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Genetics > Chromosome Structure and Transposons (Lec 13) > Flashcards

Chromosome Structure and Transposons (Lec 13) Flashcards

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1
Q

Postive supercoiling occurs when DNA is ___ rotated, while negative supercoiling occurs when DNA is ___rotated

A

over; under

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2
Q

What is supercoiling?

A

the over-winding or under-winding of DNA causing it to twist on itself

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3
Q

supercoiling occurs when the strain of over rotation or under rotation cannot be compensated for by what?

A

turning of the free ends of the DNA molecule

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4
Q

In eukaryotes, the linear DNA molecules are often folded into ___ which are themselves stabilized by proteins resulting in supercoiling

A

loops

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5
Q

What are topoisomerases? What do they do?

A

enzymes that add or remove rotations from DNA;

can temporarily break nucleotide strands, rotate ends around each other, rejoin broken ends

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6
Q

True or false?

Topoisomerase’s can induce and relieve supercoiling

A

true

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7
Q

What advantage does negative supercoils DNA have over nonsupercoiled DNA?

A

makes separation of the two strands easier during replication and transcription; allows DNA to be packed into smaller space compared to relaxed DNA

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8
Q

True or false?

most DNA is positively supercoiled

A

false; negatively supercoiled

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9
Q

A combination of DNA and proteins is called what?

A

chromatin

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10
Q

True or false?

Euchromatin undergoes the normal process of condensation and decondensation in the cell cycle

A

true

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11
Q

Heterochromatin remains in what type of state throughout the cell cycle?

A

highly condensed

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12
Q

What is a nucleosome and what are its components?

A

core particle; consists of DNA + 8 histone proteins

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13
Q

Which histones form the nucleosome?

A

H2A, H2B, H3, H4 (2 each)

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14
Q

What is the function of the histone, H1?

A

attaches DNA to nucleosome

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15
Q

Nucleosome + H1 = ?

A

chromatosome

note: consists of 167 bp of DNA

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16
Q

What are chromatosomes separated by?

A

linker DNA (about 30-40 bp)

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17
Q

What is the name of the enzyme that cleaves the linker DNA (string) from the core particles (beads)?

A

nuclease

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18
Q

In regards to chromatin structure, how is a polytene chromosome created?

A

by repeated rounds of DNA replication with no cell division

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19
Q

DNase I sensitivity correlates with what?

A

gene activity

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20
Q

In regards to chromatin structure, what do epigenetic changes consist of?

A

methylation

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21
Q

Does chromatin structure change during transcription?

A

yes

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22
Q

True or false?

epigenetic change causes inherited phenotypic change without affecting the DNA sequence

A

true

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23
Q

What are centromeric sequences?

A

binding sites for the kinetochore, where spindle fibers attach

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24
Q

Centromeres are mostly heterochromatic, and probably defined by ___ changes rater than ____ ____

A

epigenetic; DNA sequnces

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25
Q

What role does CenH3 play in the organization of a centromere?

A

replaces H3 in most eukaryote centromeres

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26
Q

Centromeres signal what?

A

the beginning of anaphase

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27
Q

What is the function of telomeres?

A

stabilize ends of chromosomes

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28
Q

Describe telomeric sequences. How are they oriented?

A

usually consist of a repeated sequence of A or T nucleotides followed by several G nucleotides

string of G and C sequence is always oriented away from the centromere

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29
Q

What is the relationship of shelterin to telomeres?

A

protects ends of mammalian chromosomes from being repaired as double-stranded breaks

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30
Q

How are Het-A and Tart related to Drosophila telomeres?

A

inserted transposable elements arranged in tandem repeats

balance loss of telomeric sequences during replication

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31
Q

True or false?

Organisms differ in amount of DNA per cell

A

true

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32
Q

Define gene family

A

similar but not identical copies of unique DNA sequences that arose through duplication of an existing gene

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33
Q

Describe moderately repetitive DNA

A

150-350 bp long and thousands to 100s of thousands of repeated units; tandem repeat sequences; interspersed repeat sequences

34
Q

Describe highly repetitive DNA

A

less than 10 bp long and repeated 100s of thousands to millions of times

35
Q

In regards to DNA sequences, what does C value refer to?

A

the amount of DNA per cell in a given organism

36
Q

What does unique-sequence DNA consist of?

A

DNA sequences that are present once or only a few times in the genome

37
Q

In regards to unique-sequence DNA, genes present in a single copy represent what percentage of protein-encoding genes in most multicellular organism?

A

25%-50%

38
Q

Repetitive DNA makes up about how much of the human genome?

A

half

39
Q

___ ____ sequences are arranged in tandem and clustered at particular locations on the chromosomes

A

tandem repeat

40
Q

Interspersed repeat sequences are scattered through the genome and most are ____ elements

A

transposable

41
Q

Long interspersed sequences comprise about how much of the human genome?

A

17%

42
Q

Highly repetitive DNA is often called what?

A

satellite DNA

43
Q

Transposons make up at least how much of human DNA?

A

45%

44
Q

Most transposons are able to insert at many different locations and rely on mechanisms distinct from what?

A

homologous recombination

45
Q

True or false?

Transposons often cause mutations

A

true

46
Q

Short flanking repeats from 3 to 12 bp long are present on both sides of the transposable elements. Do these repeats travel with the transposable element? Where are they regenerated?

A

Do not travel with transposable element; regenerated at the point of insertion

47
Q

Does replication of single stranded or double stranded DNA create the flanking direct repeats?

A

single stranded

48
Q

Terminal, inverted repeats are found at ends of many transposable elements. How long are these repeats?

A

9 to 40 bp

49
Q

The terminal inverted repeats found at the ends of many transposable elements are recognized by what?

A

enzymes that catalyze transposition

50
Q

Are the terminal inverted repeats found at the ends of many transposable elements required for transposition to take place?

A

yes

51
Q

What are the two ways transposons cause mutations?

A

Inserting into another gene; promoting DNA rearrangements

52
Q

Describe the mechanism of transposition

A

staggered breaks are made in the target DNA; transposable element is joined to single-stranded ends of the target DNA; DNA is replicated at the single-strand gaps

53
Q

Why is replicative transposition process referred to as “copy and paste” transposition?

A

A new copy of the transposable element is introduced at the new site while the old copy remains at the original site; results in increase in the number of copies of the transposable element; can be either between two different DNA molecules or between two parts of the same DNA molecule

54
Q

The following are needed for what event to occur:
Single copy of the transposable ; two DNA molecules are joined; transposable element is replicated; production of cointegrate structure; crossing over = resolution of the cointegrate

A

cointegrateformation

55
Q

What two enzymes are involved in replicative transposition?

A

transposase; resolvase

56
Q

This enzyme involved in replicative transposition may be encoded by cellular gene or often by transposable element and makes single-strand breaks at each end of the transposable element and on either side of the target sequence where insertion occurs

A

transposase

57
Q

This enzyme involved in replicative transposition may be encoded by either cellular genes or by transposable element and is responsible for resolution

A

resolvase

58
Q

Why is non-replicative transposition referred to as “cut and paste” transposition?

A

transposable element is excised from old site and inserted into a new site; does not increase number of copies

59
Q

Describe how non-replicative transposition works

A

cleavage requires a transposes enzyme produced by the transposable element; joining of the transposable element is carried out by normal replication and repair enzymes; original site is typically repaired using homologous template on sister chromatid

60
Q

Elements that transpose through an RNA intermediate are called what?

A

retrotransposons

61
Q

How is RNA copied back to another DNA site by a retrotransposons?

A

reverse transcriptase

62
Q

True or false?

Retrotransposons only use non-replicative transposition

A

fase; replicative

63
Q

True or false?

Only DNA transposons are found in bacteria

A

true

64
Q

What are the two transposons found in bacteria?

A

insertion sequences; composite transposons

65
Q

This type of DNA transposon found in bacteria, also infects plasmids and viruses, carries the only information required for movement, and typically includes one or two genes that encode transposase

A

insertion sequence

66
Q

This type of DNA transposon found in bacteria, is more complex DNA segments flanked by two copies of an insertion sequence, contains DNA sequences not directly related to transposition, and designated by Tn followed by a number

A

composite transposons

67
Q

What are the two groups of eukaryotic transposons?

A

structurally similar to transposable elements found in bacteria; retrotransposons

68
Q

How much of the human genome do SINEs constitute?

A

11%

69
Q

How much of the human genome do LINEs constitute?

A

21% approx 900,000 copies

70
Q

True or false?

One of every mutations that cause significant disease results from the transposition of a SINE or LINE

A

true

71
Q

Do transposable elements provide benefit to the cell?

A

no

72
Q

What are some of the reasons that transposable elements exist?

A

may generate genetic diversity; may exist because of their relationship to telomeres

73
Q

In regards to chromosomal rearrangements, pairing by looping and crossing over between two transposable elements oriented in the same direction leads to what?

A

deletion

74
Q

In regards to chromosomal rearrangements, pairing by bending and crossing over between two transposable elements oriented in opposite directions leads to what?

A

inversion

75
Q

In regards to chromosomal rearrangements, misalignment and unequal exchange between transposable elements located on sister chromatids leads to what?

A

one chromosome with a deletion and one chromosome with a duplication

76
Q

Describe the structure of a class I transposable element

A

long terminal direct repeats, short flanking direct repeats at target site

77
Q

Describe the structure of class II transposable elements

A

short terminal inverted repeats; short flanking direct repeats at target site

78
Q

What genes do class I transposable elements encode for?

A

reverse transcriptase

79
Q

What genes do class II transposable elements encode for?

A

transposase

80
Q

Class I transposable elements execute transposition by RNA or DNA?

A

RNA intermediate

81
Q

Class II transposable elements execute transposition by RNA or DNA?

A

DNA

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