Chapter 19 Flashcards
Describe the structure of a nucleosome, the basic unit of DNA packing in eukaryotic cells.
A nucleosome is made up of eight histone proteins, two each of four different types, around which DNA is wound. Linker DNA runs from one nucleosome to the next one.
What chemical properties of histones and DNA enable these molecules to bind tightly together?
Histones contain many basic (positively charged) amino acids, such as lysine and arginine, which can form weak bonds with the negatively charged phosphate groups on the sugar-phosphate backbone of the DNA me sugar-phosphate backbone of the DNA molecule.
In general, how does dense packing of DNA in chromosomes prevent gene expression?
RNA polymerase and other proteins required for transcription do not have access to the DNA in tightly packed regions of a chromosome.
In general, what is the effect of histone acetylation and DNA methylation on gene expression?
Histone acetylation is generally associated with gene expression, while DNAV methylation is generally associated with lack of expression.
Compare the roles of general and specific transcription factors in regulating gene expression.
General transcription factors function in assembling the transcription initiation complex at the promoters for all genes. Specific transcription factors bind to control elements associated with a particular gene and,once bound, either increase (activators) or decrease (repressors) transcription of that gene.
If you compared the nucleotide sequences of the distal control elements in the enhancers of three coordinately regulated genes, what would you expect to find? Why?
The three genes should have some similar or identical sequences in the control elements of their enhancers. Because of this similarity, the same specific transcription factors could bind to the enhancers of all three genes and stimulate their expression coordinately.
Once mRNA encoding a particular protein reaches the cytoplasm, what are four mechanisms that can regulate the amount of the active protein in the cell?
Degradation of the mRNA, regulation of translation, activation of the protein (by chemical modification, for example), and Protein degradation.
Discuss the characteristics that make mammalian genomes larger than prokaryotic genomes.
The number of genes is 5-15 times higher in mammals, and the amount of non coding DNA is about 10,000 times greater. The presence of introns in mammalian genes makes them about 27 times longer, on average, than prokaryotic genes.
How do introns, transposable elements, and simple sequence DNA differ in their distribution in the genome?
Introns are interspersed within the coding sequences of genes. Many copies of each trans-posable element are scattered throughout the genome. Simple sequence DNA is concentrated at the centromeres and telomeres.
Discuss the differences in the organization of the rRNA gene family and the globin gene families. How do these gene families benefit the organism?
In the rRNA gene family, identical transcription units encoding three different RNA products are present in long, tandemly repeated arrays. The large number of copies of the rRNA genes enable organisms to produce the rRNA for enough ribosomes to carry out active protein synthesis. Each globin gene family consists of a relatively small number of nonidentical genes clustered near each other. The differences in the globin proteins encoded by these genes result in production of hemoglobin molecules adapted to particular developmental stages of the organism.
Describe three examples of errors in cellular processes that lead to DNA duplications.
If cytokinesis is faulty, two copies of the entire genome can end up in a single cell Errors in crossing over during meiosis can lead to one segment being duplicated while another is deleted. During DNA replication, slippage backward along the template strand can result n a duplication
What processes are thought to have led to the evolution of the globin gene families
Gene duplication and divergence by mutation. Movement of genes to different chromosomes also occurred.
Look at the portions of the fibronectin and EGF genes shown in Figure 19.20 (left). How might they have arisen?
For either gene, a mistake in crossing over during meiosis could have occurred between the two copies of that gene, such that one ended up with a duplicated exon. This could have happened several times, resulting in the multiple copies of a particular exon in each gene.
What are three ways transposable elements are thought to contribute to the evolution of the genome?
Homologous transposable elements scattered throughout the genome provide sites where recombination can occur between different chromosomes. Movement of these elements into coding or regulatory sequences may change expression of genes. Transposable elements also can carry genes with them, leading to dispersion of genes and in some cases different patterns of expression. Or transport of an exon during transposition and its insertion into a gene may add anew functional domain to the originally encoded protein, a type of exon shuffling.
In a nucleosome, the DNA is wrapped around
histones.
