2.6* Flashcards
What is cytokinesis?
Cytoplasmic division following nuclear division, resulting in two new daughter cells.
What is interphase?
Phase of cell cycle where the cell is not dividing; it is subdivided into growth and synthesis phases.
What is mitosis?
Type of nuclear division that produces daughter cells genetically identical to each other and to the parent cell
What could early researchers see when observing cell division under the microscope?
They could easily see the behaviour of chromosomes during mitosis which is nuclear division followed by cytokinesis or cytoplasmic division, resulting in two daughter cells.
What could early scientists not see in cell division?
Nuclear and cytoplasmic division, called the M-phase, occupy only a small part of the cell cycle. Between each M-phase is an interphase. Interphase when studied under the microscope, appears to be uneventful. However, more sophisticated techniques have enabled scientists to learn that during interphase there are elaborate preparations being made for cell division, in a carefully ordered and controlled sequence, with checkpoints.
What are the two man checkpoints?
The G1/S checkpoint, also called the restriction point, and the G2/M checkpoint.
Name another checkpoint? (not the two main checkpoints) State its purpose.
There is one halfway through mitosis and one in early G1.
The purpose of the checkpoint is;
To prevent uncontrolled division that would lead to tumours (cancer).
To detect and repair damage to DNA (for example, damage caused by UV light).
Molecular events that control the cell cycle happen in a specific sequence, what does this ensure?
The cell cycle cannot be reversed.
The DNA is only duplicated once during each cell cycle.
Cells should usually undergo a certain number of divisions or cycles, what is this known as and how many is it?
The number is about 50 and known as the Hayflick constant.
What happens if the cell division becomes uncontrolled?
If cell division becomes uncontrolled, then a tumour can form which may become malignant or cancerous.
What helps regulate cell division?
There are proto-oncogenes that help regulate cell division by coding for proteins that help regulate cell growth and differentiation.
What happens if the proto-oncogenes fail?
If these proto-oncogenes mutate, then they may become oncogenes and can cause cells to fail to undergo apoptosis and instead to keep on dividing, leading to a tumour.
Describe the divisions of a tumour.
Tumours form because cells divide uncontrollably. The division phase, mitosis, of each cancerous cell takes the same length of time as in a non-cancerous cell. Because the cell division is uncontrolled, there is a greater proportion of cells dividing within a tumour than within normal tissue.
Why is the gene p53 important?
It triggers the two main checkpoints in the regulation of the cell cycle.
What is the gene p53 known as?
A tumour suppressor gene.
What other regulatory chemicals are there apart from p53?
Other regulatory chemicals are proteins called cyclins and CDK (cyclin-dependant kinases). Cyclins are synthesised in response to cell-signalling molecules such as growth factors.
Describe the prokaryotic cell cycle.
This occurs by a process called binary fission, the cell grows to its limit of size and then splits into two. Before the cell divides, its DNA is replicated. The new loops of DNA are pulled to opposite ends of the cell and a wall forms which begins to separate the bacterial cell. Each new cell also contains replicated plasmids and synthesised ribosome’s.
Apart from bacterial cells what other cells split by binary fission?
Mitochondria and chloroplasts within eukaryotic cells also divide by binary fission.
In the M - phase what does the chemical trigger?
A checkpoint chemical triggers condensation of chromatin.
What phase of the cell cycle if the M-phase in?
Halfway through the cell cycle, the metaphase checkpoint ensures the cell is ready to complete mitosis.
What events within the cell occur in the M-phase?
Cell growth stops.
Nuclear division (mitosis) consisting of stages : prophase, metaphase, anaphase and telophase.
Cytokinesis (cytoplasmic division)
What cells do not have a G 0 (gap 0) phase?
Epithelial cells lining the gut do not have this phase.
When and how is the G 0 (gap 0) chemical triggered?
A resting phase triggered during early G 1 at the restriction point (see below), by a checkpoint chemical.
What events happen in G 0?
In this phase, cells may undergo apoptosis (programmed cell death) differentiation or senescence.
Some types of cell (e.g. neurones) remain in this phase for a very long time or indefinitely..
At what phase of the cell cycle and which checkpoints are involved in G1 (gap 1) phase - also called the growth phase.
A G1 checkpoint control mechanism ensures that the cell is ready to enter the S phase and begin DNA synthesis.
State all (5) of the events in G1 phase.
Cells grow and increase in size.
Transcription of genes to make RNA occurs.
Organelles duplicate.
Biosynthesis, e.g. protien synthesis, including making the enzymes needed for DNA replication in the S phase.
The p53 (tumour suppressor) gene helps control this phase.
Describe the sequence of the S phase of interphase.
Because the chromosomes are unwound and DNA is diffuse, every molecule of DNA is replicated. There is a specific sequence to the replication of genes - those which are active in all types of cells, are duplicated first. Genes that are normally inactive in specific types of cells are replicated last.
State all (4) of the events in S phase.
Once the cell has entered this phase, it is committed to completing the cell cycle.
DNA replicates.
When all chromosomes have been duplicated, each one consists of a pair of identical chromatids.
This phase is rapid, and because the exposed DNA base pairs are more susceptible to mutagenic agents, this reduces the chances of spontaneous mutations happening.
What is the purpose of G 2 (gap 2) phase of interphase.
Special chemicals ensure that the cell is ready for mitosis by stimulating proteins that will be involved in making chromosomes condense and in formation of the spindle.
State all of the events in G 2 (gap 2) phase of interphase.
Cells grow.
What are chromatids?
Replicates of the chromosomes.
What is cytokinesis?
Division of the cytoplasm of a cell following mitosis.
What is mitosis?
Type of nuclear division that maintains the chromosome number; each new daughter cell contains the same genetic information as the parent cell. They are also genetically identical to each other.
Asexual reproduction;
Give an example of how single-celled protoctists divide.
Single-celled protoctists such as Amoeba and Paramecium divide by mitosis to produce new individuals.
Asexual reproduction;
Give an example of how a plant reproduces asexually.
Some plants e.g. Strawberry, reproduce asexually by forming new plantlets on the end of stolons (runners).
How do fungi reproduce?
Fungi such as single celled yeasts, can reproduce asexually by mitosis.
Does asexual reproduction happen in animals, if so give example.
Asexual reproduction is rarer in animals but some female sharks kept in captivity without any males have produced female offspring that are genetically identical to themselves. Aphids may sometimes produce eggs, by mitosis, that do not need fertilising.
How do all multicellular organisms grow?
By producing more cells that are genetically identical to each other and to the parent cell from which they arose by mitosis.
How are animal tissues repaired?
Wounds heal when growth factors, secreted by platelets and macrophages (white blood cells) and damaged cells of the blood-vessels walls, stimulate the proliferation of endothelial and smooth muscle cells to repair damaged blood vessels.
What has only use asexual reproduction for millions of years?
Two species of stick insect, of the genus Timema, have used only asexual reproduction for millions of years.
What are the main stages of mitosis?
Although mitosis is a continuous process, scientists observing the process have defined four main stages of mitosis, namely prophase, metaphase, anaphase and telophase.
What is the first stage of mitosis?
Prophase.
What is the second stage of mitosis?
Metaphase.
What is the third stage of mitosis?
Anaphase.
What is the last stage of mitosis?
Telophase.
There are four events in Prophase, what is the first?
The chromosomes that have replicated during the S phase of interphase and consist of two identical sister chromatids, now shorten and thicken as DNA supercoils.
There are four events in Prophase, what is the second?
The nuclear envelope breaks down.
There are four events in Prophase, what is the third.
The centriole in animal cells (normally found within a region of a cell called a centrosome) divides and the two new daughter centrioles move to opposite poles (ends) of the cell.
There are four events in Prophase, what is the final event?
Cytoskeleton protien (tubulin) threads form a spindle between these centrioles. The spindle has a 3D structure and is rather like lines of longitude on a virtual globe. In plant cells, the tubulin threads are formed from the cytoplasm.
What events happen in Metaphase?
The pairs of chromatids attach to the spindle threads at the equator region.
They attach by their centromeres.
What events happen in Anaphase?
The centromere of each pair of chromatids splits.
Motor proteins, walking along the tubulin, pull each sister chromatid of a pair, in opposite directions, towards opposite poles.
Because their centromere goes first, the chromatids, now called chromosomes, assume a V shape.
What events happen in Telophase?
Diagrams for the stages of mitosis are on page 149.
The separated chromosomes reach the poles.
A new nuclear envelope forms around each set of chromosomes.
The cell now contain two nuclei each genetically identical to each other and to the parent cell from which they arose.
What happens once mitosis is complete in plant cells?
Cytokinesis
Once mitosis is complete, the cell splits into two, so that each new cell contains a nucleus.
In plant cells, an end plate forms where the equator of the spindle was, and new plasma membrane and cellulose cell-wall material are laid down on either side along this end plate.
Two new identical daughter cells are now formed. they are genetically identical to each other and to the parent cell.
What happens once mitosis is complete in animal cells?
Cytokinesis
Once mitosis is complete, the cell splits into two, so that each new cell contains a nucleus.
In animal cells, the plasma membrane folds inwards and ‘nips in’ the cytoplasm.
Two new identical daughter cells are now formed. they are genetically identical to each other and to the parent cell.
How can you examine mitosis in a plant?
Cells of root tips can be stained with acetic orcein (which stains chromosomes) and observed under a microscope, to see the stages of mitosis.
Why can you only examine mitosis in plants in their meristems?
Once plant cells have divided and differentiated and have a vacuole and ridged cellulose cell wall, they cannot divide. There are meristems, such as shoot and root tips and cambium between xylem and phloem tissue, where plant cells are undifferentiated and can divide by mitosis,
How can you stain and examine root tip cells?
Cell root tips can be stained with acetic orcein (which stains chromosomes) and observed under a microscope, to see the changes in mitosis.
What is a haploid?
Having only one set of chromosomes; represented by the symbol ‘n’.
What is homologous chromosome?
Matching chromosomes, containing the same genes at the same places (loci). They may contain different alleles for some of the genes.
What is meiosis?
A type of nuclear division that results in the formation of cells containing half the number of chromosomes of the parent cell.
Why dies sexual reproduction increase genetic variation?
Because it involves the combining of genetic material from two (usually) unrelated individuals of the same species, by the process of fertilisation.
What does genetic variation within a population increase?
Genetic variation within a population increases its chances of survival when the environment changes, as some individuals will have characteristics that enable them to be better adapted to the change.
In sexual reproduction why is a diploid zygote produced?
In many organisms, the body cells are diploid. For sexual reproduction to occur they must produce haploid gametes, so that when two gamete nuclei fuse during fertilisation, a diploid zygote is produced and the normal chromosome number is maintained through generations.
What does meiosis mean, and when does it occur?
Meiosis means reduction, and it occurs in diploid germ cells to produce haploid gametes.
Where are diploid cells that are undergoing mitosis?
The diploid cells undergoing meiosis are in specialised organs called gonads - ovaries and testes (these cells have been in interphase before they enter meiosis).
How many chromosomes do you have in your body cells?
46
Where did the chromosomes in your body cells come from?
23 came from your mother, in the egg nucleus and 23 came from your gather in the sperm nucleus.
What are homologous chromosomes?
The 46 chromosomes in your body cells can form matching pairs - one maternal and one paternal chromosome containing the sane genes at the same place on the chromosome. These matching pairs are called homologous chromosomes.
Homologous chromosomes have the same genes but contain different alleles (variants) for the genes.
What is a photomicrograph of chromosomes called?
A karyotype.
Describe a Karyotype of a singular male.
One pair of chromosomes is not fully homologous, X,Y but enough homology exists to pair them up. The chromosomes in each of the homologous pairs consist of two chromatids, because these chromosomes were observed using a light microscope abs photographed as the nucleus was dividing.
How many divisions are there in meiosis?
There are two divisions in mitosis, and in each divisions there are four stages.
What are the stages In the first meiotic division?
The four stages are prophase 1, metaphase 1, anaphase 1, and telophase 1.
Sum up the main stages of meiosis.
Before meiosis, during the S phase of interphase, each chromosome was duplicated as its DNA replicated, after which each chromosome consists of two sister chromatids. In meiosis the chromosomes pair up in their homologous pairs.
What happens in meiosis after the first meiotic division?
The cell may enter a short interphase before embarking on the second meiotic division that also has four stages. Prophase 2, metaphase 2, anaphase 2 and telophase 2. This takes places at right angles to that of meiosis one.
What may the cycle of meiosis end with?
At the end of the second division, cytokinesis.
What is the first event in prophase 1 of meiosis?
The chromatin condenses and each chromosome supercoils. In this state, they can take up stains and be seen with a light microscope.
In prophase 1 of meiosis? what happens after the chromosome supercoils?
The nuclear envelope breaks down, and spindle threads of tubulin protein form the centriole in animal cells.
What are the final stages of prophase in meiosis after the tubulin threads form?
The chromosomes come together in their homologous pairs.
Each member of the pair consists of two chromatids.
Crossing over occurs where non-sister chromatids wrap around each other and may swap sections so that alleles are shuffled.
How long does prophase in meiosis last?
Prophase 1 may last for days, mounts or years, depending on the species bs type of gamete (make or female) being formed.
What is the crossing over point in meiosis, between two chromatids?
Below the centromere, half way down the middle of the two central chromatids, one from each chromosome.
How does metaphase 1 of meiosis begin?
The pairs of homologous chromosomes, still in their crossed over state attach along the equator of the spindle.
Each attaches to a spindle thread by its centromere.
What happens in metaphase 1 of meiosis after each chromosome attaches to a spindle threads?
The homologous pairs are arranged randomly, with the members of each pair facing opposite poles of the cell. This arrangement is independent assortment.
The way that they line up in metaphase determines how they will segregate independently when pulled apart during anaphase.
What happens in anaphase 1 of meiosis?
The members of each pair of homologous chromosomes ate pulled apart by motor proteins that dead them along the tubulin threads of the spindle.
The centromeres do not divide, and each chromosome consists of two chromatids.
The crossed over area separate from each other, resulting in swapped areas of chromosome and allele shuffling.
What is the first event in telophase 1 of meiosis?
In most animal cell, two new nuclear envelopes form around each set of chromosomes, and the cell divides by cytokinesis. There is then a short interphase when the chromosomes uncoil.
In telophase 1 of meiosis what happens after chromosomes uncoil?
Each new nucleus contains half the original number of chromosomes, but each chromosome consists of two chromatids.
In most plant cells, the dell goes straight from anaphase 1 into prophase 2.
What happens in prophase 2 of meiosis?
If the nuclear envelopes have reformed, then they now break down.
The chromatids coil and condense, each one consisting of two chromatids.
The chromatids of each chromosome are no longer identical, due to crossing over in prophase 1.
Spindles form.
In prophase 2 of meiosis where do the new spindles form?
New spindle fibres form at right angles to the previous axis.
What happens in metaphase 2 of meiosis?
The chromosomes attach, by their centromere, to the equator of the spindle.
The chromatids of each chromosome are randomly arranged.
The way that they are arranged will determine how the chromatids separate during anaphase.
What happens in anaphase 2 of meiosis?
The centromeres divide.
The chromatids of each chromosome are pulled apart by motor proteins that drag them along the tubulin threads of the spindle, towards opposite poles.
The chromatids are therefore randomly segregated.
What events takes place in telophase 2 of meiosis?
Nuclear envelope forms around each of the four haploid nuclei.
In animals, the two cells now divide to give four haploid cells.
In plantsman, a tetrad of four haploid cells is formed.
What are the four ways meiosis produces genetic variation?
Crossing over during prophase shuffles alleles.
Independent assortment of chromosomes in anaphase 1 leads to random distribution of maternal and paternal chromosomes of each pair.
Independent assortment of chromatids in anaphase 2 leads to further random distribution of genetic material.
Haploid gametes are produced, which can undergo random fusion with gametes derived from another organism of the same species.
What isn’t an example of a mutation that may be considers a mutation in meiosis?
Shuffling of alleles during crossing over in meiosis leads to genetic variation, but is not an example of mutation.
In the ovaries if human female fetuses when do the cells begin to divide?
Germ cells have already begun meiosis, and these cells remain. Suspended in prophase 1 from birth until puberty, when each mounts completes meiosis 1 and enters meiosis 2. Meiosis 2 is not completed until a sperm has entered the ovum.
What is differentiation?
Process by which stem cells become specialised into different types of cell.
What are epithelial cells?
Cells that constitute lining tissue.
What is a erythrocyte?
A red blood cell.
What is a neutrophil?
A type of white blood cell that is phagocytic (can ingest microbes and small particles).
What is a stem cell?
A unspecialised cell able to express all of its genes and divide by mitosis.
What determines the division of labour in a single celled organism?
Within a single celled organism, such as an amoeba, the division of labour is determined by the organelles, each of which has a specific function.
What is an advantage of single celled organisms that helps gasses diffuse across their membrane?
Single-celled organisms are small and have a large surface to volume ratio so that oxygen can diffuse across their plasma membrane, and waste products can diffuse out via the same membrane.
