Cell reproduction

During DNA replication, both strands of the double helix act as templates for the formation of new DNA molecules. Copying occurs at a localised region called the replication fork, which is a Y shaped structure where new DNA strands are synthesised by a multi-enzyme complex.

Here, the DNA to be copied enters the complex from the left. One new strand is leaving at the top of frame and the other new strand is leaving at bottom.

The first step in DNA replication is the separation of the two strands by an enzyme called helicase. This spins the incoming DNA to unravel it: at ten thousand RPM in the case of bacterial systems. The separated strands are called three prime and five prime, distinguished by the direction in which their component nucleotides join up.

The 3 prime DNA strand, also known as the leading strand, is diverted to a DNA polymerase and is used as a continuous template for the synthesis of the first daughter DNA helix.

The other half of the DNA double helix, known as the lagging strand, has the opposite 3 prime to 5 prime orientation and consequently requires a more complicated copying mechanism. As it emerges from the helicase, the lagging strand is organised into sections called Okazaki fragments. These are then presented to a second DNA polymerase enzyme in the preferred 5 prime to 3 prime orientation. These sections are then effectively synthesised backwards.

When the copying is complete, the finished section is released and the next loop is drawn back for replication. Intricate as this mechanism appears, numerous components have been deliberately left out to avoid complete confusion.

The exposed strands of single DNA are covered by protective binding proteins. And in some systems, multiple Okazaki fragments may be present. The molecular reality is very different from the iconic image of the double helix neatly separating into two DNA copies as so often depicted.

Meiosis summary

1. Parent cell has two sets of chromosomes.
2. Parent cell goes through interphase and the chromosomes duplicate.
3. After meiosis I, two daughter cells are formed with a single set of chromosomes that are duplicated.
4. After meiosis II, four daughter cells are formed with a single set of chromosomes.

Meiosis I

1. Prophase I (top left): The nuclear envelope dissolves and chromatin condenses into chromosomes.
2. Metaphase I (top right): The chromosomes align along the equator of the cell and spindle fibres extend from the centrosomes.
3. Anaphase I (bottom left): Spindle fibres pull sister chromatids to opposite ends of the cell.
4. Telophase I and cytokinesis (bottom right): Two haploid cells with duplicated chromosomes are formed.

Meiosis II

1. Prophase II (top left): The nuclear envelope in each daughter cell breaks down.
2. Metaphase II (top right): Chromosomes align along the equator of each cell.
3. Anaphase II (bottom left): Sister chromatids are pulled to opposite poles of the cell.
4. Telophase II and cytokinesis (bottom right): Four haploid cells, each with one set of chromosomes, are formed.

Sperm meets egg, which then undergoes multiple cell divisions to make a living organism with trillions of cells, all doing their own specialised function. Impressive, but where do the egg and sperm come from. The answer is as you may have guessed from the title, meiosis.

First, it's good to know that there are two types of cell division processes.

The simpler one is mitosis which produces two identical cells with exactly the same genetic information. You can think of them as clones of each other. And to find out more, why not look at our videos about mitosis?

The other process is meiosis, is a much more complicated process creating not 2 but 4 cells with only half the number of chromosomes, and crucially, all genetically different from each other.

Both mitosis and meiosis include the same phases: prophase, metaphase, anaphase and telophase. Except, in meiosis, they happen twice, so they usually referred to as one and two.

The easiest way to remember these phases' names is to remember IPMAT. So let's look at meiosis in more detail.

As always, cellular division starts with a process called DNA replication. This involves making two identical copies of the original DNA molecule. The cell ends up temporarily with double the normal number of chromosomes. You can learn more about DNA replication in this video.

In prophase 1, the duplicated chromosomes join up with the pair from the other parnent. So the mother's pair bind with the parther's pair, forming a group of two chromosomes called homologous chromosomes.

What happens next is vital for successful meiosis. As each chromosome is lined up next to its partner's pair, one chromatid from each side gets entangled with a corresponding chromatid from the other side. This is called crossing over.

During this brief period, the two chromatids swap certain sections of DNA. This is called recombination. The sections that they trade correspond to the same location so that each chromatid retains the correct number of genes.

Recombination is really important because it creates variety. The new cells aren't identical to the other parents and they also are different to one another as well. They are new genetic combinations.

In fact, that's the whole point of sexual reproduction. It increases genetic variability.

Each chromatid is now different and as each one will end up in a separate gamete, it means each sex cell is genetically different from all others. This explains why brothers and sisters are different despite having the same appearance. Only identical twins have the same genetic makeup as they both originate from the exact same egg and sperm.

Now, back to meiosis. Next comes metaphase 1 as the chromosomes align themselves up in the middle of the cell. And in anaphase 1, the spindle fibres pull the chromosomes apart to opposite ends. And during telophase 1 and cytokinesis, the cell pinches apart in the middle and the nuclear membrane reforms around the two new daughter cells. That's the end of meiosis 1.

Now, for part 2. We start with our recombined daughter cells, each still with 46 chromosomes,. But sperm and egg cells only have 23 chromosomes, so we need to cut these cells in half.

Here goes a second round of division. The process is exactly the same as before, except that there is no DNA replication. We start straight with prophase 2 with chromatin clumping again to form chromosomes, then align in the middle of the cell during metaphase 2 and chromatids are pulled apart during anaphase 2 by the spindle fibres. Telophase and cytokinesis pinch the cells together with four new granddaughter cells being formed.

The end of meiosis gives us four different sex cells each, each with only 23 chromosomes ready for future fertilisation. But that's a story for another video.

If you like this video, give it a thumbs up and don't forget to subscribe. Comment below if you have any questions. Why not check out our Fuse School apps as well. Until next time!