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Asked by joy8501 to Anouk, Chris, Judith, Leisha, Seyyed on 15 Jun 2012.
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Judith Sleeman answered on 15 Jun 2012:
DNA replication is really incredible! The cell has to replicate all of its DNA before it can divide and we need to make millions of new cells every day, so the cells needs to do this pretty quickly as well as very accurately.
Each nucleus has thousands of different ‘replication centres’ and that’s where replication starts. There are special proteins (licencing factors) that recognise the replication centres and ‘tell’ each one when to start replicating the DNA so that everything doesn’t get into a tangle.
DNA is a double-stranded molecule and the genetic code (written as bases: A,C,G,T) is held in the middle of the two strands (like the teeth on a zip). Proteins called DNA helicases unwind the DNA and separate the two strands, just a very short stretch at a time, so that enzymes called DNA polymerases can ‘see’ the bases of the genetic code and copy them into fresh strands of DNA. Then each of the new strands twists back together with one of the old strands. This means that each new double-stranded DNA molecule is actually made up of one of the old strands and one new one.
The DNA polymerase can only work in one direction, so one of the new strands has to be made sort of ‘backwards’. This is really clever and needs RNA (which is cooler than DNA, honest!). I couldn’t explain it without pictures, so for more info, try this:
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Chris Kettle answered on 15 Jun 2012:
Joy8501.
Mitosis and meiosis – love it. Too detailed to go into detail here but any science book will guide you through mitosis and meiosis.
In short – Mitosis leads to an exact copy of the DNA within the nucleus. Meiosis halves the chromosome number of the daughter cell compared to te parent cell and introduces this thing called receombination which increases the variability of their children.
Mitosis – factors cause the DNA to creat an identical copy of itself (now 92 chromosomes instead of the normal 46) in the same nucleus then the DNA starts contracting until it can be seen as chromosomes. The middle of the chromosomes have special structures called kinetochores which attach to specific areas of the nuclei of these thin fibres called spindles. Very clever mechanical action then moves the chromosomes to different sides of the nucleus so that the DNA at each side is identical. The nuclues then splits and forms two separate nuclear mebranes and then the cell divides to give two identical daughter cells.
Meiosis is very similar but there is no DNA duplication and the division process takes place twice.
Its all clever stuff!
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Seyyed Shah answered on 16 Jun 2012:
As the nucleus is found only in animal and plant cells, and not in bacteria, the replication in the nucleus is very tightly controlled. This is because animal and plant cells are much more complex than bacteria, and they contain many more genes.
To make it easier to copy all of the DNA, it is organised into units called chromosomes. Each chromosome is a long string of DNA containing many genes, wound up very tightly around proteins called histones. During DNA replication, the DNA is unwound so that it can be reached by an enzyme (protein than makes a reaction work) called DNA polymerase. The DNA polymerase (and a few other proteins) bind to the now loose DNA.
DNA is a very versatile molecule. It has two strands. Each strand is made of different combinations of four bases, A, C, T and G. These are four different chemicals found in DNA. You can have any combination of these in any order in DNA. For example: ACTGGGTGCCAAATTGAC. On the other strand we have a different sequence made from the same four bases. The two strands can only fit together because A binds to T and C to G. So, we have:
ACTGGGTGCCAAATTGAC on one strand and:
TGACCCACGGTTTAACTG on the other, and note the way they fit together so A is above or below T and G is above or below C. The A and T interact, as do C and G.Now, DNA polymerase is going to use each strand as a template. So, DNA polymerase will run across the DNA and separate the top strand from the bottom strand, and different free bases floating nearby will bind to each strand. Only A can bind to T and G to C, and DNA polymerase will join the bases of the new DNA strand together. So, we will have two new strands of DNA. See the diagram below:
AACTGGGTGCCAAA (top strand)
TTGA (new strand forming using the top strand as a template. Note that the bases in the new strand have come in and bound to the top strand. The new bases are then joined to each other).And the same for the bottom strand………
Comments
Chris commented on :
I love the fact that Judith and myself answerred the question in two different ways and have complemented each other so well! Technically Judith answerred the actual question though!