Lecture 5 Flashcards
DNA transposons are also powerful mutagens, T or F
T
What specialised DNA polymerase enzyme is responsible for replication of telomeres
Telomerase
What are the four different specialised sequences contained within chromosomes
Telomeres, centromeres, replication origins and kinetochores
What happens to the original site from which the DNA transposon was initially excised from
It is repaired by DNA repair mechanisms
Not all of the genetic information in eukaryotes is encoded in the nucleus, where else is some genetic information stored and how
Some genetic information is contained in the mitochondria and chloroplasts in the form of small circular chromosomes
Each de-condensed chromosome occupies a specific region in the interphase nuclei, explain this phenomenon
As genes are transcribed the relative position of the chromosome in the nucleus changes. At interphase transcriptionally inactive regions of/chromosomes become localised at the periphery of the nucleus. In contrast, transcriptional activation of a gene is accompanied by movement of the gene towards the centre of the nucleus
Explain the role of linker histones such as H1
Linker histones act as straps that connect the incoming and outgoing strands of DNA that wrap around the core nucleosome. This helps to stabilise the formation of the 30nm chromatin fibres
Retrotransposons also move throughout the genome/chromosomes, T or F
F – retrotransposons never move. They act through RT converting RNA back to dsDNA at random points in the genome
Define what is meant by a kinetochore
Protein complex that binds to the microtubules in the mitotic spindle
Who discovered chromosomes in 1902
Sutton and Boveri
What are the two constituent parts of each nuclear chromosome
Linear DNA molecule and proteins that confer specialised functions called chromatin
Give an example of a specialised histone that mediates the attachment of the chromosome to the kinetochore inner plate
CENP-A
During which specific stage of the cell cycle can chromosomes easily be distinguished
Metaphase of mitosis
Telomeres define chromosome ends and maintain chromosome integrity, T or F
T
Explain how the chromatin is indirectly linked to the microtubules during chromosome segregation
The CENP-A histone containing chromatin physically binds to the kinetochore inner plate. An interaction between the kinetochore inner and outer plates links this chromatin to the microtubules of the mitotic spindle
What are alpha-satellite DNA repeats that are found within centromeres
Alpha-satellite DNA repeat sequences are repeat sequence elements around 170bps that act as specific binding sites for a set of specialised histones localised to centrometic sequences. These act as target interaction proteins for kinetochores
Which type of transposons make up the majority of transposons
Retroviral retrotransposons
How do retroviral retrotransposons act
Function via the production of an RNA intermediate
What is the main benefit of carrying mobile genetic sequence information around
Acts as a genome shuffling mechanism that breaks up and reassembles the genome, providing new combinations of DNA sequences and facilitating rapid genomic evolution
What is significant about the N-terminals of core histones
These project out from the nucleosome core and are free to interact with other proteins. These tails are rich in lysine residues and facilitate regulation of chromatin structure and function. They interact with proteins that effect the ability of the chromatin to be de-condensed, re-condensed and transcribed
The organised representation of all of the chromosomes in a eukaryote at metaphase is canned the karyotype, T or F
T
What is meant by the term transposon
Mobile genetic elements that can replicate themselves and jump around the genome
DNA transposons make up the largest proportion of transposable DNA elements in the human genome, T or F
F – they only account for 5%
What are the five main functions of chromatin
Packaging and unfolding DNA in the nucleus, control of DNA replication, repair and recombination, maintenance of chromosome integrity, governing chromosome segregation and the regulation of gene expression
Describe the structure/composition and role of centromeres
Higher order repeat sequences that contain subsets of repeat sequences called alpha-satellites. Centromeres facilitate chromosome segregation during cell division
Retrotransposons are abundant in invertebrate genomes, T or F
F – they are abundant in vertebrates
What are the three types of transposons
DNA Transposons, Retroviral retrotransposons, Non-retroviral PolyA retrotransposons
Structurally and in terms of proteins encoded by them, how are retrotransposons similar to retroviruses and what is a significant difference
Retrotransposons contain DNA sequences that code for envelope and capsid proteins just like retroviruses do. However, these proteins are usually defective but without preventing the replication capability of the retrotransposon
Of what order can the telomeric repeat sequences at the ends of chromosomes be
Hundreds
How do retrotransposons act in a similar way to retroviruses
They replicate via RNA intermediates and produce new DNA copies that integrate at new genomic locations. Retrotransposons self-encode the reverse transcriptase enzyme required for this mechanism
Which technique is useful for visualising chromosomes in interphase nuclei
Chromosome painting
Chromatin is a protein, T or F
T
Describe the composition of the core nucleosomes which the DNA sequence wraps around
Core nucleosomes are proteins that consist of 8 distinct subunits called histones. The DNA sequence wraps twice around each core nucleosome
What happens as a result of selective removal of linker histone H1 from the core nucleosome
Causes de-condensation of the chromatin during interphase that allows gene transcription
The process of DNA transposon movement throughout the genome is also referred to as non-replicative transposition, T or F
T
Which specialised sequence within chromosomes is the DNA sequence located at the ends of linear chromosomes and maintain chromosomal integrity
Telomeres
What is meant by PolyA
The RNA transcript produced by the non-retroviral retrotransposon is poly-adenylated
Describe the structure of the 10nm chromatin fibre
Single strand of DNA coiled around core nucleosomes
What features define transposons and are the site for enzyme binding
Short simple sequence repeats that lie at each end of the transposon where transposase binds
Alu and B1 are human non-retroviral retrotransposons, explain what is significant about these two transposable DNA sequences
They evolved from the same sequence element – 90million years ago. Alu and B1 originate from a single copy of the 7SL RNA gene
What do telomeres consist of
Hexanucleotide repeat sequences that create a 3’ overhang in the linear chromosome
More complex organisms have more protein coding genes and less non-protein-coding DNA, T or F
F – they contain more protein and non-protein encoding genes
What is significant about the enzyme required by DNA transposons to move throughout the chromosome/genome
Transposase is encoded by the DNA transposon itself
How do DNA transposons act
Transposase binds to the short simple repeat sequences that flank the transposon itself. These enzymes then induce double-stranded breaks in the DNA sequence to excise the gene from its initial location. This mobile intermediate produced is then able to move to a new location in the chromosome/genome and insert at a random locus
What mechanism do DNA transposons move by
Cut-and-paste – without self-duplication
Which specialised sequence within chromosomes is the DNA sequence at which the kinetochore assembles to mediate chromosome segregation at mitosis and meiosis
Centromeres
Non-retroviral retrotransposons have expanded hugely in numbers during evolution of higher mammal genomes, T or F
T
Which specialised sequence within chromosomes is the DNA sequence at which DNA replication is initiated
Replication origin
What enzyme is required by DNA transposons in order to move throughout the genome in the way that they do
Transposase
Describe the structure of the 30nm chromatin fibre
Causes by a supercoiling of the 10nm chromatin fibre into a superhelix
Recall the specific DNA sequence found in telomeres
TTAGGG
Give some examples of transposable DNA elements
Ac-Dc in maize, P-elements from flies and Tn3/Tn10 from E.coli
Explain why non-retroviral PolyA retrotransposons are referred to as stripped down retrotransposons
They lack the envelope and capsid proteins encoded within the retrotransposon sequence
The kinetochores consist of out and inner plate proteins, explain the different roles of these constituent parts
The kinetochore inner plates bind to alpha-satellite DNA and specific centromeric histones. The kinetochore outer plate proteins bind to protein components of the mitotic spindles such as microtubules
Explain how the reverse transcriptase self-encoded by non-retroviral PolyA retrotransposons also acts as an endonuclease
As well as producing the new DNA copy of the RNA intermediate created from the non-retroviral retrotransposon, the RT self-encoded by it alsi creates a nick in the target DNA. This allows for the physical insertion of the newly synthesised DNA copy of the PolyA transcript into this cleavage site
