Why chromosome shape varies

 

Why chromosome shape varies: in respect of centromere and evolution

 

Abstract

I argue here that the hypothesis is based on an imperfect model by the author, and thus need further work. This essay explored the shape of human chromosomes mainly with respect to the evolution. I assumed that the shape of all chromosomes would be same, at the very first time. Since the organism, human being, prefer more variety and diversity, their evolutionary tract made introns and transposons be increased. During the evolution, location of centromere might be different compared to the original one.

 

Introduction

 (Figure of Human Chromosomes)

Chromosome is a complex of each DNA molecule and its associated proteins. This is conserved form for prokaryotic and eukaryotic cells, and even for viruses. DNA is packaged into a chromosome to compact the large information and protect DNA from damage. Every cell maintains a characteristic number of chromosomes. One thing I was interested in was the genome size and gene density, because they are closely related to the organism complexity. The other thing was the shape of the packed genome, chromosomes. The chromosomes are all in the same human being, but they all looks different. However, I couldn't find the reason why their shape is not same to each other. Thus, I tried to figure it out and made a hypothesis. The shape of chromosomes is different because of the evolution.

 

Result

 (Overview of Chromosomes of Human GRCh38p.10)

The basic means of my model is just calculating the proportion of length of p-arm in every human chromosomes. Their shapes are different as the location of centromere varies, where the p-arm and q-arm are separated. I found a figure of overview of whole genome as chromosome from Ensembl. It shows the exact location of centromere so it was easy to check where the p-arm is and to analyze the data.

 

   

These are the chart of my data. First one is describing the proportion of the p-arm length of each chromosomes(1~22, X and Y). The values are calculated by a simple equation (p arm length)/(total length). The mean of the values is 0.318. Then I ordered the values as ascending order and plotted the graph. Interestingly, the graph said that the values almost fit to the linear function. That is, the proportion of p-arm is not randomly proposed, and it might have some specific reason to be formed that way.

 

Discussion

I believe that the very first chromosomes would not be different to each other. The centromere is located approximately at the 31.8% of the chromosome. During the evolution, organisms had to regulate lots of proteins and various mechanisms in vivo. Therefore, the introns are joined to sequences and transposons create or reverse the mutation so that the diversity maintained and even increased. This process might change and modify the locus of each gene, as well as the centromere. Centromere could be moved upward or downward, and eventually they got some stable stage, thus shaped like today's human chromosomes.

 Yet, I couldn't made any logically thoughts of the tendency of the size and p-arm proportion. As the size decreases, proportion of p-arm also seems to be decreased. In addition, my model is not perfect and not accurate. The measurement is too simple, and so does the analysis. I hope I could do more advanced work as I learn more techniques and knowledge.

 

Reference

1. A Guide to Writing Scientific Essays

https://www.st-andrews.ac.uk/media/school-of-psychology/teachingdocs/currentstudents/A_Guide_To_Writing_Scientific_Essays.pdf

2. Wikipedia about Chromosome

https://en.wikipedia.org/wiki/Chromosome#Human_chromosomes

3. Ensembl

http://www.ensembl.org/index.html

 

 

Dayoung Kim

kdayoung@unist.ac.kr

UNIST