Question

The human diploid genome contains 6.4x109 bp. For B-form DNA, assume an average distance of 0.34nm per base pair along the DNA molecule and an average diameter of a DNA molecule to be 2.4nm. Does the genome, in B-form, fit inside a nucleus (approximate a sphere of 6µm diameter)? If so, what fraction of the nucleus is taken up by DNA? If not, how many nuclei do you need to contain one diploid human genome?

Answer 1

There are 22 pairs of autosomes and 1 pair of sex chromosomes. The total DNA is measured in base pairs. There are approximately 3 billion base pairs of DNA in a human cell. The nucleus consists of the chromatin, the basic unit of which is the nucleosome complex (an octamer of histone proteins H2A, H2B, H3 and H4 wrapped around by 146bp of DNA). These nucleosomes are present at a distance of approximately 200bp and they transform DNA into chromatin which is about 1/3rd of its length and is present inside the nucleus.

The Y chromosome is smaller than the X chromosome, meaning that Y chromosome-bearing sperm cells contribute fewer DNA base pairs than X chromosome-bearing sperm cells. Human chromosomal DNA is also cleverly packaged and compacted so that it perfectly fits inside of the cell nucleus.

Basic functions of the nucleus during one cell cycle

1. Information Storage and Retrieval

A. Storage (the packing problem)

B. Retrieval

i. mRNA for each protein required by the cell needs to be synthesized and processed selectively with the right number of copies, while other mRNAs are not made

ii. rRNA must be made and assembled into ribosomes iii. tRNA must be made and processed for transport to cytoplasm

2. Replication (total DNA needs to be copied once and only once, ends-Telomers)

3. Division and reformation (one copy needs to be transferred to each daughter)

DNA and nuclear size: 6 x 109 base pairs at 0.3 nm/bp or 2 m of DNA in 46 chromosomes is packaged in a 5-6 mm diameter nucleus 2. DNA is a helical polymer with complimentary strands that is unwound in reading (10 bp/360°). Also, it has 2 neg. charges per bp or 0.3/nm of length. 3. Number of mRNAs encoded (about 50,000) 4. Number of different mRNAs translated (6-10,000) - 5. Total number of mRNAs produced per day is 6. 46 chromosomes must be separately packaged with daughter chromosomes and separated.