Genomes and Chromosomes: Part Two
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Individual metaphase chromosomes have distinct morphologies
Typical appearance of a metaphase chromosome: __
A meta-phase chromosome therefore contains __
two chromosomes joined together because by the stage of the cell cycle when chromosomes condense and become visible by light microscopy, DNA replication has already occurred.
two daughter chromosomes linked together at their centromeres
the position at which the chromosome attaches to the microtubules that draw the daughters into their respective nuclei during cell division
The position of the centromere is characteristic for a particular chromosome
- Can be in the middle (metacentric)
- Can be a little off center (submetacentric)
- Can be toward one end of the chromosome (acrocentric)
- Can be located very close to one end (telocentric)
The regions on either side of the centromere are called the __
The differing positions of the centromere mean that __. The set of chromosomes possessed by an organism can therefore be represented as a __, in which the banded appearance of each one is depicted.
chromosome arms or chromatids
individual chromosomes can be recognized
Centromeres contain __ and __
DNA is present along the __ including __, which is made up largely of repeat sequences. In humans, these repeats are 171 bp in length and are called __. There are 1500 to 30,000 copies of the __ per chromosome, spanning a region of between 0.9 and 5.2 Mb.
repetitive DNA and modified nucleosomes
entire length of a chromosome
within the centromeres
alphoid DNA x2
A few genes are also present in the __. The frequency is usually less than 100 genes per Mb of DNA
The centromeres of most eukaryotes also contain __, similar to those in other regions of the chromosomes but some of them containing the protein __ instead of __.
Explain CENP-A containing nucleosomes .
They are more compact and structurally rigid than those containing H3.
the arrangement of CENP-A and H3 nucleosomes along the DNA is such that the CENP-A versions are located on the surface of the centromere. Here they form an outer shell on which the kinetochores are constructed
What are kinetochores?
These are the structures that act as the attachment points for the microtubules which draw the divided chromosomes into the daughter nuclei
Kinetochores form on the metaphase chromosomes, each of the daughter chromosomes constructing its own kinetochore on the conjoined centromere
One kinetochore is on one side of the centromere, and one on the other
__ protect chromosome ends
The ends of a chromosome are called the __, which are specialized structures with a number of important roles.
What are the roles?
Protect the ends of the DNA molecule contained within the chromosome from attack by exonuclease enzymes
If the enzymes gain access to the chromosomal DNA, what happens?
The natural ends also need protection from __. If these systems recognize a natural chromosome as a break then they might __.
then they will degrade some of the terminal region
DNA repair systems
join together two chromosomes
Telomeres contain special DNA sequences. Telomeric DNA is made up of hundreds of copies of a short sequence, __ in humans. On the other strand, the sequence is __, which means that one of the strands in the telomeric region is rich in __ while the other is rich in __.
Though this is true for all eukaryotes, the actual sequence varies.
The G-rich strand, which contributes the __, does what?
3’ end of the DNA
extends for up to 200 nucleotides beyond the terminus of the C-rich strand, giving a single-stranded overhang
__ are not present in the telomeric regions of chromosomes. Instead, two special proteins bind to the repeat sequences. In humans, these are called __, which helps to __, and __, which __
- regulate the length of the telomere
- maintains the signle-strand extension
If TRF2 is inactivated then what happens?
Other telomeric proteins are thought to do what?
this extension is lost and the two polynucleotides fuse together in a covalent linkage.
form a linkage between the telomere and the periphery of the nucleus, the area in which the chromosome ends are localized
Chromosomes should get progressively shorter during multiple rounds of replication.
Telomeres must also overcome a problem posed by DNA replication. In theory, every round of DNA replication should result in __
a slight decrease in the length of a chromosome, something that does not happen in practice.
Why do they get smaller?
DNA synthesis is 5’à3’, meaning the lagging strand is synthesized discontinuously, as Okazaki fragments.
This presents two problems at the ends of a linear DNA molecule. What is the first?
The extreme 3’ end of the lagging strand might not be copied because the final Okazaki fragment cannot be primed, the natural position for the priming site being beyond the end of the lagging strand.
The absence of this Okazaki fragment means that the lagging-strand copy is shorter than it should be. If the copy remains this length, then when it acts as a parent polynucleotide in the next round of replication, the resulting daughter molecule will be shorter than its grandparent
This presents two problems at the ends of a linear DNA molecule. What is the second?
A second problem arises if the primer for the last Okazaki fragment is placed at the extreme 3’-end of the lagging strand. Shortening will still occur, although to a lesser extent, because this terminal RNA primer cannot be converted into DNA by the standard processes for primer removal. This is because the methods for primer removal require extension of the 3’ end of an adjacent Okazaki fragment, which cannot occur at the very end of the molecule because the necessary Okazaki fragment is absent.
Telomeres are designed to solve these problems in the following way:
Most of the telomeric DNA is copied in the normal fashion during DNA , but this is not the only way in which it can be synthesized. To compensate for the limitations of the replication process, telomeres can be extended by an independent mechanism catalyzed by telomerase
This enzyme consists of both __ and __. In the human enzyme the __ is __nucleotides in length and contains near its 5’ end the sequence 5’-CUAACCCUAAC-3’. The central region of this sequence is the __
- protein and RNA
- RNA component
- reverse complement of the human telomere repeat 5’-TTAGGG’-3’.
Telomerase RNA can do what?
base-pair to single-stranded DNA overhands that are present at the end of the telomeres
Base pairing provides a __that enables the 3’ end of the DNA to be __. The telomerase RNA then does what?
The process can be repeated until the chromosome end has been extended by a sufficient amount.
- extended by a few nucleotides
- translocates to a new base-pairing position slightly further along the DNA polynucleotide and the molecule is extended by a few more nucleotides
Telomerase can add nucleotides to the end of only the __. It is not clear how the other polynucleotide-__- is extended, but it is presumed that when the__ is long enough, the __ attaches to the end of the __and does what?
- G-rich strand
- the C-rich strand
- G-rich strand
- primase-DNA polymerase alpha complex
- C-rich strand
- initiates synthesis of a new Okazaki fragment
This requires the use of a new __, which explains why the __. The important point is that the __
- RNA primer
- C-rich strand is always shorter than the G-rich one
- overall length of the chromosomal DNA has not been reduced
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