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Molecular biology chapter 9


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[Front]


What is a chromosome?
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A discrete unit of the genome carrying many genes. Each consists of a very long molecule of duplex DNA and an approximately equal mass of proteins. It is visible as a morphological entity only during cell division.

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Molecular biology chapter 9 - Marcador

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Molecular biology chapter 9 - Detalles

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What is a chromosome?
A discrete unit of the genome carrying many genes. Each consists of a very long molecule of duplex DNA and an approximately equal mass of proteins. It is visible as a morphological entity only during cell division.
What is a nucleoid?
The structure in a prokaryotic cell that contains the genome. The DNA is bound to proteins and is not enclosed by a membrane.
What is the chromatin?
The state of nuclear DNA and its associated proteins during the interphase (between mitoses) of the eukaryotic cell cycle.
What is the packing ration?
The ratio of the length of DNA to the unit length of the fiber containing it.
What is a capsid?
The external protein coat of a virus particle.
What is icosahedral symmetry?
A structure with 60 rotational symmetries (such as a soccer ball).
How do filamentous RNA viruses assemble?
Condense the RNA genome as they assemble the headshell around it.
What is a nucleation center?
A duplex hairpin in TMV (tobacco mosaic virus) in which assembly of coat protein with RNA is initiated.
How do spherical DNA viruses assemble?
Insert the DNA into a preassembled protein shell.
What is terminase?
An enzyme that cleaves multimers of a viral genome and then uses hydrolysis of ATP to provide the energy to translocate the DNA into an empty viral capsid starting with the cleaved end.
What are the properties of the bacterial nucleoid?
The bacterial nucleoid is ~80% DNA by mass and can be unfolded by agents that act on RNA or protein. -The nucleoid has ~400 independent negatively supercoiled domains.
What does the bacterial genome consist of
The bacterial genome consists of a large number of loops of duplex DNA, each of which forms an independent structural domain.
Properties of interphase DNA in eukaryotes?
DNA of interphase chromatin is negatively supercoiled into independent domains of ~85 kb.
Properties of the metaphase chromosomes?
Metaphase chromosomes have a protein scaffold to which the loops of supercoiled DNA are attached.
What is MARs and SARS?
DNA is attached to the nuclear matrix at specific sequences called MARs or SARs. The MARs are A-T-rich but do not have any specific consensus sequence.
What is chromosome scaffold?
A proteinaceous structure in the shape of a sister chromatid pair, generated when chromosomes are depleted of histones.
What are the two types of chromatins?
Euchromatin and Heterochromatin
When can we see individual chromosomes?
Individual chromosomes can be seen only during mitosis.
What is euchromatin?
During interphase, the general mass of chromatin is in the form of euchromatin, which is slightly less tightly packed than mitotic chromosomes.
What is heterochromatin?
Tightly packed form of DNA (condensed) Regions of heterochromatin remain densely packed throughout interphase.
What is a chromocenter?
An aggregate of heterochromatin from different chromosomes.
Where does chromosomes sit in the nucleus?
Chromosomes occupy chromosome territories in the nucleus and are not entangled with each other.
What are G-bands?
Certain staining techniques cause the chromosomes to have the appearance of a series of striations, which are called G-bands.
What’s the difference between bands and interbands?
The bands are lower in GC content than the interbands.
Where in chromosomes bands are genes concentrated the most?
Genes are concentrated in the G-C-rich interbands.
What are chromomeres?
Densely staining granules visible in chromosomes under certain conditions, especially early in meiosis, when a chromosome may appear to consist of a series of chromomeres.
What are lampbrush chromosomes?
A lampbrush chromosome is a meiotic bivalent in which the two pairs of sister chromatids are held together at chiasmata.
How does sites of gene expression on lampbrush chromosomes look like?
Sites of gene expression on lampbrush chromosomes show loops that are extended from the chromosomal axis.
What is polytene chromosomes?
Giant chromosomes found in larvae of certain dipterans. They are the largest and most cytogenetically useful. have a series of bands that can be used as a cytological map.
What is in situ hybridization?
Hybridization performed by denaturing the DNA of cells squashed on a microscope slide so that reaction is possible with an added single-stranded RNA or DNA; the added preparation is radioactively labeled and its hybridization is followed by autoradiography.
What’s the relationship between polytene chromosomes and in situ hybridization?
Individual bands containing particular genes can be identified by in situ hybridization.
What are puffs?
Bands that are sites of gene expression on polytene chromosomes expand to give “puffs.”
How is eukaryotic chromosomes held on the mitosis spindle?
A eukaryotic chromosome is held on the mitotic spindle by the attachment of microtubules to the kinetochore that forms in its centromeric region.
What is the microtubule organizing center (MTOC) ?
A region from which microtubules emanate. In animal cells the centrosome is the major microtubule organizing center.
What is a centromere?
A constricted region of a chromosome that includes the site of attachment (the kinetochore) to the mitotic or meiotic spindle. It consists of unique DNA sequences and proteins not found anywhere else in the chromosome.
What is acentric fragment ?
A fragment of a chromosome (generated by breakage) that lacks a centromere and is lost at cell division.
How are centromeres characterized?
Centromeres are characterized by a centromere-specific histone H3 variant, and often have heterochromatin that is rich in satellite DNA sequences.
What is satellite DNA?
Consists of a very large arrays of tendemly repeating, non coding DNA Is the main component of functional centromeres and form the main structural constitute of heterochromatin.
What makes the centromeres different in higher eukaryotic chromosomes?
Centromeres in higher eukaryotic chromosomes contain large amounts of repetitive DNA and unique histone variants.
What are telomeres?
A region of repetitive nucleotide sequences at each end of a chromosome. A typical telomere has a simple repeating structure with a G-T-rich strand that extends beyond the C-A-rich strand.
Why are telomeres important?
The telomere is required for the stability of the chromosome end. Telomeres Seal the Chromosome Ends and Function in Meiotic Chromosome Pairing
What does telomeres consist of?
A telomere consists of a simple repeat where a G-rich strand at the 3ʹ terminus typically has a sequence of (T/A)1–4 G>2.
What happens in the reaction catalyzed by the protein TRF2 in the telomeres?
The protein TRF2 catalyzes a reaction in which the 3′ repeating unit of the G+T-rich strand forms a loop by displacing its homolog in an upstream region of the telomere.
How can we visualize the telomeres cluster?
(FISH) fluorescence in situ hybridization
What is telomerase?
A Ribonucleoprotein Enzyme that adds as pieces dependent telomere repeat sequence to the 3’ end of telomeres.
How does telomerase work?
Telomerase uses the 3′–OH of the G+T telomeric strand to prime synthesis of tandem TTGGGG repeats. The RNA component of telomerase has a sequence that pairs with the C+A-rich repeats. One of the protein subunits is a reverse transcriptase that uses the RNA as template to synthesis the G+T-rich sequence. -Telomerase positions itself by base pairing between the RNA template and the protruding single-stranded DNA primer.
Where is telomerase expressed?
Telomerase is expressed in actively dividing cells and is not expressed in quiescent cells.
What happens if telomeres are lost?
Loss of telomeres results in senescence.
How senescence is escaped?
Escape from senescence can occur if telomerase is reactivated, or via unequal homologous recombination to restore telomeres.
What is the result of Telomerase mutations?
Mutation in telomerase causes telomeres to shorten in each cell division.
Can crossing over happen in telomeres?
Crossing-over in telomeric regions is usually suppressed by mismatch-repair systems, but can occur when they are mutated.