exam 3: the eukaryoric chromosome

Question 1

**eukaryotic chromosomes vs prokaryotic chromosomes

Answer 1

eukaryotic:
-multiple chromosomes
-linear chromosome (more non-coding DNA)
-complexed with a lot of protein
many origins of replication

prokaryotic:
-one chromosome
-circular chromosome
-complexed with very little protein
-one origin of replication

Question 2

what is a chromosome?

Answer 2

an organelle (not membrane bound) that contains chromatin (dna+protein)

eukaryotic chromosome:
made of 1 long linear molecule of DNA complexed with
1) histones ( primary level of DNA organization)
2) non histones
3)coding (extrons)
4) non-coding [introns, promoters (promote transcription where RNA will bind) , control elements (control transcription) , telomeres, centromeres, repetitive DNA)]regions

Question 3

Chromosomal proteins: non-histones & functions

Answer 3

make up other half of chromatin protein by weight, function varies

1. structure: forms scaffold of chromosome (responsible for condensing DNA)
2. segregation: motor proteins of kinetochores (found near centromere)
3. transcription regulation and RNA processing
   -interact with DNA to affect when, where, and at what rate genes are transcribed
   (help+ how much DNA is needed to transcribe with hoe a coding sequence is being transcribed into a coding protein)

Question 3

Chromosomal proteins: histones

Answer 3

-primary level of protein organization
-+ charged proteins that make up half of all chromatin protein by weight

Q: why pos charged?
A: DNA is neg charged-favored by natural selection

Q: what are the five types?
A: H1, H2A, H2B, H3, and H4
*H2A, H2B, H3, and H4 -form core of proteins in DNA packaging)
-total of 8: 2 molecules of each type
-DNA also wrap around 2 times
*H1 links DNA to core (sits outside: 1 molecule)

+histone amino acid sequence higly conserved among diverse organisms…what does this tell you abt the importance of histones?
A: essential for life since it has not experience change since organism with mutations will not be passed on

Question 4

chromosomal proteins: histones vs nonhistones

Answer 4

histones: fairly uniform in distribution among different cell types (cant tell them apart: bran vs liver)

nonhistones: not uniform in distribution (can tell 1 cell from the next)

Q: why the differences?
-histone: responsible for DNA packaging and compaction
-nonhistones: vary in function, many functions related to gene expression in which will vary among cell types

Question 5

chromosome structure: nucleocome

Answer 5

-DNA is compacted to fit in the nucleus (non compacted DNA is 2m long)

nucleosome: fundamental unit of chromosome packaging
(name that make up of 8 histones wrapping + 1 histone outside)
-DNA wraps around core of 8 histones
-H1 lies outside core, links DNA to core
-DNA wraps around twice
-an additional 40-50 bp of linker DNA connects 1 mucleosome to the next

Question 6

what are 2 types of model for DNA compaction?

Answer 6

1) supercoiling
2) radial loop scaffold model

Question 7

DNA compaction: supercoiling

Answer 7

nucleosomes compacted into supercoiled helix (linker DNA get twisted pulls nucleosomes tgt)

Question 8

DNA compaction: Radial loop scaffold model

Answer 8

non-histoe proteins tether (tie) supercoiled DNA into loops (during mitosis/ meiosis)

-complex of non-histone protein (condensins: compact DNA even more) during chromosome segregation in mitosis/meiosis

-condensins gather loops into rosettes (forming central points of attachment)
-rosettes packed into bundles

Question 9

how do chromosomes get their patterns? *colors

Answer 9

-chromosome means colored body

-they absorb diff dyes in variable amts along their length–> produced banding patterns

*Bands (geisma bands: g bands) reproducible from one cell to the next, even after mitosis-used to identify specific chromsomes because pattern is unique for each chromosome

*banding pattern intrinsic protperty of chromosomes

on a probe: diff part of chromosme will take up different dyes- get these patterns due to proteins+ tightness

Question 10

what can G bands do?

Answer 10

-same for specific chromosomes within a species, but may differ between species

*can be used to identify species/ establish evolutionary relationship between species
–> common ancestor: common banding pattern

EX: humans and chimps
-banding pattern of chromosome 2 is similar between humans and chimp
-appears that 2 smaller
homologous chromosomes were from a common ancestor and fused to form 1 chromosome in humans

Question 11

in addtion to finding common ancestor, what can g bands be used for

Answer 11

to study disease. chromosomal abnormalities
-aneuploidies (wrong # of chromosomes)
- translocations (combination where a piece of chromosome break off and attach to another)
-insertions
-deletions

Question 12

how are chromosome replicated and moved in a cell

Answer 12

-origins of replication (site where DNA replication begins)
-telemeres (protective caps at the end of chromosomes)
-centromeresi

Question 13

in eukaryotes, why are there 10000 of origins of replications?

Answer 13

-it would take too long to copy all of DNA if there was only 1 origin per chromosome

ex: in humans, DNA polymerase (highly efficient) copes abt 50 bp/sec; would take about 1 month to copy DNA in 1 nucleus if theres only 1 origin

Question 14

chromosomal replication: how are chromosomes being replicated?

Answer 14

RECALL: dna replication occurs in opposite direction on both strands (5’ to 3’)
why? dna is antiparallel

-origin forms bubbles after replication has begun (fork: at the ends )
-replication continues on both strands until one bubble runs into an adjacent bubble
Q: why do origin dont have nucleosomes?
A: protein is large; would block enzymes/ proteins from replication machinary
-after DNA is copied, it is immediately complexed with histone and non histone proteins

Question 15

what are telemeres? + functions

Answer 15

highly repeatitive

function: protect ends of chromosomes from degradation & prevent chromosomes from fusing
Q: remove telemeres and chromosomes fuse yielding strcutres with 2 centromeres: why is this bad?
A: recombination will be difficult, sincen complementary sequences will not segregate equally

Question 16

what happens to telomeres after replication

Answer 16

every time DNA replicatio occurs, telomeres get shorter
(longer u live, the more DNA replication, the shorter telomeres become)

-DNA polymerase can only add to the 3’ end of the growing nucleotide chain
-when primer (made of RNA; unstable ;short ss nucleic acid) at end of chromosome removed after replication, no 3’ end to add onto

Question 17

shortened telomeres: what happens when telomeres are too short?

Answer 17

depends on cell types
-most cells go thru apoptosis and die
-some cells have telomerase: RNA/protein enzyme that helps lengthen telomeres
–> create extension of telomeres to create new place for primer to attach’ DNA polymerase fills in gap using 3’ end of primer
—> most somative cells do not have active telomerase gene
Q: what type does that should not? cancer cells

*does not extend @ 5’ of short strand ; instead extend 3’ of oppsote strand -allow primers to come in and make it long engouh physically for primers to function

Question 18

telomeres are highly repetitive & concerved. sequence is same among mammals, birds, reptiles, etc. what does this tell u abt the importance of telomeres

Answer 18

essential for life
if mutation occurs, organisms die and do not reproduce

Question 19

how do chromosomes segregate after replication?

Answer 19

centromeres (non-coding)
-in regions of highly repetitive DNA
-can occur anywhere along chromosome except at ends
Q: why?
A: telomeres are already there (no telomeres-> cant survive)

Question 20

• what are the 2 functions of centromeres that ensure proper segragation?

Answer 20

1) hold sister chromatids tgt until anaphase (mitosis)/ anaphase II (meiosis)

2) site of kinetochore formation where spindle fibers attach during mitosis & meiosis

Question 21

what affect gene expression

Answer 21

chromosomal structures and DNA packaging

-heterochromatin:
some regions of DNA are more highly condensed than others; DNA is unavailable for transcription

-euchromatin:
less condensed; DNA is available for transcription

Question 22

what are cool example of gene expression

Answer 22

1) positional effect variegation in drosophila
2) X chromosome inactivation in mammalian females

Question 23

postitional effect variegation

Answer 23

variegation: patches of color

if a gene is translocated/ inverted, this will make the gene move from an area of euchromatin to heterochromatin (gene will not be expressed when in or near heterochromatin)

ex: if eye color gene in red eyes fly moved to area of heterochromatin, wont be expressed, fly will have white eyes
-can have variegated eyes: w+ allele expressed in some but not all

Question 24

Inactivatio of X chromosome

Answer 24

males have 1X; females have 2

females only need to express gene on 1 X, so they will inactivate 1X in evey cell= dosage compensation

barr body: inactive X is mass of heterochromatin
Q: if a male w anueploid with XXY, how many bar bodies?
A: 1

XY male: no barr body

*# of X chromosomes - 1 (XXXXX-1= 4 barr bodies)

Question 25

when does inactivation of X occurs

Answer 25

during embryonic development
-inactivated at random
—> females have abt 1/2 cells with X from mom inactivated & abt 1/2 cells of X from dad inactivated=> gene mosaics
==> This is why heterozygous females with 1mutated X can still appear phenotypically normal

how many X chromosomes if given # of barr bodies?
3 barr bodies-> 4 X chromosomes