It’s been over a month since I published the last post. Long time, huh? Sorry for that! I guess my studies didn’t give me much spare time to update my blog. Never the less when I manage to grab some spare time I will enjoy writing an interesting, mindblowing text.
In the previous post, we discussed DNA more on the molecular level, but we didn’t talk much about the biological role of DNA. We only mentioned that this extraordinary molecule is the hereditary material in almost every living organism. Now, the story continues, and it will be told from a different point of view.
Genetics is a branch of biology that studies heredity, which is a process where a parent passes certain genes (units of heredity) onto their children or offspring. Every child carries genes coming from both of their biological parents, and these genes express specific traits. Some of them may be physical (e.g. hair, eye, and skin color), and some of them are intellectual (e.g. higher IQ). However, some genes could also be carrying the risk of certain diseases and disorders.
Genes are tightly packed in chromosomes. Each chromosome is made up of DNA tightly coiled around histones – proteins that support its structure. Histones’ basic amino acids bind to the acidic phosphate groups in DNA, making DNA to be more stable structure. Humans have 23 pairs of these small structures in the nucleus of every cell. Half of the total number (46) comes from the mother while the other half comes from the father. Plus, don’t forget the small amount of non-nuclear DNA that comes only from the mother – mitochondrial DNA.
Twenty-two of these pairs are called autosomes, and they look the same in both males and females. The 23rd pair – the sex chromosomes – are different than the rest, though. Females have two copies of the X chromosome, while males have one X and one Y chromosome.
Each chromosome is made up of two chromatids (chromosomal arms) which are joined together at a small constricted region called the centromere. The centromere helps the chromatids attach to the spindle fibers during cell division (e.g. mitosis).
In order to understand how chromosomes work, first you must know what happens during mitosis.
What happens during mitosis?
During mitosis, a eukaryotic cell undergoes a carefully coordinated nuclear division that results in the formation of two genetically identical daughter cells. Mitosis itself consists of five active steps, or phases: prophase, prometaphase, metaphase, anaphase, and telophase. Before a cell can enter the active phases of mitosis, however, it must go through a period known as interphase, during which it grows and produces the various proteins necessary for the division. Then, at a critical point during interphase (called the S phase), the cell duplicates its chromosomes and ensures its systems are ready for cell division. If all conditions are ideal, the cell is now ready to move into the first phase of mitosis.
Prophase is the first phase of mitosis. During this phase, the chromosomes inside the cell’s nucleus condense and form tight structures. In fact, the chromosomes become so dense that they appear as curvy, dark lines when viewed under a microscope. Because each chromosome was duplicated during S phase, it now consists of two identical copies called sister chromatids that are attached at a common center point called the centromere.
Important changes also take place outside of the nucleus during prophase. In particular, two structures called centrosomes move to opposite sides of the cell during this phase and begin building the mitotic spindle. The mitotic spindle plays a critical role during the later phases of mitosis as it orchestrates the movement of sister chromatids to opposite poles of the cell.
After prophase is complete, the cell enters prometaphase. During prometaphase, the nuclear membrane disintegrates and the mitotic spindle gains access to the chromosomes. During this phase, a protein structure called the kinetochore is associated with the centromere on each sister chromatid. Stringlike structures called microtubules grow out from the spindle and connect to the sister chromatids at their kinetochores; one microtubule from one side of the spindle attaches to one sister chromatid in each chromosome, and one microtubule from the other side of the spindle attaches to the other sister chromatid.
Following prometaphase, metaphase begins. At the start of metaphase, the microtubules arrange the chromosomes in a line along the equator of the cell, known as the metaphase plate. The centrosomes, on opposite poles of the cell, then prepare to separate the sister chromatids.
After metaphase is complete, the cell enters anaphase. During anaphase, the microtubules attached to the kinetochores contract, which pulls the sister chromatids apart and toward opposite poles of the cell. At this point, each chromatid is considered a separate chromosome.
Finally, once anaphase is complete, the cell enters the last stage of the division process — telophase. During telophase, the newly separated chromosomes reach the mitotic spindle and nuclear membrane forms around each set of chromosomes, thus creating two separate nuclei inside the same cell. As the image below illustrates, the cytoplasm then divides to produce two identical cells.
I hope you enjoyed this text, because if you did you’re gonna enjoy the next few as well. I will try to dive deep into DNA science, so you can expect more content related to genetics, proteins, etc… But don’t worry! It will be both fun and educational, as always.
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Scitable (by Nature Education) – Replication and Distribution of DNA during Mitosis