07 Genomes (JC)

Learning Objectives

• Contrast the size and organization of prokaryotic versus eukaryotic genomes
• Explain why genome size does not predict organismal complexity or phylogeny, and vice versa
• Describe the content of the human and mammalian genomes
• Describe the current and potential applications of massively parallel DNA sequencing technology

Prokaryotic Genomes

• The genomes of Bacteria and Archaea are compact; essentially all of their DNA is functional.
• The sizes of prokaryotic genomes ranges from about 1 million to 10 million base pairs of DNA, usually in a single, circular chromosome
• Genes in a biochemical pathway or signaling pathway are often clustered together and arranged into operons, where they are transcribed as a single mRNA that is translated to make all the proteins in the operon.

Eukaryotic Genomes
 

• The genome sizes of eukaryotes are tremendously variable, even within a taxonomic group (so-called C-value paradox).
• Eukaryotic genomes are divided into multiple linear chromosomes; each chromosome contains a single linear duplex DNA molecule.
• Eukaryotic genes in a biochemical or signaling pathway are not organized into operons; one mRNA makes one protein.
• Eukaryotic genes are split; non-coding introns must be removed and the exons spliced together to make a mature mRNA.
• The multiple exons in a eukaryotic gene can be spliced in different ways to make multiple mRNAs and multiple proteins from a single gene (alternative splicing).

What accounts for the variation in genome size?
There is no good correlation between the body size or complexity of an organism and the size of its genome. Eukaryotic genomes sequenced thus far have between 6,000 and 30,000 protein-coding genes. The human genome has about 21,000 protein-coding genes. Therefore, the variation in genome size is due to varying amounts of non-coding DNA.
What’s in the human genome?

• Protein-coding genes comprise less than 2% of the human genome.
• Introns make up just over 1/4 of the human genome.
• Transposable elements and DNA derived from them make up about 1/2 of the human genome. They are the DNA transposons, LTR retrotransposons, LINEs and SINEs.
• One family of SINEs, called the Alu element, is a 300-nucleotide sequence that is present in over 1 million copies.

DNA sequencing
The human genome project was accomplished by large banks of automated sequences that used the Sanger dideoxy sequencing technology. In recent years, however, massively parallel sequencing technologies have brought down the cost and throughput of DNA sequencing much faster than computing speed and power has increased (Moore’s Law).

The implications for being able to obtain huge amounts of DNA sequence quickly and cheaply has startling implications for biological research in all fields, and for human health.

Put it all together:

DNA sequencing and personal genomics case study
B1510_module4-7_Genomes__questions_2012

Additional resources:

Ralston, A. (2008) Operons and prokaryotic gene regulation. Nature Education 1(1)