Cell Division (Edexcel A Level Biology (A) SNAB): Flashcards
Exam code: 9BN0
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- FrontThe Cell Cycle
Define cell cycle.
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Define cell cycle.
The cell cycle is the regulated sequence of events that occurs between one cell division and the next.
Name the three phases of the cell cycle.
Interphase, nuclear division (mitosis) and cytokinesis (cell division).
What are the three stages of interphase?
G₁ phase, S phase and G₂ phase.
What happens during the S phase of interphase?
The DNA is replicated.
Each chromosome ends up as two identical sister chromatids.
What happens during the G₁ phase?
The cell grows and makes the RNA, enzymes and proteins needed for growth.
A signal to divide is received during this phase.
What happens during the G₂ phase?
The cell continues to grow, and the new DNA is checked and repaired.
Proteins such as tubulin are made ready for the spindle.
Define cytokinesis.
Cytokinesis is the division of the whole cell after mitosis, producing two genetically identical daughter cells.
How does cytokinesis differ between animal and plant cells?
In animal cells the cytoplasm constricts between the two nuclei.
In plant cells a new cell wall forms.
What triggers a cell to move from one phase of the cell cycle to the next?
Chemical signals called cyclins.
Interphase is made up of the G₁ phase, the phase and the G₂ phase.
Interphase is made up of the G₁ phase, the S phase and the G₂ phase.
True or False?
DNA replication occurs during mitosis.
False.
DNA is replicated during the S phase of interphase, before mitosis begins.
During which phase is a cell making the most mRNA, and why?
The G₁ phase.
This is when protein synthesis is most active, and mRNA is made during transcription.
Define mitosis.
Mitosis is nuclear division producing two genetically identical daughter nuclei, each with the same number of chromosomes as the parent.
Name the four stages of mitosis in order.
Prophase, metaphase, anaphase and telophase (PMAT).
What happens during prophase?
Chromosomes condense and become visible, each as two sister chromatids.
Spindle fibres form and the nuclear envelope breaks down.
What happens during metaphase?
Chromosomes line up along the equator of the spindle.
Spindle fibres attach to the centromeres of each chromosome.
What happens during anaphase?
The centromeres divide and the sister chromatids separate.
Spindle fibres shorten, pulling them to opposite poles.
What happens during telophase?
Chromosomes reach the poles and decondense.
A nuclear envelope reforms around each set and the spindle breaks down.
What holds the two sister chromatids of a chromosome together?
The centromere.
Why is mitosis important for growth?
It produces genetically identical cells with the full chromosome number.
This allows a zygote to grow into a multicellular organism.
Give two roles of mitosis besides growth.
Repair of damaged tissue and replacement of dead cells.
Asexual reproduction, producing genetically identical offspring.
The two daughter cells produced by mitosis are genetically to each other and to the parent cell.
The two daughter cells produced by mitosis are genetically identical to each other and to the parent cell.
True or False?
Cytokinesis is one of the four stages of mitosis.
False.
Cytokinesis is a separate part of the cell cycle — the four stages of mitosis are prophase, metaphase, anaphase and telophase.
What is asexual reproduction?
The production of new individuals from a single parent.
The offspring are genetically identical to the parent.
Why is a root tip used to study mitosis?
Root tips contain a meristem — a zone of actively dividing cells.
This means many cells undergoing mitosis can be seen.
Which plant is most commonly used for the root tip mitosis practical?
Garlic or onion (Allium cepa).
Why are root tips warmed in hydrochloric acid during the practical?
The acid breaks up the tissue and separates the cells.
This makes it easier to squash them into a thin layer.
Why is a stain such as acetic orcein added to the root tip?
It stains the chromosomes (a deep purple).
This makes the stages of mitosis visible under the microscope.
Why is the stained root tip gently squashed on the slide?
To spread the cells into a single thin layer.
This lets light pass through and individual cells be seen clearly.
Define mitotic index.
The mitotic index is the proportion of cells in a sample that are undergoing mitosis.
State the formula for the mitotic index.
mitotic index = number of cells with visible chromosomes ÷ total number of cells
A sample has 42 cells, of which 32 have visible chromosomes. Calculate the mitotic index to 2 decimal places.
mitotic index = 32 ÷ 42
= 0.76
How would you express a mitotic index as a percentage?
Multiply the mitotic index by 100.
Which cells count as 'undergoing mitosis' when calculating the mitotic index?
Any cell with visible chromosomes.
That is, cells in prophase, metaphase, anaphase or telophase.
The region of a plant root where cells are actively dividing by mitosis is called the .
The region of a plant root where cells are actively dividing by mitosis is called the meristem.
True or False?
A higher mitotic index means a greater proportion of cells are dividing.
True.
The mitotic index is the proportion of cells undergoing mitosis, so a higher value means more cells are dividing.
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