Cell Biology

(c) (a) is true but (b) is false.

Mitochondria and chloroplasts are semi-autonomous: they have their own DNA and divide by fission, so (a) is true. Statement (b) is false because they also contain ribosomes (70S) and the machinery to synthesise some of their own proteins.

(b) Kinetochore of the chromosome

Spindle fibres attach to kinetochores, protein discs on either side of the centromere. The centromere is the primary constriction that carries the kinetochores, not the attachment site itself; telomeres are chromosome ends.

(b) Spindle fibres, Centrioles and Cilia

Microtubules (tubulin polymers) make up spindle fibres, the 9+0 triplets of centrioles, and the axonemes of cilia and flagella. Peroxisomes, chromatin and nucleosomes are not microtubular, which rules out the other options.

(c) Zygotene

Per the NEET key, crossing over is initiated at zygotene, when homologues begin synapsis and the synaptonemal complex forms. The recombination nodules complete exchange at pachytene and chiasmata become visible at diplotene, but initiation is placed at zygotene; leptotene is only chromosome condensation.

(c) Lysosomes

Lysosomes are bound by a single membrane. Mitochondria and chloroplasts have a double membrane (envelope), and the nucleus is enclosed by a double-layered nuclear envelope. Lysosomes arise from the Golgi apparatus and hold hydrolytic enzymes.

(d) Anthocyanins

Anthocyanins are water-soluble pigments dissolved in the cell sap of vacuoles, giving red, blue and purple colours to flowers and fruits. Chlorophylls, carotenes and xanthophylls (carotenoids) are lipid-soluble and are located in plastids, not in the vacuole.

(d) Synapsis

Synapsis, the pairing of homologous chromosomes, occurs in zygotene of prophase I of meiosis, not in mitosis. Spindle fibres, disappearance of the nucleolus in prophase, and chromosome movement during anaphase are all features of mitosis in somatic cells.

(b) Nuclear envelope

Prokaryotes have no true nucleus, so they lack a nuclear envelope. Plasma membrane and ribosomes (70S) are present in all cells, and mesosomes are infoldings of the plasma membrane in bacteria.

(c) Ribosomes

Ribosomes are made of rRNA and proteins and have no surrounding membrane. Mesosomes, vacuoles and lysosomes all have a membrane (or are membranous folds).

(a) without cell wall

A protoplast is a plant, bacterial or fungal cell whose cell wall has been removed, usually by enzymes such as cellulase and pectinase. It still has its plasma membrane and nucleus and is not a dividing cell.

(b) (b), (a), (d), (c)

Meiosis I prophase order: synapsis (zygotene), crossing over (pachytene), disappearance of the nucleolus (late diplotene/diakinesis), then terminalisation of chiasmata (diakinesis). The other options place crossing over before synapsis or terminalisation before nucleolar disappearance.

(c) (iii), (iv), (i), (ii)

Thylakoids are flat membranous sacs in the chloroplast stroma (iii); cristae are mitochondrial inner membrane infoldings (iv). Cisternae are disc-shaped sacs of the Golgi apparatus (i), and chromatin is the condensed DNA structure (ii).

(d) amount of DNA doubles in each cell.

S (synthesis) phase is when DNA replicates, so DNA content goes from $2C$ to $4C$. The chromosome number stays the same (sister chromatids stay attached at the centromere), so option (b) is wrong.

(a) Microfilaments

Microfilaments are solid rods about 6 nm wide made of one protein, actin. Microtubules are hollow, about 25 nm wide, and made of alpha and beta tubulin; intermediate filaments are about 10 nm and made of various proteins.

(d) Pachytene

Crossing over occurs at pachytene, where recombination nodules form on the synaptonemal complex and the enzyme recombinase catalyses the exchange of segments between non-sister chromatids. Zygotene is only synapsis (pairing), while diplotene and diakinesis show chiasmata and terminalisation, not the exchange itself.

(a) Vacuoles

In a plant cell kept in water, water enters by osmosis mainly into the large central vacuole, whose tonoplast regulates solute and water content and so the turgor and expansion of the cell. Plastids, ribosomes and mitochondria do not regulate osmotic water uptake.

(d) (iv), (ii), (i), (iii)

Centriole forms the basal body of cilia and flagella (iv), chlorophyll is located in the thylakoids (ii), cristae are infoldings of the inner mitochondrial membrane (i), and ribozymes are catalytic nucleic acids (RNA) (iii). This sequence is given only by option (d).

(c) $G_2$ and M

DNA is replicated in S phase, so a cell goes from 2C to 4C by the end of S and stays at 4C through $G_2$ and M (until the chromosomes separate at anaphase and the cell divides). $G_1$ and $G_0$ cells are at 2C, and in S the amount is increasing, not fixed at 4C.

(b) SER

The smooth endoplasmic reticulum is the main site of lipid synthesis (and steroids). The rough ER makes proteins on its ribosomes.

(c) Bivalent

In zygotene homologous chromosomes pair up by synapsis, and the pair is a bivalent (a tetrad of four chromatids). The equatorial plate is the metaphase plane, and the kinetochore is the centromere's protein disc.

(a) Meiocyte

Meiosis occurs in the meiocyte (spore mother cell, i.e. a diploid cell that divides to give haploid cells). Conidia and gemmules are asexual reproductive units, and a megaspore is a product of meiosis.

(b) Condensation → nuclear membrane disassembly → arrangement at equator → centromere division → segregation → telophase

In mitosis the chromosomes condense, the nuclear envelope breaks down, and the chromosomes align at the metaphase plate. The centromeres then divide and the chromatids separate (segregation) before telophase. Crossing over occurs only in meiosis I, which rules out options (a) and (c). Option (d) omits nuclear membrane breakdown.

(d) Mitochondrion

Mitochondria oxidise carbohydrate-derived pyruvate through the Krebs cycle and the electron transport chain, and make most of the cell's ATP. Chloroplasts make ATP from light, not by breaking down carbohydrates. Lysosomes digest materials and ribosomes make proteins.

(c) The DNA is condensed into a Chromatin Fibre

Histone H1 binds the linker DNA at the nucleosome and helps pack nucleosomes into the higher-order chromatin fibre. Condensed chromatin is not accessible for transcription, so (a) and (d) are wrong. H1 binding does not indicate replication.

(c) Chromosomes will not segregate

The anaphase promoting complex degrades securin and cyclin so that sister chromatids can separate at anaphase. If APC is defective, the chromatids stay joined and the chromosomes will not segregate. Condensation happens earlier and does not need APC.

(c) Prior to fission

Bacteria are prokaryotes with no S-phase or nucleolus. Their circular DNA replicates before binary fission so that each daughter cell gets a copy. Transcription is a separate process.

(b) Diplotene

In diplotene the synaptonemal complex dissolves and the paired homologous chromosomes begin to separate, except at chiasmata where crossing over occurred. Pachytene is when crossing over takes place, zygotene is synapsis, and diakinesis is terminalisation of chiasmata when the nuclear envelope breaks down.

(d) It is a site for active ribosomal RNA synthesis

The nucleolus is a non-membrane-bound region in the nucleus where rRNA is synthesised and ribosomal subunits are assembled. It is larger in cells with high protein synthesis, not specifically dividing cells (it disappears during division), and it takes no part in spindle formation.

(b) Formation of secretory vesicles

The Golgi complex modifies, packages and dispatches materials, forming secretory vesicles from its trans face. Fatty acid breakdown occurs in peroxisomes, respiration in bacteria occurs at the plasma membrane/mesosomes, and amino acid activation occurs in the cytoplasm.

(d) Phospholipid synthesis

Phospholipid and other lipid synthesis occurs in the smooth ER. The rough ER, studded with ribosomes, handles protein folding, glycosylation and cleavage of the signal peptide.

(a) Polysome

Several ribosomes attached to a single mRNA and translating it at once form a polysome (polyribosome), giving many copies of the polypeptide quickly. A nucleosome is DNA wrapped around histones, and plastidome is the total plastids of a cell.

(d) Polytene chromosomes – Oocytes of amphibians

Polytene chromosomes occur in the salivary glands of Drosophila and other dipterans, while lampbrush chromosomes are found in amphibian oocytes. Lampbrush chromosomes are diplotene bivalents, allosomes are sex chromosomes, and submetacentric chromosomes are L-shaped at anaphase.

(a) Lysosomes and Vacuoles

Lysosomes and vacuoles are single-membrane compartments with no DNA. Mitochondria and chloroplasts have their own DNA, and the nuclear envelope is a membrane, but the nucleus inside carries DNA, which is why options with mitochondria or chloroplast are wrong.

(c) exit the cell cycle

Cells in the G0 (quiescent) phase have exited the cell cycle and are metabolically active but do not divide unless called on to do so. They have not terminated or suspended the cycle, and they are not entering it.

(b) G1 → S → G2 → M

Interphase has G1 (growth), S (DNA replication) and G2 (preparation for division). It is followed by the M phase (mitosis). The order is always G1, S, G2, M.

(d) p-arm and q-arm respectively

In chromosome nomenclature the short arm is p (petit) and the long arm is q. A submetacentric chromosome has its centromere slightly off the middle, so one arm (p) is shorter and the other (q) is longer. The names m, n, s and l are not used.

(d) Enzymes of electron transport are embedded in outer membrane.

The electron transport chain enzymes and ATP synthase sit in the inner mitochondrial membrane, which is folded into cristae. The outer membrane is smooth and permeable to small molecules, and the matrix holds circular DNA and ribosomes. Statement (d) is therefore incorrect.

(b) Lysosomes are formed by the process of packaging in the endoplasmic reticulum

Lysosomes are membrane-bound vesicles made by packaging in the Golgi apparatus. The enzymes are made on rough ER and then processed in the Golgi, so packaging does not occur in the ER. The other statements are correct: lysosomes are membrane-bound, hold many hydrolases, and these work best at acidic pH.

(c) Rudolf Virchow

Schleiden and Schwann proposed the cell theory. Rudolf Virchow (1855) added that new cells arise from pre-existing cells ("Omnis cellula-e cellula"). Aristotle is not linked to this idea.

(a) Golgi bodies

Proteins and lipids from the ER are modified in the Golgi apparatus. NCERT states that the Golgi is the important site of formation of glycoproteins and glycolipids. The ER makes the proteins and lipids, polysomes make proteins, and peroxisomes handle oxidative reactions.

(a) Diplotene

The synaptonemal complex forms during zygotene and is complete in pachytene. It dissolves at diplotene, when the homologous chromosomes start to separate and chiasmata become visible. Leptotene is the stage when chromosomes condense and become visible.

(d) a-(iv), b-(iii), c-(ii), d-(i)

Zygotene is the stage of synapsis. Crossing over occurs in pachytene. Chiasmata are seen in diplotene, and diakinesis shows terminalization of the chiasmata. Only option (d) has all four right.

(d) These are involved in ingestion of food particles

Inclusion bodies are reserve materials such as glycogen granules, cyanophycean granules and gas vacuoles in prokaryotes. They lie free in the cytoplasm and are not bounded by a membrane. They do not ingest food, so statement (d) is incorrect.

(d) G1 phase

Cells that will not divide again leave the cycle from the G1 phase and enter the quiescent stage G0. They remain metabolically active but do not proliferate unless called on to do so. Exit does not occur after S, G2 or M.

(a) Cell is metabolically active, grows but does not replicate its DNA

G1 is the gap between mitosis and the start of DNA replication; the cell grows and is metabolically active. DNA synthesis occurs in the S phase, nuclear division in M phase, and reorganisation of cell components in G2/M.

(a) S phase

In eukaryotes DNA replication occurs in the S (synthesis) phase of interphase, between G1 and G2. The DNA content doubles from 2C to 4C, while the chromosome number stays the same.

(b) Anaphase II

Centromeres divide only in anaphase II of meiosis, when sister chromatids separate. In anaphase I whole homologous chromosomes separate with centromeres intact.

(a) Pachytene

Recombination nodules appear at pachytene, where crossing over occurs with the enzyme recombinase. Synapsis occurs in zygotene, and chiasmata become visible later in diplotene.

(b) A-IV, B-I, C-II, D-III

M phase is equational division, G2 is where proteins are synthesised for mitosis, the quiescent stage (G0) is inactive, and G1 is the interval between mitosis and the start of DNA replication.

(d) 80

The ribosome consists of about 80 different proteins plus rRNAs, which is the NCERT-based figure expected here. 20 corresponds to amino acid types, a tempting but wrong match.