IMP QNA
Q. Describe cell cycle and diagrammatically represent the check points of cell cycle.
Answer:
1. cell cycle is a series of events that takes place in a cell as it grows and divides.
2. check points of cell cycle are stages in cell cycle at which cell examines internal and external cues and devides whether or not to move forward with division.
3. The cell cycle consists of:
G1 phase - This is first growth phase where cell increases in size and cellular contents are duplicated.
S phase- DNA replications happens in this phase, each of 46 chromosomes are replicated by the cell.
G2 phase - This is second growth phase where organelles and proteins develop for preparing for cell division.
M phase- where mitosis occurs and cell seperates forming two identical daughter cells.
4. There are a number of check points, but three most important ones are
- G1 checkpoint :
- it is present at G1/S transition
- here the cell choses to divide or not to divide.
- to make the decision, the cell checks internal conditions like size, nutrients present in cell, whether DNA is damaaged or not and external conditions like molecular signals.
- once cell passes this phase, it irreversibly commits to the division.
- if cell dosen't get cues to make decision to go ahead, it leaves the cell cycle for rests in G0 phase.
- G2 checkpoint :
- it is present at G2/M transition
- it wants to ensure that cell division should go smoothly by checking DNA damage and whether DNA was replicated completely in S phase.
- if errors are found then G2 checkpoint allows for repairs.
- If damage is not repairable, the cell may undergo apoptosis or programmed cell death.
- Spindle checkpoint:
- it is present at metaphase to anaphase transition
- at this checkpoint, the cell checks if chromosomes are correctly attached to spindles in metaphase.
- the cycle will not proceed until all chromosomes are firmly attached to atleast two spindle fibres from opposite poles of cell.
- this checkpoint works by looking for chromones that are in wrong place (example:: flaoting around.)
- if chromosome is misplaced, then cell will pause mitosis until soindle captures the chromosome.
Q. Justify: ‘Maternal messenger RNAs are critical to the formation of the anterior-posterior axis in Drosophila’.
Q. Explain gastrulation process in drosophila
answer:
1. gastrulation is a a process in which the embryo transforms from one-dimensional layer of epidermal cells (blastula ) into multi-dimensional layer of epidermal cells called as gastrula.
2. Drosophila is a two winged insect which have a life span of about 40-45 making them excellent organisms for genetic study.
3.The synthesis of three germ layers - endoderm (innermost layer) , mesoderm (middle layer) , and ecto derm (outermost layer) happens in gastrualtion process.
4. Gastrulation:
- The primordia refers a tissue in early stage of development.
- At blastoderm stage, the primordia lies at surface of embryo(top).
- It begins with formation of mesoderm on ventral side of embryo.
- after few minutes, begins the formation of endoderm on lateral side of embryo.
-after 45 mins of beginning of gastruation, mesoderm layer has been fully formed.
- Now gastrulation movements take place by blastoderm cells which sends the mesoderm inside the embryo.
- Once inside the embryo, the mesoderm loses its strcture and disperses into single cells.
-. These cells on outer layer begin to divide and form the ectoderm layer.
- Now the anterior endoderm also inavginates together with mesoderm.
- The posterior endoderm remains single layered for a bit longer time and then disperse into single cells.
- posterior endoderm cells use mesoderm layer as support to get towards the middle of embryo.
- These posterior endoderm then meet mesoderm to form the continuos endoderm.
- This way the blastula has been formed.
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