DNA Replication

The S phase

The S phaseThe DNA synthesis phase of the cell cycle where chromosomes are replicated. of the cell cycleThe sequence of events in a cell’s life, including growth, DNA replication, and division. is so named because it marks the time for DNA synthesis in the cell’s life. This is when replication takes place. The synthesis phase is placed carefully between two gap phases. This positioning allows the cell to complete enough growthAn increase in size and number of cells. to perform DNA replication. It also allows the cell to complete any growth required before the energy-intensive days of mitosis or meiosis.
In the S phase, each chromosome or piece of DNA unwinds. It is then used as a template to create an identical copy. Strangely, though, the identical copy remains attached to the original copy at the centromereThe region of a chromosome where sister chromatids attach and spindle fibers bind..

Recall that the centromere is kind of like the waist of a chromosome. I mean waist like I wear a belt around my waist. Also, remember that chromosomes might be the same length. However, they can have their centromeres in different areas, which makes them different.
Take a look at these two pictures here in gap one and gap two. It’s very subtle. You can see that all of the chromosomes have been duplicated. I want to share something crucial with you. If you ever want to count chromosomes in a cell, count the number of centromeres you see for accuracy.

DNA Polymerase

As is with many processes in the body, DNA replication is done by an enzyme. And this enzyme is called DNA polymeraseAn enzyme that synthesizes new DNA strands by adding nucleotides to a template.. The ending ASE indicates to us that this is an enzyme, and it’s the enzyme that makes the DNA polymer. And so it knows the base pairingThe specific hydrogen bonding between complementary bases (A-T, C-G in DNA). rules only of DNA. When it encounters guanine, it will look for a matching cytosineA pyrimidine nitrogenous base that pairs with guanine in DNA and RNA. and opposite. When it encounters an adenineA purine nitrogenous base found in DNA and RNA, pairs with thymine (DNA) or uracil (RNA)., it will look for a matching thymine and opposite.

I can never find an appropriate picture of DNA replication. None suit my preferences. So, I made this awkward one here. I have one chromosome. Here you can see it as one length of DNA with a centromere. The DNA’s two strands each become replicated by a DNA polymerase. What ends up getting produced is this large structure on the right. It is one chromosome, just in its duplicated state.

You might be arguing with me. You might be saying, “No Amy, that’s two chromosomes.” That is not correct; it is one chromosome in its duplicated state. I know this because the previous slide in this mini lecture explained it. If I want to know chromosomes, I should count the number of centromeres.

DNA Replication

DNA replication is known as semiconservative. Look at picture number one on the top left of this slide. Then, compare it to picture number six on the bottom right. Notice how on the bottom right we have two new strands of DNA or two new double helices of DNA. Each new double helixThe twisted-ladder structure of DNA molecules. contains one strand from the old or original DNA. It also has one completely new strand of DNA. This is what semiconservative means. It means that we are conserving 50% of the original DNA in each newly synthesized DNA molecule.

This is different from non-conservative. In this method, we would make two completely brand new moleculesGroups of atoms bonded together. of DNA. We would then discard the original. This is different from completely conservative. We would maintain our old copy of DNA. We would make one completely new copy of DNA. Semiconservative is like the best of both worlds.

DNA Replication

As you saw from the previous slide, all of the steps between picture 1 and picture 6 are pretty intense. This is true; it’s also beyond the level of detail that we need for this class period. Not just DNA polymerase that acts in DNA replication. There are enzymesProteins that speed up chemical reactions in the body. like helicase that break the hydrogen bondsWeak attractions between hydrogen and electronegative atoms like oxygen or nitrogen.. Other enzymes like topoisomerase stabilize the unwinding DNA. Various versions of DNA polymerase also play a role.

DNA replication on the top strand is occurring in the opposite direction of DNA replication on the bottom strand.
This is true; DNA replication actually involves two very different ways to synthesize new DNA.

Nucleosome Assembly

When you make DNA, you can’t just let the double helix hang out in the nucleusThe control center of the cell that contains DNA and directs cellular activities.. You have to condense the DNA back into a nicely packaged chromosome. You need those histones. They are just proteinsLarge molecules made of amino acids with various functions in the body.. We are going to wind the DNA around those histones to make those nucleosomes. Then we’re going to coil up the nucleosomes to make the supercoilThe coiling of chromatin into highly condensed chromosomes. that condenses into a chromosome.

This type of packaging is only present in complex organisms called eukaryotic organisms. Bacteria handle all of the needed events of life. They don’t have this complicated packaging of their one chromosome of DNA. Bacteria do with one chromosome what takes us as humans 46 chromosomes to get done. Bacteria are amazing. I don’t even need to keep a copy of this one chromosome anywhere. I’m not gonna ruin it. Humans, however, think any of their chromosomes could get ruined at any moment. Therefore, they have an extra one just in case.