A2 Revision – Photosynthesis



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.

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).