TOPIC 3B Flashcards
Cell Cycle
The cell cycle is the regulated sequence of events that occurs between one cell division and the next
HOw many phrases does the cell cylce have?
The cell cycle has three phases:
Interphase
Nuclear division (mitosis)
Cell division (cytokinesis)
Cyclins
The movement from one phase to another is triggered by chemical signals called cyclins
What happens during interphase?
During Interphase the cell increases in mass and size and carries out its normal cellular functions, e.g. synthesising proteins and replicating its DNA ready for mitosis
What are the three phases that interphase consist of?
Interphase consists of three phases:
G1 phase
S phase
G2 phase
Examiner Tips and Tricks
Make sure you know the order of the phases of the cell cycle but also what specifically occurs during the different phases. Don’t forget, interphase is itself made up of three distinct stages (G1, S and G2) and you need to know what happens during each of these.
For example, an exam question might ask you to identify the stage of the cell cycle during which a cell would be producing the most mRNA molecules and explain why. The correct answer would be the G1 phase, as this is when protein synthesis is occurring and the production of mRNA occurs during transcription (the first part of protein synthesis).
Definition of Mitosis
And how many main stages can mitosis be divided into?
Mitosis
Mitosis is the process of nuclear division by which two genetically identical daughter nuclei are produced that are also genetically identical to the parent cell nucleus (they have the same number of chromosomes as the parent cell)
Although mitosis is, in reality, one continuous process, it can be divided into four main stages
These stages are:
- Prophase
- Metaphase
- Anaphase
- Telophase
What happens during prophase?
Prophase
Chromosomes condense and are now visible when stained
The chromosomes consist of two identical chromatids called sister chromatids (each containing one DNA molecule) that are joined together at the centromere
The two centrosomes (replicated in the G2 phase just before prophase) move towards opposite poles (opposite ends of the nucleus)
Spindle fibres (protein microtubules) begin to emerge from the centrosomes (which consist of two centrioles in animal cells)
The nuclear envelope (nuclear membrane) breaks down into small vesicles
What happens during metaphase?
Metaphase
Centrosomes reach opposite poles
Spindle fibres (protein microtubules) continue to extend from centrosomes
Chromosomes line up at the equator of the spindle (also known as the metaphase plate) so they are equidistant to the two centrosome poles
Spindle fibres (protein microtubules) reach the chromosomes and attach to the centromeres
Each sister chromatid is attached to a spindle fibre originating from opposite poles
What happens during anaphase?
The sister chromatids separate at the centromere (the centromere divides in two)
Spindle fibres (protein microtubules) begin to shorten
The separated sister chromatids (now called chromosomes) are pulled to opposite poles by the spindle fibres (protein microtubules)
Telophase
Chromosomes arrive at opposite poles and begin to decondense
Nuclear envelopes (nuclear membranes) begin to reform around each set of chromosomes
The spindle fibres break down
The significance of mitosis
The process of mitosis is of great biological significance and is fundamental to many biological processes, including:
The growth of multicellular organisms
The replacement of cells and repair of tissues
Asexual reproduction
What happens during the growth of multicellular organisms?
The two daughter cells produced are genetically identical (clones) and have the same number of chromosomes as the parent cell.
This process allows unicellular zygotes to grow into multicellular organisms by mitosis.
Growth can occur throughout the organism’s body or be restricted to regions like meristems (growing points) in plants.
How are damaged tissues repaired and cells replaced in organisms?
Damaged tissues are repaired by mitosis followed by cell division.
Cells are continually replaced by genetically identical cells as they die.
In humans, cell replacement is particularly rapid in the skin and the lining of the gut.
Some animals, like zebrafish and axolotls, can regenerate body parts, such as fins, legs, and tails.
What happens during Meiosis I?
The original ‘parent’ cell is diploid (2n) with two sets of chromosomes.
Before meiosis I, chromosomes replicate.
During meiosis I, homologous pairs of chromosomes are separated, creating two haploid (n) nuclei.
Each chromosome still consists of two chromatids at this point.
The chromosome number halves (from 2n to n) in meiosis I, not in meiosis II.
What happens during Meiosis II
During meiosis II, the chromatids that make up each chromosome separate to produce four haploid (n) nuclei
At this point, each chromosome now consists of a single chromatid
Meiosis I and II
During meiosis, one diploid nucleus divides by meiosis to produce four haploid nuclei
Having genetically different offspring can be advantageous for natural selection
Meiosis has several mechanisms that increase the genetic diversity of gametes produced. The two main mechanisms are:
Independent assortment
Crossing over
