Meiosis

In response to Charlotte and Emma or Emily, or possibly both, a little clarification of meiosis.

I think  it is probably the language that becomes confusing here, since there are chromosomes, chromatids and bivalents all floating around in the mix.

Unlike mitosis, there is no cell cycle. This is because after mitosis each daughter continues in a cycle like the parent cell of growth, DNA and organelle replication etc. Gametes end up with  a haploid chromosome number, so they do not divide again. However, it is worth considering the cell cycle in terms of the cell just prior to the gamete (primary spermatocytes and primary oocytes) since it is during interphase that the DNA is replicated. Remember that you can’t actually see that the DNA strand (and therefore each chromosome) has replicated until the supercoiling during prophase. Nevertheless, the replication occurs during the cell cycle so that each chromosome strand now consists of two sister chromatids.

Refer to it as a chromosome when single strand, a chromosome comprising two sister chromatids after DNA replication.

The first stage of meiosis is similar to mitosis (PMAT) but are numbered, e.g. prophase 1, metaphase 1. During P1 the chromosomes condense and become visible. This is when they take on the characteristic ‘X’ shape with the centromere holding the two sister chromatids together. Each of the homologous chromosomes (e.g. pair number 22, or pair number 9) pair up near the other (remember each of the pair of chromosomes is made up of two sister chromatids, so there are now 4 strands of DNA involved). When the homologous pairs are adjacent they are referred to as bivalents.

The bivalents then undergo crossing over, or chiasmata (singular chiasma) where portions of genetic material are swapped between non-sister chromatids within a bivalent. The 23rd pair of chromosomes only show a small amount of crossover. 

in M1, the bivalents line up opposite each other at the cell equator, and are pulled apart by spindle fibres in A1. This is the key step to remembering the difference between mitosis and meiosis stages: in meiosis the bivalents are opposite each other, in mitosis the chromosomes all line up across the equator. Remember that in each bivalent, there is a chromosome (made of two sister chromatids that have undergone chiasmata) from the mother and one from the father. It is random as to which side of the equator the maternal and paternal chromosomes line up, another source of variation.

After T1, the second phase of meiosis occurs (P2, M2, A2, T2), to produce the 4 haploid gamete cells. The second part od meiosis proceeds in pretty much the same way as mitosis, but with only half the number of chromosomes present.

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It’s all about the lymph…

In response to Sophie’s question…

 

Tissue fluid (or interstitial fluid) is the fluid that surrounds our cells. It is basically water with dissolved solutes like sugars, salts, amino acids and waste products from the cells. Blood vessels have pores in them that allow the movement of molecules between the tissue fluid and the blood plasma (which is pretty much the same composition as the tissue fluid. You can think of the cells as balloons full of water sitting in a bath also full of water. Molecules are moving between tissue fluid and the cells (osmosis, diffusion for example) constantly; the presence of the cell membranes allow some molecules to build up in concentration and to separate the cell from its surroundings.

Lymph is tissue fluid that has entered the lymph channels (the system of vessels that lymph fluid moves through).  Lymph transports excess tissue fluid and proteins back to the blood. Lymph nodes are ‘blobs’ along the lymph channels filled with white blood cells that destroy pathogens. The lymph is moved along the channels by the incidental movement of skeletal muscle and some peristalsis.  Unfortunately, lymph channels also sometimes act as a quick route around the body for cancerous cells that have broken off from a tumour, which are carried around the body and may start growing in other tissues (metastasis).

C3 and C4 plants

C3 plants are simply the plants we have been looking at already in photosynthesis. In C3 plants, CO2 is fixed from the atmosphere into the 3 carbon compound GP using the RuBisCO enzyme. Remember the problem here; above 25OC the RuBisCO starts to prefer oxygen over CO2 fixing (the CO2 and O2 are competing), reducing the rate of photosynthesis. C4 plants avoid this problem by having a more efficient way to fix the CO2.

Essentially, the CO2 is used in another biochemical pathway using pyruvate (3C) which is converted eventually to oxaloacetate (4C), and eventually to the compound malate (4C). This allows a ‘store’ of CO2 to be built up that can be released directly into the Calvin cycle. In these higher concentrations the CO2 is more readily absorbed by the RuBisCO in preference to O2.

The problem is the CO2 has to be fixed twice, which requires more energy than a C3 pathway. C3 plants require 18 molecules of ATP to synthesise 1 molecule of glucose; C4 plants require around 30 ATP per molecule.  It turns out to be a balancing act – C4 plants are found in places where there is a higher temperature that would favour RuBisCO taking O2, for example the tropics. C3 plants are found in more temperate regions. C4 plants are also more water efficient, doing better in dry conditions.

Despite being only around 3% of all terrestrial plants, C4 plants are responsible for around 30% of terrestrial carbon fixation.

Abominable research

You may be wondering why there isn’t much in the course on cryptozoology. This is the study of animals that have not yet been shown to exist, for example unicorns and the Loch Ness monster. This week has seen some attention on the hunt for the Abominable Snowman, or Yeti, and claims from some scientists that they have uncovered evidence for their existence. Unfortunately (and this will explain why we don’t cover cryptozoology in A levels) they have done no such thing.

Science relies on the methodical gathering of evidence, testing and re-testing of hypotheses. The ‘scientists’ involved have gone straight past the boring bit of gathering valid data and gone straight to the headline grabbing phase. Their evidence consists of some hair (unidentified as yet, but it wouldn’t be difficult to do), an alleged nesy made from twigs (Really? Do you have any photos? No? How odd.) and footprints (guess they didn’t get any photos of those either). Science does not get to the truth by making unsubstantiated claims or listening to anecdotes (the Royal Society, probably the oldest scientific society in the world, has for its motto ‘Nulius in verba’, translated as ‘not by words’. This is a good description of the core of science; it is not enough to simply say or assert something, you must back it up with evidence. The evidence must be clearly available to other people to test (it’s no good making special pleading claims that only you can see the evidence, or ‘it did that when I tried it.’

It is possible that the yeti exists, but no valid evidence has been presented to demonstrate this. The scientific position remains that the yeti does not exist. If valid evidence emerges for the existence of these animals, then scientists will change their minds. It is however unlikely to be found. Resorting to boring old things like facts and evidence, creatures of the size proposed (and the same goes for Nessie) would have well understood need for territory, mating groups and reasonably sized gene pools to avoid in-breeding problems. From what we know of comparable sized animals, it is unlikely that something this big would have remained undetected.

New animals are discovered all the time, but it is rare indeed for legendary creatures like bigfoot and Nessie to be found living happily in our world.