Gene Expression and Protein Synthesis

Study Notes

Gene expression is the process by which the information in a gene is used to direct the assembly of a protein.
Messenger RNA (mRNA) is transcribed from the DNA if the gene is active.
The sequence of the gene is read in groups of three bases, with each group (codon) corresponding to one of 20 different amino acids used to build the protein.
Transfer RNA (tRNA) molecules match their “anticodon” to the mRNA codon, and add the corresponding amino acid to the protein during translation.

Epigenetic modifications allow for a gene expression profile to be passed from a cell to its daughter cells.
Epigenetic marks do not change the DNA sequence and are referred to as being “above the genome”.
Epigenetic mechanisms include direct modifications to the DNA (DNA methylation) and modifications to the packaging of DNA (Histone modification).

The epigenome is the full set of chemical modifications in a cell that tell the genome what to do, by directing transcription.
The epigenome is passed on through cell division, helping cells to remain specialized.
The epigenome can also develop over time, for example during embryonic development.

Totipotent cells can form a whole organism (embryo and placenta), with few epigenetic marks.
Pluripotent cells can form all layers of the trilaminar embryo but not the placenta, with more epigenetic marks.
Multipotent cells can form cells within a particular lineage, with more epigenetic marks specific to that lineage.

Differentiated cells have extensive epigenetic marks specific to that tissue.
If the cells are still dividing, the epigenetic marks are carried over to daughter cells.

DNA methylation is key to some important cellular events, including genomic imprinting and X-chromosome inactivation.
In females, X-chromosome inactivation occurs through DNA methylation, resulting in the inactivation of one X-chromosome, which appears as the Barr body.

The reference sequence developed by the Human Genome Project allows for identification of genetic variations.
Types of variants include single nucleotide variants (SNV) and common variants, which are likely to represent normal variation and are unlikely to be deleterious to the individual.
Common variants are also referred to as polymorphisms, occurring in >1% of copies of alleles.

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