The Nucleus and DNA

Understanding genetics and DNA (deoxyribonucleic acid) can be quite challenging and so here are some notes on the basic foundations of genetics – we will look at the nucleus and its functions.

The Nucleus

The nucleus is generally the largest organelle in an animal cell and contains the majority of the genetic material in that cell. It is separated from the surrounding cell through the presence of an envelope.

This envelope also contains pores, allowing controlled movement (using ATP) of substances in and out of the nucleus. Within the nucleus, there is also the nucleolus, which is the location for rRNA assembly.

The nucleus has 3 key functions:

1. Storage of genetic material
2. Replication of DNA
3. Transcription of RNA

What is DNA and how is it stored?

DNA contains the genetic information of a cell. It is an extremely stable double-stranded helix. It consists of nucleotides, of which there are 4: adenine, guanine, cytosine and thymine. These nucleotides are made up of a phosphate group, a deoxyribose sugar and a nitrogenous base. The bases have complementary and specific base pairing – adenine or pairs with thymine and guanine only pairs with cytosine.

The bases pair together with phosphodiester bonds and this leads to the formation of a sugar-phosphate backbone on the outside of the DNA – adding to its stability.

Obviously, DNA is very long and so there has to be an efficient way of storing it within the nucleus. This is done as follows:

• DNA is first wrapped around histone proteins to form a nucleosome
• Nucleosomes then form coils called solenoids
• These solenoids gather to form chromatin
• Chromatin is then further organised to form chromosomes (the main structures carrying the DNA) – chromosomes are only visible for a short period during metaphase in the cell cycle.

There are generally 22 pairs of chromosomes plus a pair of sex chromosomes (i.e. 23 pairs in total, or 46 chromosomes).

What are genes?

Genes are the crucial ‘unit’ of genetic information. A gene is essentially a section of DNA that codes for a particular protein. As an individual has 2 copies of each chromosome, they also have two versions of a gene (these may be the same or different). These versions are referred to as alleles.

Therefore, if these alleles are the same it is referred to as homozygous otherwise if they are different, it is referred to as heterozygous.

Genes contain exons, expressing sequences, and introns, interrupting sequences. When a gene is being processed, the introns are removed through splicing as they are of no use.

Already, from this, we can understand how changes in genes can have dramatic consequences. A single mutation of a base in a gene may result in a different protein product and, therefore, may result in a disease process.

DNA Replication

This occurs during the S phase of the cell cycle (the process a cell goes through, including replication and growth). Replication occurs at several points and involves DNA polymerases (which will attempt to correct any potential errors).

Summary

• The nucleus stores majority of the cells genetic information
• Its main functions include storage and replication of genetic material and transcription of RNA
• DNA is a stable, double-stranded, helical structure
• It is stored in the form of chromosomes
• Humans have 23 pairs of chromosomes – which includes a pair of sex chromosomes
• Genes are the single, crucial unit of DNA and code for proteins

References and Recommended Reading

• Basic Sciences for Dental Students