prokaryotic and eukaryotic mobile genetic elements or transposons.

A.      Most mobile genetic elements in bacteria transpose via an RNA intermediate
B.      Most mobile genetic elements in bacteria are DNA
C.      Mobile genetic elements in eukaryotes are only retrotransposons
D.     Both, RNA and DNA transposons are found in eukaryotes

Choose the correct combination

a.       A and C
b.      B and C
c.       A and D
d.      B and D

(32).  Copying errors occurring during replication are corrected by the proof reading activity of DNA polymerase that recognize incorrect bases

a.       At the 5’ end of the growing chain and remove them by 5’- 3’ exonuclease activity
b.      At the 3’ end of the growing chain and remove them by 3’ – 5’ exonuclease activity
c.       At the 3’ end of the growing chain and remove them by 3’ – 5’ exonuclease activity
d.      At the 5’ end of the growing chain and removed then by 3’ – 5’ exonuclease activity

(33).  During each cycle of chain elongation in translation, how many conformational changes does the ribosome undergo that are coupled to GTP hydrolysis?

a.       Zero
b.      One
c.       Two
d.      Three

(34).  Collagens are the most abundant component of the extracellular matrix. In order to maintain normal physiological processes like wound healing, bone development, etc., which one of the following type of enzyme is MOST important

a.       Peptidases
b.      Proteases
c.       Amylases
d.      Lipases

(35).  Which one of the following event NEVER activates the G protein coupled receptor for sequestering Ca2+ release?

a.       Interaction of binding to sperm receptors
b.      Activation of Frizzled by Wnt
c.       Cortical reaction blocking polyspermy
d.      DNA synthesis and nuclear envelope breakdown

(36).  The main difference between normal and transformed cells are

a.       Immortality and contact inhibition
b.      Shorter generation time and cell mobility
c.       Apoptosis and tumour suppressor gene hyper function
d.      Inactivation of oncogenes and shorter cell cycle duration

(37).  When bacteria growing at 20^oC are warmed at 37^oC, they are most likely to synthesize membrane lipids with more

a.       Short chain saturated fatty acids
b.      Short chain unsaturated fatty acids
c.       Long chain saturated fatty acids
d.      Long chain unsaturated fatty acids

(38).  Hydra shows morphological regeneration and involves which one of the following signal transduction pathway in its axis formation?

a.       Wnt/β-catenin pathway
b.      Retinoic acid pathway
c.       FGF pathway
d.      Delta-Notch pathway

(39).  The mammalian oocyte prior to sperm entry is arrested at what stage of cell division?

a.       Prophase of mitosis
b.      Prophase of meiosis I
c.       G1 phase of mitotic cell cycle
d.      Metaphase of meiosis II

(40).  The pluripotency of the inner cell mass in mammals is maintained by a core of three transcription factors namely,

a.       Oct 4, Sox 2 and Nanog
b.      Oct 4, Sox 2 and Cdx2
c.       Sox 2, Nanog and Cdx2
d.      Oct 4, Cdx2 and Nanog

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Answer key with detailed explanations:

21.  Ans. (d). 2.5 X 10 ⁵

Turnover number of enzyme: Number of substrate molecules converted to product in a given unit of time (one second) by a single enzyme molecule when the enzyme is fully saturated with its substrate. Turnover number of different enzyme varies. Turnover number can be considered as the speed of an enzyme for its catalytic reaction. Catalase enzyme is known to have the highest turnover number, which is 40,000,000. Turnover number of enzyme is represented as K_cat in enzyme kinetics. K_cat is calculated as:

K_cat = V_max/[E]_T

Where Vmax is maximum velocity, [E] Concentration of enzyme

22.  Ans. (c). CO of n and NH of n+4 amino acid

Alpha helical conformation is a type of secondary structure of protein. In alpha helical structure, the polypeptide backbone is tightly wound around an imaginary axis drawn longitudinally through the middle of the helix, and the R groups of the amino acid residues protrude outward from the helical backbone. Each turn in the alpha helix is about 5.4 Å long and each helical turn is with 3.5 amino acids. Alpha helix is stabilized by internal hydrogen bonds. Hydrogen bond formation occurs between the hydrogen atom attached to the electronegative nitrogen atom of a peptide linkage and the electronegative carbonyl oxygen atom of the fourth amino acid on the amino-terminal side of that peptide bond. Thus, virtually all amino acids in alpha helix are in hydrogen bonds. Naturally occurring alpha helices are right handed.

23.  Ans. (b). (c) > (a) > (b)

24.  Ans. (d). Dihydrolipoyl oxidase

Pyruvate dehydrogenase enzyme (PDC) complex is a complex enzyme catalyzing the ‘link’ reaction of respiration which convert the pyruvic acid (end product of glycolysis) to acetyl co-A by a process called pyruvate decarboxylation. As the name suggests, PDC complex enzyme consists of three different enzymes namely E1, E2 and E3 and each of these three enzymes have different sets of cofactors.

Enzymes in the PDC complex:

1.      Pyruvate dehydrogenase (E1): cofactor: TPP (Thiamine pyrophosphate)

2.      Dihydrolipoyl transacetylase (E2): cofactors: Lipoate and coenzyme A

3.      Dihydrolipoyl dehydrogenase (E3): cofactors: FAD and NAD+

25.  Ans. (b). Immunodeficient

V(D)J Joining: A site specific recombination process during B cell development, where the V and J gene segments (for light chain) and the V, D, and J gene segments (for heavy chain)  are joined together to form a functional VL- or VH-region coding sequence. This site specific joining is mediated by an enzyme complex called the V(D)J recombinase, consisting of two parts:

(1) Lymphocytes specific enzyme

(2). Enzymes help in DNA repair)

Lymphocyte-specific proteins of V(D)J recombinase are encoded by two linked genes called rag-1 and rag-2 (rag = recombination activating genes). The RAG proteins first introduce double-strand breaks at the flanking DNA sequences, and this is followed by a rejoining process that is mediated by both the RAG proteins and the enzymes involved in general DNA double-strand repair. Individuals who are deficient in either rag gene are immunodeficent because they are unable to V(D)J joining and thus they highly susceptible to infection. These individuals do not have functional B or T cells. T cells use the same recombinase to assemble the gene segments that encode their antigen-specific receptors.

26.  Ans. (b). Central spindle

Cytokinesis is the division of cytoplasm after the karyokinesis (division of nucleus) of cell division. The process of cytokinesis is different in plants and animals. In animals, cytokinesis proceeds by the furrowing and invagination of plasma membrane at the plane of spindle equator. However in plants, due to the presence of cell wall, the simple furrowing process of plasma membrane is not possible and here the cytokinesis is assisted by the formation of phragmoplast and cell plate.

The key determinant of the plane of cytokinesis in animal cells is the position of central spindle. Spindle fibres are the part of spindle apparatus which assists in the chromosomal segregation during mitotic and meiotic cell divisions. There are three types of spindle fibres.

(1). Chromosomal spindles (or kinetochore microtubule): spindle fibres attached to the kinetochore of chromosome and they pull the chromosomes towards the pole to assist in chromosome segregation.

(2). Astral fibres (or astral microtubules): they are the spindles which are not attached to any chromosomes, they simply radiate from the centrosome.

(3). Central spindle (or pole-to-pole spindle, or polar microtubules): they are the microtubules emerging from the opposite poles and arranged as overlapping, parallel bundles in the centre (equator of the cell) during anaphase. They are regarded as the key determinant of the plane of cytokinesis in animal cells. Central spindle recruit various proteins to the central portion of the cell to assist the furrowing of plasma membrane. They are also involved in the formation of ‘midbody’ or Flemming body during cytokinesis. Midbody of Flemming body is the transient structure found in mammalian cells just near the end of cytokinesis.

27.  Ans. (d). Nexin

Eukaryotic cilial axoneme shows (9 + 2) organization. They composed of nine outer units and two central units. Each of the nine outer units is composed of one complete unit (called A tube) and one incomplete unit (called B tube). The central two units composed of single complete units. The dynein arms are seen as ‘fussy’ projections from the complete microtubule (A tube). There are two types of dynein arms, outer three headed dynein arms and inner two headed dynein arms. The nine peripheral units in the axoneme are interconnected by nexin bridge. Nexin bridge regulate the beating of cilia. The dynein arms are the motor proteins which convert the chemical energy of ATP into mechanical energy for movement.

Cofilin: it is an actin binding protein which assists the disassembly of actin filaments in the cells. They are very essential in the actin filament organization.

Actin: they are the globular proteins which form microfilaments and they are the components of cytoskeleton. In the cells, actin can exists as globular free monomeric units called G actin (globular actin) or as linear polymer called F actin (filamentous actin). Actin participate in many cellular events such as muscle contraction, cell motility, cell division, cytokinesis, vesicular transport, movement of cell organelles, cell signaling and maintaining the structure and shape of the cell as cytoskeleton. There are three types of actin isomers in vertebrates including human. They are α-actin, β-actin, γ-actin. α-actin is commonly found in the muscles. β-actin, γ-actin are present in the cytoskeleton microfilaments.

Actin concentration in the cells is also used as an internal control in many molecular biology techniques such as quantitative PCR and western blotting

Myosin: Myosin is an ATP dependent motor protein involved in the contraction of muscles. Structurally myosin consists of a head, neck and a tail. The head portion possesses the ATPase activity and it associate with actin filament.

28.  Ans. (d). Cl^–

CFTR (Cystic fibrosis Transmemrane Conductance Regulator) is a trans-membrane transporter, which transport chloride and thiocyanate ions across plasma membrane of epithelial cells. They belong to the ABC Transporter (ATP Binding Cassette transporter) of the ion channel protein category. Functioning of CFTR is very essential for the proper maintenance of chloride ion channel functioning in the cells and the damage in the functioning causes problems in epithelial fluid transport. Cystic fibrosis is a recessive autosomal genetic disorder in human caused by the mutations in the CFTR gene.

29.  Ans. (b). Sigma 32 (σ32)

30.  Ans. (b). A free 2’O of an internal adenosine makes a nucleophilic attack on 5’P of first base of intron

Type two splicing is a type of RNA splicing method and it belongs to the category of self-catalytic ribozymes splicing. The major difference of type II splicing from type I splicing is that, in type II splicing, GTP is not required and also here a lariat is formed. Group II introns are found in rRNA, tRNA and mRNA of mitochondria and chloroplasts of fungi, plants, protists and also in bacteria.

31.  Ans. (d). B and D

Mobile genetic elements are also called as transposons or transposable element or jumping genes, as they name suggests they change their position in the genome either by ‘cut-copy’ or ‘copy-paste’ mechanism. There are three classes of transposons, they are:

(1). Class I transposons (retro-transposons): transposition by copy paste mechanism, there will be an RNA intermediate

(2). Class II transposons (DNA transposons): transposition by cut copy mechanism, no RNA intermediate is formed

(3). Autonomous and non-autonomous Transposable elements

32.  Ans. (c). At the 3’ end of the growing chain and remove them by 3’ – 5’ exonuclease activity

33.  Ans. (c). Two

34.  Ans. (b). Proteases

35.  Ans. (b). Activation of Fizzled by Wnt

36.  Ans. (a). Immortality and contact inhibition

37.  Ans. (c). Long chain saturated fatty acids

Saturation and chain length of fatty acid is indirectly proportion to the melting point. A fatty acid with many unsaturation and long chain shows relatively lower melting point, which means that these fatty acids will tend to stay as liquid under lower temperature condition. Organisms which are adapted to cold environmental conditions show relatively higher concentration of short and unsaturated fatty acids in the plasma membrane. A bacteria growing in at 20^oC will be having their plasma membrane with short chain and unsaturated fatty acids to maintain the fluidity of plasma membrane. When this bacterium is immediately transferred to higher temperature (here 37^oC) the fluidity of the plama-membrane will increase and it will disrupt the membrane. So in order to maintain the integrity of plasma membrane, the cell immediately change its plasma-membrane composition with large number of long and saturated fatty acids, these fatty acids will have higher melting points.

38.  Ans. (a). Went/β-catenin pathway

39.  Ans. (d). Metaphase of Meiosis II

40.  Ans. (a). Oct 4, Sox 2 and Nanog

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