Life Science Blogs

What is a cell?
The simple definition for a cell is a fundamental unit of life. A cell is a living system, capable of metabolism, carrying out its biochemical reactions essential for the normal functioning of the cells. There are various type of cells, e.g. 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?
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 boat with oars at the side. 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 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 boat), which assist in attaching cell surfaces

• Flagella(motor of the 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.

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, e.g. food.

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 the unknown. By breaking down the words into parts (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. The chemical is an acid located in the nucleus, 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, control the operation of the MRT stations (functions of the cell). To ensure smooth operation of the MRT station, these officers have 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). Apparently CGAT doesn’t refer to any Japanese idols of your fantasy. CGAT stands for Cytosine, Guanine, Adenine and Thymidine respectively. C will pair with G, and A will pair with T. 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?
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, 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, 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: 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, and CEO of A*STAR’s newly formed Experimental Therapeutics Centre (ETC). Cancer Research UK also announced that it will appoint Professor Lane as its first Chief Scientist from January 2008. The protein of p53 gene regulates programmed cell death, apoptosis. In the psrence oef 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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