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Meiosis
A specialised form of nuclear Division in which there two successive nuclear divisions (meiosis i and II) without any chromosome replication between them. Each division can be divided into 4 phases similar to those of mitosis pro, meta, ana and telophase). meiosis reduces the starting number of 4n chromosomes in the parent cell to n in each of the 4 daughter cells. Each cell receives only one of each homologous chromosome pair, with the maternal and paternal chromosomes being distributed randomly between the cells. This is vital for the segregation of genes. During the prophase of meiosis i (classically divided into stages: leptotene, zygotene, pachytene, diplotene and diakinesis), homologous chromosomes pair to form bivalents, thus allowing crossing over, the physical exchange of chromatid segments. This results in the recombination of genes. meiosis occurs during the formation of gametes in animals, which are thus haploid and fertilization gives a diploid egg. In plants meiosis leads to the formation of the spore by the sporophyte generation. (genetics) cell division that produces reproductive cells in sexually reproducing organisms; the nucleus divides into four nuclei each containing half the chromosome Number (leading to gametes in animals and spores in plants).A type of nuclear Division where four [[haploid cells are created from one [[diploid cell, and is the process used to create [[gametes in humans.
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The Cell

What is a cell?
This may sound like a stupid question. However the answer to this question, will allow us to better understand all living things around us. The simple definition for a cell is the fundamental unit of life. A cell is a living system, capable of metabolism, which is carrying out of biochemical reactions essential for the normal functioning of the cells. There all are kinds of cells, bacteria cell, animal cells, plant cells etc. Animal cells can be further divided into skeletal cells, muscle cells, epithelium and the list goes on.

Cells are classified according to their cell structure. Scientists classify them as prokaryotes and eukaryotes. Prokaryote is subdivided into bacteria (or eubacteria) and archaea (or archaebacteria). Eukaryotes usually refer to animal cells. Virus is NOT a cell. Refer to the diagram below for the comparison of the size of various cells and virus.

What is a prokaryote?
To answer this question, would mean asking the question what does an organism has which confer the name prokaryote? Prokaryote is the most abundant organism on Earth, subdivided into bacteria (or eubacteria) and archaea (or archaebacteria). The size of prokaryote is about 1-10um in diameter. Structurally, a prokaryote is like a zodiac boat with oars and the side (refer to diagram below). Typical structure of a prokaryote comprises:

• Cell/Plasma membrane, which is a lipid bilayer (2 layers of oil), with embedded proteins, which control entry and exit of molecules into the cell (Imagine a zodiac boat with taps at the side to regulate in and out of seawater).
• Cell wall (much like an amour shield) to protect the cell from any environmental changes.
• Pilli(oars at the side of the zodiac boat), which assist in attaching cell surfaces
• Flagella(motor of the zodiac boat), literally rotates to propel the bacteria
• Nucleoid, a single, circular compacted chromosome, attached to cell membrane
• Ribosomes, macromolecules, organic compounds and ions: for cellular metabolism

Compared to eukaryotes, prokaryotes are much simpler in structure. Prokaryotes reproduce by asexual reproduction known as binary fission, NOT mitosis. Recall, mitosis is nuclear division. Prokaryotes do NOT have nucleus. During binary fission, the chromosome replicates, and the two copies pulled apart as the cell grows.

What are bacteria?
Bacteria are prokaryotes which can be found almost everywhere, in soil, water and even in our body. They are different from archaea mainly in their biochemistry. Example of a bacterium beneficial to humans is Lactobacillus casei strain Shirota, the bacteria found in Yakult. Some bacteria is also capable of forming spores, one notorious example is the anthrax spores, formed by Bacillus anthracis

What are archaea?
Archaea are prokaryotes which mostly thrive in environments deemed as extreme to human beings, e.g. hot springs, acidic/alkaline soil and extreme saline water.

What is a eukaryote?
Eukaryotes refer to cells which have membrane-bound organelles (small organs within the cells) with specialized metabolic functions. They are about 10-100um in diameter. Eukaryotes are divided into 4 kingdoms, animals, plants, fungi and protists (algae and protozoa). Note that protists have a rigid cell wall.

Typical structure of a eukaryote compriseses:

• Convoluted plasma membrane, to increase surface area
• Cytoskeleton controls shape and movement of cells. Include microtubules, made of tubulin, and microfilaments made of actin
• Specialised organelles, e.g. rough endoplasmic reticulum, and golgi apparatus. Their individual functions will be elaborated in upcoming posts.

Eukaryotes undergo mitosis. Below is a video clip of mitosis in a plant cell, as view under a light microscope.

What is a virus?
As mentioned earlier on, a virus is NOT a cell. They are just DNA or RNA (ribonucleic acid) surrounded by protein and occasionally other macromolecular components. They depend on cell’s functions for protein synthesis. In the extracellular(outside the cell) state, a virus is known as a virion which is metabolically inert. Upon entering the cell, its intracellular(inside the cell) state is initiated and virus replication occurs. A virus only has a simplest structure, yet capable of harming us. How certain virus harm us would be described in future postings.

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.

Your comments are important to me. Are the content meaningful to you? Do you understand the terms? Feel free to drop your comments!

Do come back to this blog as I update you more knowledge on life science.

Mission of B3 - Introduction

Hello! Thank you for visiting this blog. The mission of this blog is to disseminate essential knowledge of life science in bite sizes to the masses. Before you start associating life science with those boring lectures, I can assure you that the following would be entertaining. In fact, it is ESSENTIAL for you to know the TRUTH. Carry on reading to find out more.

Why do I blog on life science, when there’s plenty of website loaded with tons of biology facts and figures? Yes, the internet is flooded with life science information and most of which is too detailed to be appreciated by the general public. Not to mention most of us, including myself in the past, won’t be searching for such information unless I am working on a school assignment or suffering from a major disease which I am not familiar of. Touch Wood!

After speaking to numerous people from researchers to cab-drivers, I’m perturbed by the fact that the gap in terms of life science knowledge between a researcher and a layman is far too wide. Most people do not understand the word DNA or cancer. A researcher can be telling me how p53 is involved in the cancer pathway, and a cab driver is asking me, ‘What is cancer? Will you die?’

As Singapore develops into a world-recognized life science hub, and technologies such as stem cell treatment and genetic therapy becomes the norm in years to come, there is a NEED for the masses to understand how our body functions, not just at the cellular level but also at the molecular level. Only with knowledge, can the public understand how these near future treatments works and slowly accept them.

In short, this blog aims to achieve the following goals:

1. Give you a basic understanding of how our body functions, at a cellular and molecular level.
2. Make you understand and explain common life science terms, e.g. cancer. It defeats the purpose if you can’t explain these terms, and share the knowledge with your friends and family members
3. To develop your interest in life science.

To achieve these goals, I will be sharing with you how I comprehend this seemingly complicated subject in the most fun and interactive way to ensure these knowledge anchors in your brain. Contrary to school teaching, which offers you all the information you need to know, the information will be presented in a Q&A format. This is to develop critical thinking and the habit of asking the right questions. To effectively understand the concepts, it would be good to figure out your own answers for each question before reading the given answers. The whole idea is to compare and contrast with what you have already known and coming up with you own understanding of the concepts.

Be INFORMED and SHARE the knowledge