Please answer a layman's questions - Very basiscs of Cell and Genetics (Nov/25/2004 )

Dear Experts:

I am a computer scientist and know almost nothing about Cell Biology and Genetic Engineering. PBS documentary on Genetic Engineering excited my curiosity about Cell Biology and Genetics. I would be very thankful if you could answer my questions briefly.

I am mostly interested in human cell.

1. Is there any molecule in human body that is not part of Cell? What are those?
2. Is the physical size of all cells are equal? What is/are the dimension?
3. What is the size of nucleus?
4. Is all human being cells’ size equal, obese or skinny?
5. Is there protein inside of nucleus?
6. What outer layer of cell (outside nucleus) composed of? [I guess protein only!!]
7. After cell splits, does it travel to different part of the body? Or stays just next to the mother cell.
8. Does each cell produce protein for it own cell only?
9. Does cell keep track of the count of splitting? How and when does it stop replicating itself?
10. When cell dies how it is flushed out of our body? [Let’s say few of my muscle cells are dead. How would it be thrown out of my system?
11. How far cell can travel in human body? [Not talking about cell’s blood flow.]
12. Does cell has time clock too?
13. Does each cell aware of the total count of cells in the body? Does it know the number of different kind of cells in the body?

Most Important question:
During the very early stage of growth of a baby in mother’s womb:
Life starting with a single cell; cell splitting process continues. Cell must create at least one cell of each kind in the beginning by turning on certain type of genes (gene for eye, ear, brain, leg, cells) in the nucleus and turn off all other type of genes. From then on each type of cell can take care of itself by continue to multiply of it’s own kind. But in the beginning cell must produce one of each kind of cell. How does it keep track of the count?

Hi,

I'm not a cell biologist, but the naswer to most of your questions can be found in any general biology textbook. I recommend "biology, by Campbell, there are many editions and any university or probably public library in the region will have copies of it or similar texts.

Besides this as far as I know:

1) Yes, there are molecules in the body that are not part of the cells, they may be produced (such as hormones) or used (such as oxygen) by the cells in the body, but do not count as cells.
2) Cells have a range of sizes and shapes, some are long and skinny, others are approximately spherical,
3) the nucleus will generally be less than 1/10th the volume of the cell.
4)see 2 above
5)Yes there is protein outside the nucleus, proteins compose roughly 30% (I think I'm right there) of the cell, they are found in all parts of the cell.
6)The outer layer of the cell is called the cell membrane (as opposed to the nuclear membrane) and is composed of a range of proteins and lipids (fats), with the proteins embedded in the fats. The lipids are arranged in a bilayer with a hydrophobic tail in the middle and a hydrophilic head on the outside.
7)Varies, some cells migrate, others stay put.
8)No, proteins can be shared around as can other components of the cells, such as minerals.
9) Hard one here, sort of, the cells do die after a time and have a definite life span, but it is hard to say that they keep track of the divisions. I don't know the answer to how and when it stops replicating, but it will be controlled by a range of factors such as nutrient levels, contact with other cells, hormone and other chemical gradients.
10) Cells when they die undergo a process known as apoptosis (cell death) that basically dissolves the cell and the components are recycled or removed from the body via the blood.
11) I don't know, some can move millimeters in distance I think.
12)see answer 9
13) Probably not aware as such of the number of other cells ect in the body, but there are limits on how many cells of each tissue type there are, cells that have lost this limiting ability continue to divide and become what we know as cancers.

Most important Q:
These processes are controlled by a huge range of factors all competing to switch on and off genes at the right time. The evidence suggests that there is no absolute way for the egg after fertilisation to keep track of the number of cells produced, indeed, it is possible to remove cells from the very early embryo (8-16 cell stage) and the embryo will still develop normally. Sometimes the cell division processes and regulatory systems go wrong and produce mutations visible to us as people with no arms or a cleft lip or anything, even heart defects (hole in the heart for example). It is hard to explain without a lot of background, but any general biology text will steer you towards further reading if you want to know more details.

Most of the questions you asked are still not fully understood, and are the subject of research worldwide, so I suspect that no-one will be able to give you complete answers to all the questions. But these sorts of questions are often what inspire people to become biologists.

Anyway,
Hope this is of some use to you

9. Does cell keep track of the count of splitting? How and when does it stop replicating itself?

Yes. Cells keep count of how many times they divide by sensing the shortening of the ends of their chromosomes (telomeres). Each time a cell divides it cannot copy the chromosomes right to the very end. When the telomeres get too short the cell senses this and the cell either stops dividing (senescence) or destroys itself (apoptosis). Some cells need to divide more than this would allow. They have an enzyme called telomerase which lengthens the ends of the chromosomes. Cancer cells are sometimes referred to as immortal because they have bypassed these control systems.

1) There are many molecules outside of cells. The body is composed of cells and extracellular spaces, such as the blood vessels, the gut, the airways etc. There are certainly molecules in these: mostly water, proteins, lipids, carbohydrates.

2) Cells have a great range of functions and accordingly vary in size. The oocyte (egg cell?) is usually considered the biggest human cell. Ordinary body cells are in the micrometer range, e.g. 20-40 micrometers in diameter for human cervical cancer cells (a widely investigated cell line).

4) As mentioned: all cells are different. In obese people, the excess amount of fat is stored in specialised cells, fat cells. Fat people will have more and bigger fat cells, than skinnier people. I dont think that obesity affects the other cells of the body.

5) There are many proteins inside the nucleus, e. g. for processing and duplication of the genomic information.

6) Protein, RNA, carbohydrates, lipids, water, many metabolic intermediates, nucleotides, steroids , and its not called outer layer, but cytoplasm. The "outer layer" of a cell would be the cell membrane, composed of lipids and proteins, or the extracellular matrix. At this point you should consider looking into a 10th grade biology textbook.

8) Think of hormones, they travel all the way from the brain to different organs, for example.

10) Dead cells enter the blood stream and are removed by the spleen.