exam 3: the eukaryoric chromosome Flashcards
**eukaryotic chromosomes vs prokaryotic chromosomes
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
what is a chromosome?
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
Chromosomal proteins: non-histones & functions
make up other half of chromatin protein by weight, function varies
- structure: forms scaffold of chromosome (responsible for condensing DNA)
- segregation: motor proteins of kinetochores (found near centromere)
- 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)
Chromosomal proteins: histones
-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
chromosomal proteins: histones vs nonhistones
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
chromosome structure: nucleocome
-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
what are 2 types of model for DNA compaction?
1) supercoiling
2) radial loop scaffold model
DNA compaction: supercoiling
nucleosomes compacted into supercoiled helix (linker DNA get twisted pulls nucleosomes tgt)
DNA compaction: Radial loop scaffold model
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
how do chromosomes get their patterns? *colors
-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
what can G bands do?
-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
in addtion to finding common ancestor, what can g bands be used for
to study disease. chromosomal abnormalities
-aneuploidies (wrong # of chromosomes)
- translocations (combination where a piece of chromosome break off and attach to another)
-insertions
-deletions
how are chromosome replicated and moved in a cell
-origins of replication (site where DNA replication begins)
-telemeres (protective caps at the end of chromosomes)
-centromeresi
in eukaryotes, why are there 10000 of origins of replications?
-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
chromosomal replication: how are chromosomes being replicated?
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
what are telemeres? + functions
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
what happens to telomeres after replication
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
shortened telomeres: what happens when telomeres are too short?
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
telomeres are highly repetitive & concerved. sequence is same among mammals, birds, reptiles, etc. what does this tell u abt the importance of telomeres
essential for life
if mutation occurs, organisms die and do not reproduce
how do chromosomes segregate after replication?
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)
- what are the 2 functions of centromeres that ensure proper segragation?
1) hold sister chromatids tgt until anaphase (mitosis)/ anaphase II (meiosis)
2) site of kinetochore formation where spindle fibers attach during mitosis & meiosis
what affect gene expression
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
what are cool example of gene expression
1) positional effect variegation in drosophila
2) X chromosome inactivation in mammalian females
postitional effect variegation
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
Inactivatio of X chromosome
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)
when does inactivation of X occurs
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
