CSIR-NET Life Sciences Unit 5 PYQs: Developmental Biology Questions with Solutions

Drosophila segmentation is a cascade. Maternal gradients (Bicoid, Nanos, Hunchback) switch on the gap genes in broad bands. Gap proteins together set the seven stripes of the pair-rule genes (eve, ftz). Pair-rule proteins then fix the 14 stripes of the segment-polarity genes (engrailed, wingless). Hox genes give each segment its identity.

Drosophila Hox genes show colinearity: their order on the chromosome matches the order of the body regions they specify. Antennapedia (Antennapedia complex) acts in the thorax, especially T2. Ultrabithorax acts in T3 and A1. abdominal-A acts in the middle abdominal segments. Anterior to posterior: Antp, Ubx, abd-A.

Answer: (1) The third thoracic segment is transformed to another second thoracic segment and a fly with four wings.

Ubx gives the third thoracic segment its identity: haltere instead of wing, and the T3 leg. Without it, T3 falls back to the identity of T2, so the haltere becomes a second wing and the fly has four wings. This is the famous Lewis bithorax fly.

Answer: (3) Transformation of one segment into another

Homeotic (Hox) genes do not create segments. They tell each existing segment what to become. A loss or gain of function therefore turns one segment into the likeness of another, for example legs in place of antennae in Antp gain of function. Missing every other segment is a pair-rule defect, missing contiguous blocks is a gap defect, and polarity defects in every segment come from segment-polarity genes.

Answer: (2) AP2

In the ABC model, A-class APETALA2 is transcribed in all four whorls. Its protein is confined to whorls 1 and 2 because microRNA172 suppresses its translation in the inner whorls. AP1 is expressed in whorls 1–2, AP3/PI in 2–3, and AGAMOUS in 3–4.

Answer: (4) Centrioles

In most animals, including humans, the egg discards its centrioles during oogenesis. The sperm supplies the centriole (the proximal centriole of its basal body), which becomes the zygote’s first centrosome and organises the sperm aster and first spindle. Mitochondria come from both gametes, but paternal ones are destroyed. The mouse is an exception and builds centrosomes from maternal material.

Answer: (3) It leads to digestion of the zona pellucida.

When the sperm meets the zona pellucida (ZP3/ZP2 in mammals), the acrosome fuses with the sperm plasma membrane and releases its enzymes, such as acrosin and hyaluronidase. These digest a path through the zona pellucida, exposing the inner acrosomal membrane, which carries Izumo for fusion with the egg. Membrane fusion itself is a later step.

Answer: (1) The first stage of commitment is termed as specification, while the second stage is determination.

Commitment has two stages. A cell is first specified: it will follow its fate if cultured alone in a neutral environment, but transplanting it can still change that fate. It then becomes determined: the fate is irreversible even after transplantation. Options 2, 3 and 4 swap or garble these definitions.

Answer: (3) stem cell-mediated regeneration

Planarian regeneration relies on neoblasts, adult pluripotent stem cells (cNeoblasts) spread through the body. They migrate to the wound and form the blastema, so this is stem-cell-mediated regeneration. Epimorphosis, as in the salamander limb, uses dedifferentiation of mature cells. Morphallaxis, as in Hydra, mostly repatterns existing tissue.

Answer: (1) epiblast and hypoblast

The chick blastoderm separates into an upper epiblast and a lower hypoblast. The fluid-filled space between these two layers is the avian equivalent of the blastocoel. The subgerminal cavity, between the blastoderm and the yolk, is a separate space.

Answer: (4) Female mice with a mutation in the IGF-1 and IGF-2 show reduced lifespan.

Reduced insulin/IGF-1 signalling extends lifespan, from daf-2 worms to mice. Female mice heterozygous for the IGF-1 receptor live about a third longer and resist oxidative stress better. So a shorter lifespan after IGF mutation is not a feature of ageing. Telomere shortening, insulin/IGF control of lifespan and the benefit of stress resistance all are.

Answer: (3) β-catenin will accumulate in the nuclei of all blastula cells leading to animal cells getting specified as endoderm and mesoderm.

GSK-3 targets β-catenin for degradation. Normally only the vegetal micromeres keep β-catenin and send it to the nucleus, which specifies endoderm and mesoderm. Blocking GSK-3 (with lithium, for example) stabilises β-catenin in all cells, so animal cells are also vegetalised into endoderm and mesoderm. The opposite treatment produces an ectodermal ball.

Answer: (3) Rotational

Mammals have rotational cleavage, slow and holoblastic. At the second division one blastomere divides meridionally and the other equatorially, so the second-cleavage planes are rotated 90° to each other. Radial cleavage is typical of sea urchins and amphibians, spiral of molluscs and annelids, and bilateral of tunicates.

Answer: (2) Inner cell mass

Mammalian embryonic stem cells come from the inner cell mass of the blastocyst, which forms the embryo proper and is pluripotent. The trophectoderm forms the placenta. The blastocoel is a fluid-filled cavity, not a group of cells.

Answer: (1) Epiboly

Epiboly is the spreading of an epithelial sheet, usually ectoderm, to cover the deeper layers. Examples are the animal cap of the frog and the zebrafish blastoderm spreading over the yolk. Involution is inward rolling at the lip, ingression is single cells leaving an epithelium, and emboly is a general term for the inward movements.

Answer: (2) The cells will become respecified to more proximal position.

Retinoic acid acts like a proximalising signal in the regenerating newt or axolotl limb. Blastema cells treated with RA are respecified to a more proximal position, so a limb cut at the wrist can regrow a whole limb from the shoulder. The effect is dose-dependent and acts through Meis and Prod1.