Barr body is found in the cytoplasm during
(a) interphase in cell of female mammal.
(b) interphase in cell of male mammal.
(c) prophase in cell of female mammal.
(d) prophase in cell of male mammal.
(a) : Barr body is also known as sex chromatin, X chromatin. Barr body is partially inactivated and development of this facultative heterochromatin occurs in one of the two X chromosomes in interphase nuclei. Any of the two X-chromosomes can become heterochromatic.
When synapsis is complete all along the chromosome, the cell is said to have entered a stage called
(b): The synapsis, pairing of homologous chromosomes takes place during zygotene, Synapsis results in the formation of bivalents during zygotene. The formation of special proteinaceous structure called synaptonemal complex occur. After zygotene stage cells entered in pachytene stage in which the bivalents become spiralled, shortened and thickened.
Many cells function properly and divide mitotically even though they do not have
(a) Plasma membrane
(d) The presence of plastid is a feature of plant cells but animal cells are devoid of them. Even then they function properly and divide mitotically similar to plant cells.
Which of the following is not a characteristic of meiosis?
(a) It involves two stages of DNA replication one before meiosis-I and another before meiosis-II
(b) It involves recombination and crossing over
(c) Sister chromatids separate during anaphase- II
(d) Nuclear membrane disappears during prophase.
(a) : Chromosome replication occurs once but meiosis has two M-phases each with its own Karyokinesis and Cytokinesis. As a result chromosome number is halved. The transition period between M-phase II (meiosis II) is short and without DNA replication. It is called interkinesis.
Microtubule depolymerizing drug such as colchicine is expected to
(a) inhibit spindle formation during mitosis
(b) inhibit cytokinesis
(c) allow mitosis beyond metaphase
(d) induce formation of multiple contractile
(a) : Colchicine is an alkaloid derived from the crocus, Colchicum autumnale. It inhibits spindle formation in cells during mitosis so that chromosomes cannot separate during anaphase, thus inducing multiple sets of chromosomes. It does not affect cytokinesis.
Which of the following statements is incorrect about G0 phase.
(a) Mitosis occurs after G0 phase.
(b) Biocatalysts can be used to exit G0 phase.
(c) Cell volume keeps on increasing during this phase.
(d) Cell metabolism occurs continuously in G0 phase.
(a): G0 phase is the stage of inactivation of cell cycle due to non-availability of mitogens and energy rich compounds. The cells in this phase remain metabolically active and usually grow in size assuming particular shape (cell differentiation). Cell enters G0 phase from a cell cycle checkpoint in the G1 phase. G1 phase checkpoint (restriction point) takes the key decision whether the cell should divide, delay division or enter resting stage. Cells then remain in G0 phase until there is a reason for them to divide. Several biocatalysts can help a cell in G0 phase to proceed through cell division when required.
Beads on string like structures of A are seen in B, which further condense to form chromosomes in C stage of cell division.
A B C
(a) Chromonema Chromatin Metaphase
(b) Chromatin Chromatid Metaphase
(c) Chromonema Chromosome Anaphase
(d) Chromonema Chromatid Anaphase.
(a) : The "beads-on-a-string" structure is seen in electron microscope of isolated metaphase chromosomes. The chromonema form the gene bearing portions of the chromosome. Basically chromonema is made up of nucleosome chains. Nucleosome chain gives a beads on string appearance under electron microscope. Nucleosome is the fundamental packaging unit in eukaryotic chromosomes.
In a diploid cell, at which stage of cell cycle, the amount of DNA is doubled?
(a) G1 and G2 phase
(b) G0 phase
(c) S, G2 and M phase
(d) S phase
(d): S-phase is known as synthetic phase. In this stage replication of DNA takes place on the template of the existing DNA and thus the amount of DNA per cell doubles. If the initial amount of DNA is denoted as 2C, then it increases to 4C.
Which is the longest phase of the cell cycle?
(b): Cell cycle is divided into 2 phase. (i) Interphase (a period of preparation for cell division), (ii) M phase (the actual period of cell division). The interphase cell is metabolically quite active. Interphase is the long non dividing phase further divided into G1, S and G2. It occupies 75 to 90 % of entire cell division time.
The checkpoint in cell cycle plays important role in
(a) repair DNA damage
(b) apoptosis initiation
(c) assess DNA damage
(d) inhibit cell damage.
(c)The cell cycle checkpoints play an important role in the control system by sensing defects that occur during essential processes such as DNA replication or chromosome segregation, and inducing a cell cycle arrest in response until the defects are repaired.