Stage of meiosis during which homologous pairs of chromosomes are arranged in equatorial plane?
Correct Answer: Metaphase
Description: Ans. a (Metaphase) (Ref. Langman Embryology 9th/pg. 5; Harper's 27th/pg. 326; Ganong 23rd/Figure 1-13)MITOSISIt is the process whereby one cell divides, giving rise to two daughter cells that are genetically identical to the parent cell.Each daughter cell receives the complete complement of 46 chromosomes. Before a cell enters mitosis, each chromosome replicates its deoxyribonucleic acid (DNA). During this replication phase the chromosomes are extremely long, they spread diffusely through the nucleus, and they cannot be recognized with the light microscope. With the onset of mitosis the chromosomes begin to coil, contract, and condense; these events mark the beginning of prophase. Each chromosome now consists of two parallel subunits, chromatids, that are joined at a narrow region common to both called the centromere. Throughout prophase the chromosomes continue to condense, shorten, and thicken, but only at prometaphase do the chromatids become distinguishable. During metaphase the chromosomes line up in the equatorial plane. And their doubled structure is clearly visible. Each is attached by microtubules extending from the centromere to the centriole, forming the mi- totic spindle. Soon the centromere of each chromosome divides, marking the beginning of anaphase, followed by migration of chromatids to opposite poles of the spindle. Finally, during telophase, chromosomes uncoil and lengthen, the nuclear envelope reforms, and the cytoplasm divides. Each daughter cell receives half of all doubled chromosome material and thus maintains the same number of chromosomes as the mother cell.Meiosis is the cell division that takes place in the germ cells to generate male and female gametes, sperm and egg cells, respectivelyPermanent cellsRemain in G0, regenerate from stem cells.Neurons, skeletal and cardiac muscle cells, and RBCs remain in G0.Stable (quiescent) cellsEnter G1 from G0 when stimulated.Hepatocytes, lymphocytes.Labile cellsNever go to G0, divide rapidly with a short Gl.Bone marrow, gut epithelium, skin, hair follicles.Also remember:# The centromere is an adenine-thymine (A-T) rich region ranging in size from 102 to 106 base pairs. It binds several proteins with high affinity. This complex, called the kinetochore, provides the anchor for the mitotic spindle. It thus is an essential structure for chromosomal segregation during mitosis.# The ends of each chromosome contain structures called telomeres. Telomeres consist of short, repeat TG-rich sequences.# Telomerase, a multisubunit RNA-containing complex related to viral RNA-dependent DNA polymerases (reverse transcriptases), is the enzyme responsible for telomere synthesis and thus for maintaining the length of the telomere.# Since telomere shortening has been associated with both malignant transformation and aging, telomerase has become an attractive target for cancer chemotherapy and drug development.
Category:
Biochemistry
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