✨Module 2: Cell division Flashcards
What is the cell cycle?
Sequence of events that lead to the division of the cell and the formation of two genetically identical daughter cells.
A chromosome is made up of …
A DNA molecule has …
A gene has …
Lots of DNA molecules.
Lots of genes.
Lots of base pairs.
Describe the structure of a chromosome.
Made of DNA and proteins. DNA is wrapped around proteins called histones. This results in a structure called chromatin. Chromatin is then coiled into chromosomes.
What is a chromatid?
An arm of a chromosome. Two arms are held together by the centromere. Aka sister chromatids.
What are telomeres?
Regions of structural DNA at each end of a chromosome. The chromosome will become shorter after replication. Telomeres protect the genes on the chromosome.
Non-coding telomeres at the end of chromosomes prevent the coding DNA from being lost in cell division.
What is the difference between homologous chromosomes and sister chromatids?
Homologous chromosomes are pairs of chromosomes that contain the same genetic information, and they are present in diploid cells. Sister chromatids are identical copies of a single chromosome that are produced during replication, and they are present in both diploid and haploid cells.
What are the 3 growth phases in interphase?
G1, S, G2
Interphase is referred to as the resting phase as cells are not actively dividing. Why is this an inaccurate description?
Interphase is a very active stage of the cell cycle. It involves:
=> DNA is replicated and checked for errors.
=> Cell gets bigger and makes more organelles.
=> Mitochondria grow and divide, increasing in number.
=> Protein synthesis occurs in the cytoplasm.
What happens in the G1 and G2 phase?
(stands for Gap 1/2 phase)
G1 - the cell grows and makes new organelles for the daughter cells. (chromosomes are NOT duplicated here).
G2 - cell growth continues, duplicated DNA is checked for errors.
What happens in the S phase (synthesis phase)?
DNA is replicated in the nucleus. We will have 92 chromosomes, making it a tetraploid. The new copies of chromosomes are called homologous chromosomes.
There is also checking of genetic material in this phase.
Why is the S phase important?
Ensures the two daughter cells can each get the full chromosome set.
What happens after interphase?
The M/mitotic phase - nucleus divides and chromatids separate.
What is the G0 phase?
When the cell leaves the cycle, either temporarily or permanently.
Describe the reasons why a cell may enter the G0 phase.
=> Cell becomes specialised to carry out a particular function so doesn’t need to divide.
=> DNA may be damaged so it’s no longer able to divide. This cell enters cell apoptosis/permanent cell arrest.
=> Cells become senescent (grow old).
Some cells that enter G0 can be stimulated to go back into the cell cycle such as lymphocytes in an immune response.
Explain why checkpoints are important in the cell cycle.
It ensures the cell has grown to the right size, replicated DNA is error free.
Having checkpoints reduce the risk of copying any damaged DNA into the newly replicated cell.
What does the G1, G2, spindle assembly checkpoint check for?
G1 - this checkpoint is before the S phase. Checks fault in DNA structure, cell size. Ensures entry into the M phase is stopped until repaired. if not, the cell enters G0 phase.
G2 - end of the G2 phase. Checks for cell size, DNA replication and DNA damage.
Spindle assembly checkpoint - checks whether the chromosomes (centromeres) have attached to the spindles and have aligned (in metaphase). Mitosis cannot proceed until this checkpoint is passed.
What is the difference between the centrosome and the centromere?
Centrosome - moves to the opposite poles of the cell and controls the formation of the spindle in mitosis. It contains a pair of centrioles.
Centromere - 2 chromatids attach here and attaches to the spindle.
The importance of mitosis?
Growth and repair of cells, asexual reproduction in plants and fungi (also some animals like amoeba).
How to observe mitosis in a plant?
Growing root tips (or shoot tips) can be treated with a chemical to allow the cell to be separated. Then they can be squashed to form a single layer of cells on a microscopic slide. Stains that bind to DNA make chromosomes visible.
These growing root or shoot tips are called meristems. meristems are totipotent, so they can differentiate into various cells.
Explain what happens in each stage of mitosis.
Prophase - chromosomes condense and become visible. Nucleolus disappears. Centrioles (centrosome) move to opposite poles of the cell, which produce spindle fibres that extends across the cell. Nuclear membrane breaks down.
Later prophase - nuclear membrane disappears completely, spindle microtubules attach to the centromeres and start to move the chromosomes to the centre.
Metaphase - chromosomes line up along the equator/metaphase plate and the spindle fibres attach to the centromeres of each chromosome.
Anaphase - the centromere DIVIDES and the sister chromatids are pulled to the opposite poles of the cell by the spindle fibres shortening. This splits the chromosome into two V-shaped sister chromatids.
Telophase - chromatids reached opposite poles of the cell, so are now known as daughter chromosomes. The nuclear envelope reforms around the chromosomes - one on each pole of the cell. Therefore the cell now has two nuclei, each with a complete set of identical DNA. Nucleolus reforms. Spindle fibres break down. A cleavage furrow begins to form (the point at which the cytoplasm and cell membrane will divide).
What happens in cytokinesis?
Cytoplasm divides to form 2 separate daughter cells.
What happens to animals in cytokinesis?
A cleavage furrow forms at the cells equator by a ring of actin. The cytoskeleton pulls the cell surface membrane inwards and fuses, forming 2 cells.
What happens to plants in cytokinesis?
They have cell walls so they can’t form a cleavage furrow. Instead, a cell plate develops across the equator (formed by vesicles from the Golgi). These fuse with each other, dividing in 2.
What does the mitotic index tell us?
The number of visible chromosomes. It is calculated by:
Cells going through mitosis divided by all cells in the sample.
