DNA Replication

A little background information about the structure of the DNA:

The DNA consists of nucleotides. A nucleotide consists of a phosphate group, the deoxyribose sugar and the nitrogenous base. There are four nitrogenous bases in the DNA: Adenine (A), Thymine (T), Cytosine (C), Guanine (G).

As you can see on the picture below, the Nucleotides are bound together by hydrogen bonds (I am going to make another video and/or blogpost about hydrogen bonds:)).

Let’s talk about the numbers on the picture. What does 5′ and 3′ mean? Well, a deoxyribose molecule contains carbon, right? Here is how a deoxyribose molecule looks like: 

(There is a carbon at 1′, 2′,3′,4′ and 5′!)

The process of replication:

Let’s move on and talk about the DNA replication. DNA replication is semiconservative. The double helix unwinds and the  new nucleotides are added on each strand. However, it doesn’t happen in the same way, because the strands are running in the opposite directions. The strand that is running (chemically mean) in the 5′ to 3′ direction is called the leading strand, and the strand that runs in the 3′ to 5′ direction is called the lagging strand. (see the picture above)

When DNA replication starts, an enzyme called helicase separates the double helix (more specifically: it separates the hydrogen bonds between the nucleotides). The single strand binding proteins hold each strand in place. Let’s see how the nucleotides are added on each of these strands:

(look at the picture below while you read :))

The leading strand: 

An enzyme called DNA polymerase III runs down the leading strand and adds on the new nucleotides.

The lagging strand: 

Since new nucleotides can only be added on the 3′ end, the DNA polymerase III cannot just add new nucleotides on, like on the leading strand. First, an enzyme called RNA primase adds an RNA primer (a little bit of RNA). Then, the DNA polymerase III can start adding on new nucleotides. The process repeats over and over. The DNA fragments between the RNA primers are called the Okazaki fragments. The DNA polymerase I replaces the RNA primers with DNA, and the DNA ligase links the Okazaki fragments. 

Please check out my video about the DNA replication HERE :)

Also, it's very helpful (at least in my opinion :)) to watch animations about this topic. Maybe I will be able to make animations in the future (I will do my best :)).

Greetings!

The mitosis app :)

Hello!
I've recently made an app about mitosis! Here is how you can download it:
1. Launch the app called PapTap on App Store. (it's free)
2. Search mitosis
3. And there you have it! All you need to do now is to click on it :)
I hope you like it :)

Here is what you should
get after completing stages 1., 2. and 3.:

The phases of meiosis

Meiosis refers to the division of gametes. Note: All the things explained below happen after the interphase has been successfully completed.

Meiosis I

Prophase I: 
During prophase I, the chromosomes condense, just like during mitosis. However, there is also a process called synapsis. During synapsis, the chromosomes come together and then they exchange some segments. That gives us more variety, because the chromosomes that are produced are unique. Another thing that happens during prophase I is that the nuclear envelope disappears and the mitotic spindle starts to form (see previous post).

Metaphase I:
As you could predict, the chromosomes meet in the middle during metaphaseI. However, they line up in a different way compared to the way they line up during mitosis. During metaphase I, the homologous chromosomes lie side by side, because the sister chromatids are not going to separate during metaphase I. There are several ways of the arrangement of these chromosomes. The blue chromosome could be on the right side, or on the left side etc. The random arrangement of chromosomes is called independent assortment, and it gives us variety.

Anaphase I:
During anaphase I, the homologous chromosomes are moving apart from each other.

Telophase I:
The telophase I is very similar to the telophase in mitosis. The nuclear envelope reforms, the clevage furrow forms, chromosomes decondense and the organelles migrate to the opposite poles.

 Cytokinesis completes meiosis I. When these two cells above separate, the meiosis I is finished, and the cell is ready to enter meiosis II.

Meiosis II

Prophase II: 
During prophase II, the chromosomes condense, the nuclear envelope fragments and the spindle starts to form.

Metaphase II: 
Metaphase II is just like metaphase in mitosis. The chromosomes meet in the middle, prepared to separate.

Anaphase II: 
During anaphase II, the homologous chromosomes separate. In other words, sister chromatids migrate to the opposite poles of the cell, just like in mitosis.

Telophase II:  
New nuclei form on each side, the clevage furrow forms and the organelles migrate to the opposite poles.

  
Cytokinesis completes meiosis II, and thus the whole process of meiosis. The products of meiosis are therefore four haploid daughter cells, because the DNA replication happens only once.
PLEASE CHECK OUT THE VIDEO I MADE ABOUT MEIOSIS HERE :)

The phases of mitosis

Let's look more closely what happens when a cell divides. The phases of cell division are also called the phases of mitosis (cell division = mitosis). There are five phases of mitosis: prophase, prometaphase, metaphase, anaphase and telophase. Mitosis ends with cytokinesis, which is the split of the cell who has gone through all the phases listed above. As a result of cytokinesis, we get two new cells, or daughter cells, from one cell, the mother cell.

Prophase:

During prophase, the chromosomes condense, so they are packaged up more tightly. The centrosomes (on the picture: the orange rectangles with blue lines sticking out of them) migrate to the opposite poles of the cell and start to attach the microtubules to each other (microtubules = the blue lines). The attached microtubules are known as the mitotic spindle.

 

Prometaphase:

In this phase, the nuclear envelope fragments, and the microtubules start to attach to the kinetochore (the middle part of a chromosome).

  

   

Metaphase: 

During metaphase, the chromosomes that are now attached to the microtubules meet in the middle of the cell, forming the metaphase plate. The metaphase plate is one of the straightest lines in nature

Anaphase:

During anaphase, the sister chromatids start to move apart from each other. Each daughter cell will receive one sister chromatid, so that it's number of chromosomes is exactly the same as the number of chromosomes in the mother cell.

Telophase:

During telophase, nuclei start to form, and the organelles in the cell migrate to the opposite poles. The cell elongates, and a cleavage furrow forms (it is made of actin filaments).

Cytokinesis:

Cytokinesis is not considered a phase of mitosis, but it's vital in the process, because its not complete without it. During cytokinesis the cleavage furrow splits and we get two new daughter cells, that looke exactly like the mother cell.

Check out my video about the phases of mitosis here.

What happens to a cell before it enters mitosis?

Before mitosis, the cell goes through a phase called interphase. However, interphase is subdivided further into three other phases, and knowing what happens during each of them is crucial for us if we want to understand mitosis and meiosis.As menitioned above, interphase is divided into three phases: G1 (gap one) phase, S (synthesis) phase and G2 (gap two) phase. Let's look more closely what exactly happens in the cell during all these phases:

G1: During G1 phase, all the organelles in the cell replicate, except the genetic material (DNA)

S: During the S phase, the DNA is replicated. The process od DNA replication is a highly complex process, so I will not describe it more specifficaly in this post. All you need to know about the S phase is that the DNA replicates. NOTE! The DNA in our cells is packaged up into chromosomes. Here is how the chromosomes look like after the synthesis phase (after the DNA replication):

G2: During the G2 phase, the cell continues to grow. All the enzymes needed for mitosis are produced during G2 phase

During interphase, the cell goes through various checkpoints which controll that all the phases have been successfully completed. If there is an error (for example, if the DNA has been copied incorrectly), the cell will not divde.
I have also made a video about this topic. Please, check it out here: My video about interphase

Greetings!