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Regulation of Gene Expression and Cell Specialization

  • Regulation of gene expression can occur at different times. The largest point is before transcription, or pre-transcriptional regulation.

  • Transcription factors can encourage or inhibit this from happening.

  • Sometimes changes to the packaging of DNA will alter the ability of the transcription machinery to access a gene, this is called epigenetic changes.

  • In bacteria, a cluster of genes can be under the control of a single promoter; these functioning units of DNA are called operons.

  • The operon consists of four major parts:

structural genes, promoter genes, the operator, and the regulatory gene:

  • Structural genes code for enzymes needed in a chemical reaction. These genes will be transcribed at the same time to produce particular enzymes.

  • The promoter gene is the region where the RNA polymerase binds to begin transcription.

  • The operator is a region that controls whether transcription will occur; this is where the repressor binds.

  • The regulatory gene codes for a specific regulatory protein called the repressor. The repressor is capable of attaching to the operator and blocking transcription.

  • Post-transcriptional regulation occurs when the cell creates an RNA, but then decides that it should not be translated into a protein. This is where RNAi comes into play.

  • RNAi molecules can bind to an RNA via complementary base pairing. This creates a double-stranded RNA

  • Post-translational regulation can also occur if a cell has already made a protein, but doesn’t yet need to use it.

Gene Regulation in Embryonic Development

  • The cell changes shape and organization many times by going through a succession of stages. This process is called morphogenesis.

  • Fertilization triggers the zygote to go through a series of cell divisions.

  • The early genes that turn certain cells in the early embryo into future-this or future-that are called homeotic genes. A subset of homeotic genes are called Hox genes.

Mutations

  • A mutation is an error in the genetic code.

  • Mutations can occur because DNA is damaged caused by chemicals or radiation and cannot be repaired or because DNA damage is repaired incorrectly.

Base Substitution

  • Base substitution (point) mutations result when a single nucleotide base is substituted for another. There are three different types of point mutations:

  • Nonsense mutations cause the original codon to become a stop codon, which results in early termination of protein synthesis.

  • Missense mutations cause the original codon to be altered and produce a different amino acid.

  • Silent mutations happen when a codon that codes for the same amino acid is created and therefore does not change the corresponding protein sequence.

Gene Rearrangements

  1. Insertions and deletions result in the gain or loss, respectively, of DNA or a gene. Introduction or deletion of bases often results in a change in the sequence of codons used by the ribosome (called a frameshift mutation) to synthesize a polyprotein.

  2. Duplications can result in an extra copy of genes and are usually caused by unequal crossing-over during meiosis or chromosome rearrangements. This may cause a new trait

  3. Inversions can result when changes occur in the orientation of chromosomal regions

  4. Translocations occur when two different chromosomes break and rejoin in a way that causes the DNA sequence or gene to be lost, repeated, or interrupted.

  5. Transposons are gene segments that can cut/paste themselves throughout the genome. Its presence can interrupt a gene and cause errors in gene expression.

  • Bacteria are prokaryotes that come in many shapes and sizes.

  • Bacteria divide by fission; however, this does not increase their genetic diversity. Instead, they can perform conjugation with other bacterial cells and swap some of their DNA.

  • Viruses are nonliving agents capable of infecting cells since they require a host cell’s machinery in order to replicate.

  • A virus has two main components:

    • a protein shell (the capsid)

    • genetic material made of DNA or RNA.

  • The thing infected by a virus is called a host.

  • Bacteriophages undergo two different types of replication cycles, the lytic cycle and the lysogenic cycle.

  • In the lytic cycle, the virus immediately starts using the host cell’s machinery to replicate the genetic material and create more capsid proteins.

  • The transfer of DNA between bacterial cells using a lysogenic virus is called transduction.

  • Viruses with a lipid envelope are called enveloped viruses.

  • Retroviruses like HIV are RNA viruses that use an enzyme called reverse transcriptase to convert their RNA genomes into DNA so that they can be inserted into a host genome.

Biotechnology

  • Recombinant DNA is generated by combining DNA from multiple sources to create a unique DNA molecule that is not found in nature.

  • A common application of recombinant DNA technology is the introduction of a eukaryotic gene of interest into a bacterium for production for research and to cure diseases

  • This technology that produces new organisms or products by transferring genes between cells is called genetic engineering.