so i just finished my ap biology test corrections for ch. 12,13,18.5 and i got to the last question that i got wrong and i did some research on it and my mind is completely mind-boggled by amazement.

here is question 77 on the test:

Which of the following about the DNA in your brain cells is true?

A. Most of the DNA codes for protein.
B. The majority of the genes are likely to be transcribed.
C. Each gene lies immediately adjacent to an enhancer.
D. Many of the genes are grouped into operon-like clusters. (prokaryotic cells, not us)
E. It is the same as the DNA in your heart cells.

I originally chose A for this question because I was thinking that the genes in a cell’s DNA function by being transcribed and processed by mRNA. mRNA leaves the nucleus and is translated into a specific protein. So i was thinking that the DNA would code for the synthesis of proteins. But that is wrong.

The answer’s E. I researched this online and it amazes me. So all of the cells in your body have the same DNA and same genes since all the cells started from a single fertilized egg. Even though, the cells have the same DNA and genes, different cells read and express the genes differently. Similar to two cooks with a cookbook, the breakfast cook only has to read the breakfast chapter and the lunch cook only reads the lunch section. So the heart muscle can select which genes it wants to turn on and be expressed and which ones will be turned off and ignored.

This made me question how we are all different people if our bodies run the same way and all of our cells in our bodies have the same DNA, then why aren’t all of the people the same since we all originated from a fertilized egg?

And the answer has to do with random assortment of chromosomes during metaphase I of meiosis. the maternal chromosomes and the paternal chromosomes align randomly at the metaphase plate. instead of all the maternal chromosomes going to one side of the pole (ends of the fibers that they dangle off of), they orient themselves randomly with a mix of maternal and paternal on either side. the independent assortment of chromosomes accounts for in humans 2^23 or 8.4 million possible combinations of chromosomes. Since the number of chromosomes during random assortment is represented by 2^n where n is the number of chromosomes contained in an organism’s gametes. Humans have 46 chromosomes total and half of the chromosomes exist in the gametes which is 23.

Also crossing over in prophase I of meiosis in which the maternal and paternal chromosomes share segments with each other helps with genetic variation, you now have DNA from both parents inherited upon a single chromosome. However, say if you have a chromosome with an allele (part of a gene that expresses a trait) coding for brown hair from mom and another allele coding for blonde hair from dad. the alleles that code for hair color can exist on the same gene, but one may be expressed over the other. the dominant allele is brown (B, either Bb or BB) and the recessive is blonde (bb). since alleles can also assort randomly like chromosomes.

another factor that contributes to genetic variety is random fertilization, a sperm and egg can produce 2^23 * 2^23 or 70 trillion combinations and if we add in the crossing over part, then the number would be a lot higher.

Also a privet shrub has 46 chromosomes like a human, but the shrub reads the genes differently than we as humans do.

Science never ceases to astound me, we are truly part of a integral universe, yet each of us is unique in our own way.