Chapter 19 AP Biology Notes I.
Chapter 19.2: Gene expression can be regulated at any stage, but the key step is transcription. a. Cell differentiation: i. All organisms must regulate (turn on or turn off) their genes 1. Multicellular organisms must do this on a long term basis. ii. Cell differentiation is a process were the cells of multicellular organisms differentiate into one of 200 possible cell types 1. Example: nerve cells, muscle cells etc… b. Differential Gene Expression: i. At any given time a typical human expresses around 20% of its genes. ii. The difference between cell type is not due to differences in the genome from cell to cell(genome is the same0 but differential gene expression, or the expression of different genes by cells with the same genome iii. Eukaryotic genomes contain about 35,000 genes, but(in the case of humans), only about 1.5% of the DNA codes for proteins 1. Some of the DNA codes for RNA products such as ribosomal and transfer RNA 2. Most of the DNA does not code for any although scientist suspect there may be some coding for other RNA molecules iv. The key is that the right protein must be transcribed at the right time 1. If something in this process goes awry, then serious imbalances and diseases such as cancer can occur. c. Summarizing gene expression in eukaryotic cells: i. Regulation can occur on many levels or control points: 1. During transcription 2. Post transcription 3. During translation 4. Post translation ii. Prokaryotic cells do not have a nuclear envelop therefore their regulation occurs at the level of transcription 1. For instance, the nuclear envelop offers post transcriptional control which is absent in prokaryotes iii. Levels of gene regulation: 1. Chromatin modification: DNA unpacking involving histone acetylation and DNA demethylation II.
a. Chromatin-­‐ compact form of DNA that fits into the nucleus and also is important in helping regulate gene expression i. Genes in heterochromatin are not usually expressed b. Histone modifications: i. Histones: group of five proteins that are associated with the coiling of DNA ii. Histone acetylation: acetyl groups (-­‐COCH3) attached to positively charged lysines in histone tails iii. Deacetylation is the removeal of the acetyl groups-­‐ neutralizing the positive charge so they can no longer bind to neighboring nucleosomes iv. Creates a loser chromatin structure allowing transcription proteins to access genes 2. Transcription 3. RNA processing 4. Transport to the cytoplasm a. Degradation of mRNA 5. Translation 6. Cleavage: chemical modification, Transport to cellular destination 7. Degradation of protein Chapter 19.5: Duplications, rearrangements, and mutations of DNA contribute to genome evolution. a. Slide One: Duplication of Chromosome Sets: i. Basic change driving genome evolution is mutation 1. Evolution = increase in size of the genome = genome diversity ii. Slide Two: Evolution of genes with novel functions: 1. These genes can evolve from polyploidy organisms where one set of chromosomes provides essential functions and mutation developing in the extra sets of chromosomes 2. As long as one copy of a crucial gene is expressed, the divergence of another copy can lead to its encoded protein acting in a novel way, changing the organisms phenotype 3. Accumulations of mutations may lead to a new phenotype 4. Common in plants, rare in animals b. Slide Three: Duplication and divergence of DNA segments: i. Errors during meiosis I can lead to duplication of genes ii. Example: unequal crossing over during prophase I can result in one chromosome with a deletion and another with a duplication of a particular region. iii. Transposable elements can provide sites where non-­‐ sister chromatids can cross over when their homologous gene sequences are not correctly aligned. iv. Slippage of the DNA template causing a misalignment between the new strand and the template strand, resulting in a deleted or duplicated region of DNA c. Slide Four: Evolution of Genes with Related Functions: The Human Globin Genes i. Duplication events can lead to evolution of genes 1. Α-­‐ globin and β-­‐ globin gene families a. Evidence shows these families evolved from one common ancestral globin gene b. This ancestral gene duplicated and diverged about 450-­‐ 500 million years ago c. Duplication of these genes since has occurred several times leading to divergence of the two into two families. d. Same ancestral gene gave rise to myoglobin and leghemoglobin 2. Α-­‐ globin-­‐ this copy of the gene accumulated mutation over time a. Some mutations may have had adverse effects b. Others may have had no effect at all c. Some may have altered the function of the protein product in a way that was advantageous to the organism 3. Natural selection would have acted on these genes producing alternative forms of α-­‐ globin we see today 4. The amino acid sequences of the β-­‐ globins are much more similar to each other than to the α-­‐globin sequences 5. There are many pseudogenes among the functional genes that provides evidence for this model d. Slide Five: Evolution of genes with novel functions: i. Example: lysozyme vs. α-­‐lactalbumin 1. Lysozymes-­‐ enzymes that helps prevent infection by hydrolyzing the cell walls of bacteria 2. Α-­‐ lactalbumnin-­‐ non enzyme protein that plays a role in milk production ii. These two enzymes are very similar in their amino acid sequence and three dimensional shapes 1. Both are found in mammals 2. Only lysozymes are found in birds iii. Suggests that lysozymes underwent a duplication event in the mammalian lineage and not in the avian lineage. e. Slide Six: Rearrangements of Parts of Genes: Exon Duplication and Exon Shuffling i. The presence of introns may have promoted the evolution of new proteins by facilitating the duplication or repositioning of exons in the genome. ii. Exons within a particular gene can duplicate or be deleted(just like genes themselves) 1. A duplicated exon would code for a copy of the protein containing a second copy of the encode domain. This could alter the proteins structure 2. The change in the protein’s structure could alter its function by increasing its stability and enhancing other properties like its ability to bind to a particular ligand a. Collagen-­‐ a structural protein with highly repetitive amino acid sequence and is reflected in the repetitive exons in the collagen gene iii. Exon shuffling: occasional mixing and matching of different exons either within a gene or between two nonallelic genes owing to errors in meiotic recombination. 1. Could lead to new proteins with novel combinations of functions a. Example: TPA-­‐ tissue plasminogen activator i. Extracellular protein which limits blood clotting-­‐ limits that damage that can result from heart attacks by slowing the clotting reaction ii. Had four domains of three types, each domain is encoded by an exon(one exon being present in two copies) iii. This gene for TPA is believed to have arisen by several instances of exon shuffling because some of the exons are found in other proteins f. How Transposable Elements Contribute to Genome Evolution: i. They can promote recombination ii. They can disrupt cellular genes or control elements iii. They can carry entire genes or individual exons to new locations