How we become who we are today? – A DNA Odyssey

We become who we are today because of:

1. DNA
2. Environment. The environment shapes our thought and behavior, and numerous genes are directly and indirectly influenced by numerous environmental factors, such as the food we take.

For the start, I will be focusing on how the DNA determines who we are today.

What is DNA?
DNA (DeoxyriboNucleic Acid). I understand this word is a mouthful. If you Don’t kNow Anything about it, you’re not alone. In fact that’s the work of researchers, to find out what they do not know. By breaking down the words into pieces (de-oxy-ribo-nucleic acid), it would literally mean ‘no oxygen ribose nucleic acid’. Basically it means a ribose, which is a sugar, which lacks an oxygen atom. This chemical is an acid which can be found in the nucleus, which is the control centre of the cell. Imagine each MRT/train station (cell) has a control station (nucleus). Those officers, in the control station are the DNA, they control the operation of the MRT stations (functions of the cell). To ensure smooth operation of the MRT station, these officers have the information on how the MRT should function. Similarly, our DNA store information on how our cells should work.

How does DNA store information?
How do we take notes or record information? Isn’t it in the form of words? Words, themselves are made up of alphabets. Similarly, DNA stores information in form of chemical ‘alphabets’ known as bases. DNA only has 4 bases, CGAT (Call Girl At Tokyo). Obviously CGAT doesn’t refer to any Japanese idols of your fantasy dreamland. CGAT stands for Cytosine, Guanine, Adenine and Thymidine respectively. C will pair with G, and A will pair with T. That’s the reason for creating that acronym in that order. You don’t have to memorize the chemical name of CGAT, unless you’re out to impress your colleagues and friends. In the nucleus, DNA is packaged into chromosome. It is similar to pages of words bind together to form a book.

How much DNA do we have to determine who we are today?
Who we are today is determined by 23 pairs of chromosomes, which amounts to approximately 20,000-25,000 genes. Genes refer to DNA sequences which encode instructions in synthesing proteins. Surprisingly this number is about the same as a tiny flowering plant called Arabidopsis and barely more than the worm Caenorhabditis elegans. This amount of gene only take up 2% of the human genome, the remaining are noncoding region, in which their functions are yet fully understood.

How do the chromosomes determine a guy or a gal?
I believe most of you know the answer. Anyway if you do not know the answer, here is it. Most of the chromosomes are similar in shape, i.e. homologous except the 23^rd pair of chromosomes, which is also the sex-determining chromosome. Females carry 2 homologous X chromosome, while males carry an X and Y chromosome.

How do we maintain 23 pairs of chromosomes from one generation to the next?
All our cells have 23 pairs of chromosomes, EXCEPT our gametes, which is our specialized reproductive cells, obviously the egg and the sperm. Logically, either the egg or the sperm has to carry 23 chromosomes, not more not less, so that during the fusion of egg and sperm, the fertilized egg or zygote reconstitutes to 23 pairs of chromosomes.

How do human cells develop?
The two possible ways to develop anything,

1. Starting from scratch, meaning you create what you want by assembling the various components together, like assembling various LEGO parts to build the structure you want
2. Find a similar item and copy from it.

Our chromosomes are programmed to perform the 2^nd option which is to ‘copy’ previous cells. Apparently the 2^nd option is the faster method of development. All human cells, except our reproductive cells, develop out of mitosis. Note that mitosis, is used interchangeably with cell divison, but strictly speaking, mitosis is nucleus division. Cytokinesis is the division of cytoplasm, which usually occur after mitosis. Many fungi and fertilized eggs of many insects do not undergo cytokinesis after mitosis. During mitosis, cells which carry X number of chromosomes, give rise to more cells with X number of chromosomes. In other words, during mitosis, 1 human cell with 23 pairs of chromosomes give rise to another human cell with 23 pairs of chromosomes. Mitosis is used as a mean of cell-renewal, replacing dead cells with new cells, which carry the same number of chromosomes to carry on their purpose. This process is extremely important for skin cells which are constantly being get rid of and replaced. It is estimated that we lost 100billion of skin cells daily.

The completion of one cell division to the start of the next division is known as cell cycle. To prevent over development, there’s always a check and balance. Similarly, the cell cycle consists of mitosis (Production) and interphase (Quality Control, the ‘check and balance’). Interphase, is the interval between the two divisions. This interphase consists of various phases, which I’ll not elaborate. The key point is whether the cell proceeds with division or not is determined during the interphase. The interphase takes about 15 hours, while mitosis takes about 1 hour. This goes to show how important interphase is. Cancer occurs when the abnormal cells carries on division, by-passing the regulation process in the interphase. In layman terms, it would mean passing the QC test even though it is faulty and proceed with production.

Our reproductive cells, i.e. sperm and egg derive out of a different type of nuclear division known as meiosis which gives the each of them 23 chromosomes. During the reconstitution of 23 pairs of chromosomes in the fertilized egg, there’s crossing over of DNA sequence between the two pairs of 23 chromosomes so that we don’t look exactly the same as one of our parents. The number of possible permutations is 2²³. After the fertilization, the zygote undergoes another cell division known as cleavage, which forms 3 germ layers, which eventually develop into the various organs of our body.

Advanced Section(For those who wants to know more): p53 gene is a tumor-suppressor gene which is involved in the interphase of cell cycle. Prof Sir David Lane, one of the scientists who originally discovered p53 gene in 1979, is currently the executive director in Institute of Molecular and Cell Biology, under Proteos building in Biopolis, Singapore. The protein of p53 gene regulates programmed cell death, apoptosis. In the presence of a normal p53 gene product, a cell which contains a severely-damaged DNA will undergo apoptosis. If the p53 is mutated, damaged cell will continue proliferation. Interestingly numerous cancers contain a mutation in p53, which is why intensive research has been conducted on p53 gene in the hope that it would shed light in finding a cure for cancer.

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