Chapter 6- Cell Division, Diversity And Organisation Flashcards
Where do all the cells in a multicellular organism originate from?
Either by asexual reproduction or from a zygote formed by the fertilisation of a female gamete by a male gamete.
The purpose of mitosis
A type of nuclear division that occurs when new cells are required for growth, repair of tissues or asexual reproduction.
Purpose of meiosis
The formation of gametes
Cell cycle definition
The cycle of division, growth and maturity and then another division is called cell cycle.
The relation of specialised cells and the time for the cell cycle to complete once
More specialised the cells are, the longer its cell cycle
Certain highly specialised cells (eg. muscle and nerve cells) lose their power to divide altogether.
Mammalian red blood cells are also incapable of dividing, because they lose their nucleus during the course of development.
Embryo cells’ duration of cell cycle
8-60 minutes
Yeast cells’ duration of cell cycle
1.5 to 3 hours
Intestinal epithelial cells’ duration of cell cycle
About 12 hours
Bone marrow cells’ duration of cell cycle
About 18 hours
Stomach epithelial cells’ duration of cell cycle
About 24 hours
Hepatocytes (liver cells)’ duration of cell cycle
About 1 year
What is the first step following division
Enters the first growth phase (G1) on interphase
During this period, the cell components (apart from the chromosomes) are replicated.
What is the next stage of the cell cycle after G1
This is the synthesis stage where the DNA is replicated.
Synthesis is part of the interphase.
What follows after the synthesis stage
Second growth phase (G2)
During this stage the cells are prepared for division
The DNA is checked for errors by a suite of enzymes with specific proof-reading and repair functions. If any are detected, the cell may destroy itself to prevent passing on mutations.
Plus, the DNA starts to coil tightly, to become visible as chromosomes at the beginning of mitosis
What follows after the G2 phase of the cell cycle
Mitosis and cell division and then cytokinesis (division of cytoplasm)
Definition of cytokinesis
The division of cytoplasm after nuclear division
definition of checkpoints in a cell cycle
There are four points in the cell cycle, known as checkpoints, where these checks take place.
What is the first checkpoint in a cell cycle
- In the G1 stage, the chromosomes are checked for damage. If the damage is detected, the cell does not proceed into the S phase. It does not proceed to the next stage, until the DNA is repaired.
What is the second checkpoint in a cell cycle
During the S phase, a check is made that all the chromosomes have replicated. If they have not, the cel cycle is stopped.
What is the third checkpoint of the cell cycle
During the G2 phase, another check is made for DNA damage that may have occurred during replication. Once again, the cell maybe delayed to repair the DNA.
What is the final checkpoint in a cell cycle
This occurs during the metaphase. This check identifies whether the chromosomes have correctly attached to the spindle fibres before anaphase proceeds.
How does mitosis generally work
The new cells require a full set of chromosomes, and so those in the original or mother cell must duplicate in order to give the two sets that are needed.
The whole process is continuous, it does not pause at any stage
Prophase of mitosis
By the end of the prophase, the chromosomes have become visible, the nucleolus has disappeared and the nuclear membrane has broken down.
At the G2 phase, the chromosomes begin to appear however by the end of prophase becomes completely visible. This is because it supercoils.
What is supercoiling
The twisting of the DNA around its own axis, winding the helix more tightly.
How does the DNA look like during the S phase
During the S phase the single strands of tightly wound DNA, CHROMATIDS, link with another CHROMATID to form a CHROMOSOME. These chromatids are held together at a single point known as CENTROMERE.
How do spindle fibres form
Centrosomes (each consisting of a pair of centrioles, duplicated, like the DNA during interphase) move to opposite ends of the cells called poles. These send out microtubules to form a spindle, which forms a framework that guides the chromatids to the opposite poles.
Occurs in prophase
What happens in metaphase
The spindle fibres attach themselves to the centromere of each chromosome, at the centre of the structure, known as the equator.
This attachment involves a protein structure on each chromatid, called a kinetochore.
What happens in anaphase in mitosis
The centromeres divide and the newly-separated chromatids( now called chromosomes) are pulled by their centromeres to opposite poles of the cell. The spindle fibres shorten at both ends so pulling the chromatids apart.
What happens in telophase in mitosis
New nuclear membranes form around each group of chromosomes, which now uncoil again. New nucleoli form in each nucleus
What occurs in cytokinesis
Cytoplasm divides. The cytoplasm near the equator tuck inwards, splitting the cytoplasm, otherwise known as furrowing.
The organelles are shared between the two cells, having increased in number during interphase. The golgi apparatus produces vesicles that form the new section of plasma membrane.
The proteins- actin and myosin cause division furrow
How is mitosis in plants different
Plants do not have centrioles, even though they form spindle fibres.
Also, during cytokinesis, the Golgi apparatus forms a new cell membrane and the cell wall.
Mitosis and asexual reproduction
Mitosis occurs in asexual reproduction.
Asexual reproduction is the process of producing organism from a single parent. Whereas in mitosis only the cell divides itself to form two cells.
Advantages and disadvantages of asexual reproduction
+
Quicker
-
Lack of genetic variation. Less resistant to disease and environmental change
Chances of the whole population to be wiped out is high
Definition of asexual reproduction
Reproduction which involves the production of offspring from a single individual
Definition of homologous pair
Two chromosomes with the same sequence of genes on them.
Why are gametes haploid
When gametes fuse in sexual reproduction, their nuclei join together into one nucleus in the zygote. For this reason, each gamete must be haploid so the zygote nucleus is diploid once the gametes have fused.
Why is important that selective genes from the parents’ set are passed through the gametes?
Certain essential genes might no be passed on through random selection.
Genes in homologous pair
Since there are homologous paired chromosomes, each gene will be present on both chromosomes. These copies are not identical: for example, the gene of the eye colour might be brown on one chromosome and blue in the other. These genes will be found in the same place of each chromosome.
In a homologous pair, one chromosome comes from the egg cell (mother)and the other comes from the sperm cell (father).
Stages of meiosis
Consists of two successive divisions
First of which halves the number of chromosomes in each cell and then the two haploid cells then divide again so that a total of four cells are formed.
What happens in prophase I (meiosis)
Chromosomes condense so that they become shorter, thicker and visible.
Homologous chromosomes pair to form bivalents (one from the mother and the other from the father)
Chiasmata form to hold chromosomes together. This is the point where the one chromatid of each chromosome join, break and exchange parts in crossing over.
Nuclear membrane breaks up into small sacs of membrane which become part of the endoplasmic reticulum; centrioles replicate and move to opposite poles and form spindle microtubules.
Crossing over occurs here
Difference between chiasmata and centromere
Centromere is the point where two sister chromatids are joined to form a chromosome.
Chiasmata is the point where one chromatid of a chromosome crosses over with another chromatid from another chromosome.
Metaphase I
Bivalents move to the equatorial (or metaphase) plate across the centre of the cell
The paternal and maternal chromosomes in each bivalent position themselves independently of the others
Microtubules attach to the centromere of each chromosome
Anaphase I
Chromosomes (each with two chromatids) are pulled by shortening of microtubules towards the poles
If this was mitosis, the chromatids would be pulled to the OPPOSITE poles
Telophase I
Chromosomes reach opposite poles
Nuclear membranes reform to make two daughter nuclei that have half the number of chromosomes of the parent cell-these nuclei are haploid.
Cytokinesis occurs- the cell surface membrane pinches in leaving small cytoplasmic bridges between the cells
What happens in prophase II
Centrioles replicate and move to poles that are at right angles to those in meiosis I
Nuclear membrane breaks up
What happens in metaphase II in meiosis
Individual chromosomes align on the equator with their chromatids randomly arranged (important if crossing over has occurred in meiosis I)
Microtubules attach to the centromeres
What happens in Anaphase II
Sister chromatids break apart at the centromere and move it opposite poles
What happens with telophase II
Nuclear membranes reform
Cells divide to give four haploid cells that are genetically different to one another and from the parent cell
What are the key differences between the second division and first division of meiosis
In meiosis, the homologous chromosomes pair up and then separate into different cells. This does not occur during mitosis.
The two chromatids formed from a single chromosome remain together during the first division of meiosis, but separate in the second division. The second division of meiosis is similar to mitosis in that chromatids separate so that the number of chromosomes remains the same.
Independent assortment
When meiosis occurs, one of each homologous pair of chromosomes is transferred to the new gamete. Which of any two homologous pair of chromosomes goes into a particular gamete is completely random.
When fertilisation occurs, either of the two chromosomes in a homologous pairs may be passed on. Homologous pairs have the same genes but not necessarily the same alleles. Therefore, depending on which chromosome from each pair is passed on, each gamete has a different genetic make-up.
What happens at cross over
Crossing over occurs at prophase 1 of meiosis
Crossing over is a process by which the two chromosomes of a homologous pair exchange part of their genetic material. The two homologous chromosomes come together (a process called synapsis) and breaks appear at equivalent points on two of the non-sister chromatids at places where the chromatids cross over, called chiasmata.
The broken sections are then exchanged.
How does crossing over aid genetic variation
The homologous pairs have identical gene sequences, the exchange of broken sections does not disrupt the genetic information but does rearrange the alleles from the original paternal and maternal chromosomes.
What does G0 mean
This stage comes after G1 and this is where the cell goes if it is not dividing or preparing to divide.
How is DNA found in chromosomes
Wrapped around histone proteins
What is a chromatin
The DNA and histone proteins together are called chromatin.
When does mitosis occur
Whenever new cells are needed
Where does meiosis occur
It only occurs in the gonads
What is independent random assortment
The random orientation of bivalents on the equator in metaphase I of meiosis determines the direction in which the pairs of chromatids move in anaphase I.
In metaphase II random orientation of pairs of chromatid determines which chromatids migrate to opposite poles of the cell (segregate) during anaphase II.
What is the other name for a male gamete
Spermatozoa
Another name for a female gamete
Secondary oocyte
Gene reshuffling
Crossing over and independent + random variation all account for the genetic reshuffling that results in continuous variation
What are genetic mutuations
Sources of variation that generate major changes in genotype, capable of giving rise to new species.
What is the totipotent stem cells
These stem cells can form all the cell types in a body, including the embryonic cells. Embryonic cells within the first couple of divisions after fertilisation are the only cells that are totipotent.
What are pluripotent stem cells
These cells can give rise to all of the cell types that make up the body. Embryonic stem cells are pluripotent
What are multipotent stem cells
Can develop more than one type of cell type, but are more limited that pluripotent cells. Adult stem cells and cord blood stem cells are considered multipotent
What are unipotent stem cells
Refers to a cell that can dfferentiate along one lineage
Third point of variation in meiosis
Random fusion of gametes
