Title: InkBiology / Cell cycles, reproduction and development
On every note of this deck, unless a note says otherwise:
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Front: DEFINE
Define: Spindle.
Back: The framework of protein microtubules built across the cell before the chromosomes are moved, running from pole to pole and attaching to every centromere; shortening the fibres is what separates the chromatids.

Front: DEFINE
Define: Polyspermy.
Back: Fusion of more than one sperm with a single egg, which gives a cell with too many chromosome sets to develop.

Front: SPOT THE ERROR
Say what is wrong with this claim:
Fertilisation is a race, and the fastest sperm is the one that gets in.
Back: Speed decides nothing. Whichever sperm arrives first still has to bind to a receptor on the zona pellucida and digest a path through it, and what stops the next one is the cortical reaction: fusion alters those receptors and hardens the coat, so no other sperm can bind at all.

Front: DEFINE
Define: Stem cell.
Back: An unspecialised cell that can divide repeatedly to produce more cells like itself, and can differentiate into one or more specialised types.

Front: SPOT THE ERROR
Say what is wrong with this claim:
A stem cell can become any kind of cell, which is what makes it a stem cell.
Back: Very few cells can become anything, and only for a few days. The stem cells in bone marrow are multipotent: they make blood cells and nothing else. Even a pluripotent cell from an embryo cannot build a placenta. Potency comes in four named levels, and totipotent is the one that gets over-claimed.

Front: SPOT THE ERROR
Say what is wrong with this claim:
Cells spend most of their lives dividing, and interphase is the resting phase between divisions.
Back: Interphase is the busiest part of the cycle, not a rest. The whole genome is replicated during it and then checked, which is where most of the time goes. Mitosis is the short part at the end.

Front: DEFINE
Define: Homologous pair.
Back: Two chromosomes of the same size carrying the same genes at the same loci, one inherited from each parent. Not the same thing as a pair of chromatids.

Front: DEFINE
Define: Differentiation.
Back: The process by which a cell becomes specialised, through transcription of some of its genes and not others.

Front: DEFINE
Define: Chromatid.
Back: One of the two identical copies of a chromosome, joined to its sister at the centromere after DNA replication.

Front: CHECK YOURSELF
A zygote is found to contain 69 chromosomes. Explain how this could have arisen, why the embryo will not develop, and what normally prevents it.
Back: Two sperm have fused with the same egg, which is polyspermy. Each gamete carries 23 chromosomes, so 23 from the egg and 23 from each of two sperm gives 69, in three sets rather than two.
The extra set is fatal, but not because mitosis fails: each chromosome is copied before division, so a triploid cell can divide into triploid daughters. The problem is dosage. Every gene is present in three copies rather than two, the balance of gene products is disturbed throughout development, and the embryo and placenta develop abnormally, so development stops early.
What normally prevents it is the cortical reaction. Fusion of the first sperm raises the calcium ion concentration in the egg's cytoplasm, the cortical granules release their contents outside the cell surface membrane, and those contents alter the sperm receptors on the zona pellucida and harden it, so no further sperm can bind or pass through.
Note what the answer does not say: nothing here depends on how fast any sperm was swimming.

Front: DEFINE
Define: Zona pellucida.
Back: The glycoprotein coat outside the egg's cell surface membrane, carrying the receptors a sperm binds to.

Front: CHECK YOURSELF
A cell from a mouse has 40 chromosomes. State the number of chromosomes and the number of chromatids present in that cell at the end of G2, and in each daughter cell immediately after mitosis. Explain your answers.
Back: At the end of G2 the cell has 40 chromosomes and 80 chromatids. S phase has replicated every chromosome, so each one is now two sister chromatids joined at a centromere, but the centromere count, and so the chromosome count, has not changed.
Immediately after mitosis each daughter cell has 40 chromosomes and 40 chromatids, or equivalently 40 single-chromatid chromosomes. Anaphase separated the sisters, and once separated each is a chromosome in its own right.
The wording is what makes this hard rather than the biology. A chromatid becomes a chromosome the moment its centromere divides, so the same physical strand has two names depending on when you ask.

Front: DEFINE
Define: Self-renewal.
Back: Division that produces at least one daughter cell identical to the parent stem cell, so the population of stem cells is maintained.

Front: CHECK YOURSELF
A patient with leukaemia is treated using stem cells taken from their own bone marrow. State the potency of those cells, explain why they could not be used to repair damaged nerve tissue, and explain why they are not rejected.
Back: The cells are multipotent. Haematopoietic stem cells in bone marrow can differentiate into erythrocytes, into every kind of white blood cell and into the megakaryocytes that shed platelets.
They could not repair nerve tissue because their range is limited to that one family of related types. A neurone is not among them, so no amount of the right signal would produce one. A pluripotent cell would be needed, from an embryo or from reprogramming.
They are not rejected because they came from the patient. Their cell surface antigens are the patient's own, so the immune system does not recognise them as foreign and no lymphocyte responds to them.
That last point is the general advantage of an autologous source, and it is exactly what iPS cells were developed to give at a higher potency.

Front: DEFINE
Define: Cortical granules.
Back: Vesicles lying just inside the egg's cell surface membrane, whose contents are released at fertilisation and change the zona pellucida.

Front: DEFINE
Define: Potency.
Back: The range of cell types a stem cell is still able to become.

Front: DEFINE
Define: Mitotic index.
Back: The proportion of cells in a sample that are visibly in mitosis.

Front: DEFINE
Define: Acrosome.
Back: A vesicle at the front of a sperm head containing hydrolytic enzymes, which are released to digest a path through the layers around the egg.