Diagram 14 - Cell Cycle
(a) Explain the processes that take place during phase X. [10 marks]
· Phase X is interphase which is subdivided into G1 phase (P), S phase (Q) and G2 phase (R). [1]
· At G1 phase, proteins and new organelles are synthesized. [1]
· Chromosomes are extremely fine and are not visible. They are called chromatin. [1]
· The cell has a high metabolic rate and is preparing to divide if the external condition is conducive for growth. [1]
· At S phase, replication of DNA occurs. [1]
· Chromosomes are duplicated and each chromosome will now has two sister chromatids. [1]
· Both sister chromatids contain identical copies of the DNA. [1]
· At G2 phase, the cell continues to grow. [1]
· The cell remains metabolically active by accumulating energy. [1]
· The cell completes its final preparations for division by synthesizing more organelles and proteins. [1]
(b) State the stages within process U and explain the events that take place.
[10 marks]
· Process U refers to mitosis which consists of prophase, metaphase, anaphase and telophase. [1]
· At prophase, the chromosomes condense and become tightly coiled/ [1]
· The two sister chromatids joined together at the centromere. [1]
· Spindle fibres begin to form between the centrioles. [1]
· Nucleolus and nuclear membrane start to break down/ [1]
· Diagram of prophase [1]
· At metaphase, the centrosomes reach at the opposite poles [1]
· The chromosomes align themselves at the metaphase plate (equator) with the centromere attaching to the spindle fibres. [1]
· Diagram of metaphase [1]
· At anaphase, the spindle fibres contract and the centromere divides. [1]
· The sister chromatids are pulled apart to the opposite poles. [1]
· At telophase, the chromosomes reached at the opposite poles. [1]
· Chromosomes start to uncoil to become the non-visible chromatins. [1]
· Spindle fibres break down where the nucleolus and nuclear membrane reform/ [1]
ANY 10
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