LECTURE 3: Chromosome Structure Flashcards
Describe the concept of ploidy
a. List which cell types are haploid and diploid
indicates the number of sets of chromosomes in a cell.
Most animals are diploid – have two copies of each chromosome, one from the mother & one from father
somatic cells are diploid
Gametes are produced by meiosis and are haploid – have only one set
Describe how aneuploidy occurs, and why it can be bad for a cell or organism
Aneuploydy = abnormal number of chromosomes
This occurs through nondisjunction during meiosis, where one of the gametes receives an extra chromosome or doesn’t have a chromosome
rest have various different chromosomal abnormalities, many of them variations in sex chromosome numbers (e.g. XYY, XXX)
This is bad because of gene dosage = the cell is ‘used’ to having
the transcriptional outputs of 2 pairs of gene
List the different types of chromosome rearrangements that can occur
– Translocations – movement to new chromosome
– Inversions – portion of chromosome is “flipped”
– Deletions – section of DNA excised
– Insertions – section of DNA inserted
- beneficial in terms of evolution bc new genes can be created depending on selective pressure
- can cause errors in DNA replication->transcription-> translation -> protein production
List the parts of a chromosome
centromeres
telomeres
Describe the End replication problem
End-replication problem:the shortening of DNA molecules with each replication of DNA in S phase
DNA polymerase can’t start on its own… it can only add nucleotides onto the 3’ hydroxyl of an existing DNA or RNA molecule
During DNA replication an RNA primer is used to start the DNA synthesis
DNA polymerase builds in a 5’ to 3’ direction to make a new DNA strand
The RNA primer is later removed, leaving a small 3’ overhang on the DNA strands
the overhang folds back to form a loop (safer from nucleases)
Every time DNA is replicated the telomeres get a bit shorter – eventually this would be a problem if we ran out of telomere repeats and started losing gene sequences!
Describe how telomerase works
Telomerase is an enzyme with an RNA template attached to it
- Telomerase RNA binds to complementary sequence
- Nucleotides added to DNA 3’ terminus (5’ to 3’ synthesis as usual)
- Telomerase slides over and its RNA binds to the next complementary sequence
- More nucleotides added to DNA 3’ end
- The other strand filled in using a specific DNA polymerase (not shown)
- Note that this is DNA synthesized by an RNA template… which is called ‘reverse transcription’
chromosomes
Eukaryotic genomes are sub- divided into packets of DNA/protein complexes called…
are the carriers of genetic information and are located in the nucleus
were named in the 1880s because of dark staining when viewed (during mitosis) with a light microscope
chromo= light
*occasionally visible by light microscope when they condense during cell division
Diploid
number (2n) – number of chromosomes per somatic (i.e. non- gamete) cell.
Humans: 2n = 46
Haploid number
(n) is number of chromosomes in gametes
What is karyotype and how can it be observed?
The complete set of chromosomes in the cells of an organism
can be observed by isolating mitotic cells and staining the chromosomes with fluorescent probes (complementary DNA sequences to the chromosomes with fluorophores attached)
Describe the difference between an autosome and a sex chromosome
Human 2n: 22 pairs of autosomes (1 from mom 1 from dad)
2 sex chromosomes
Autosome = not a sex chromosome
What occurs during interphase?
is when the cell is doing its normal functions.
During S phase in the cell cycle (DNA replication phase), chromosomes duplicate – form dyads, the attached duplicated chromosomes are called sister chromatids
- These get separated during mitosis
The ‘X’ shape is only from S phase to metaphase!
When are chromosomes visible and why?
are only visible by light microscopy during mitosis!
This is due to condensation of the chromosomes during prophase through the activity of a number of proteins
*uncondensed during interphase so you cant see them with light microscope
Where are chromsomes located in the nucelus
During interphase, they do have very specific locations within the nucleus. Note that each chromosome 9 homolog is in a different area… so they all aren’t ‘paired up’ in interphase.
Even in interphase the DNA/protein complex (chromatin) of each chromosome is condensed to some degree (next lecture)
Different chromosomes labeled with different fluorescent probes and observed under a fluorescent microscope. Every chromosome has it’s own area it hangs out in.
Meiosis
Two cell divisions! 2n -> 2+2n -> 2n -> 1n
DNA replication first
followed by recombination (swapping bits between chromosome pairs from your mom and chromosome pairs from your dad)
Followed by cell division to give 2n cells
Then, another cell division without DNA replication, so each of the four resulting cells are haploid (1n)
List the types of aneuploidy
trisomy = one extra chromosome; monosomy = missing chromosome
what does aneuploidy cause?
turners syndrome- lack all or part of X chromosome
down syndrome- have an extra chromosome
Describe why chromosome translocations can be bad
Translocations are the transfer of a piece of one chromosome to a
nonhomologous chromosome
This can result in changes in phenotype by:
- Breaking a gene
- Moving gene to a region where it can be controlled by another regulatory sequence(results in increase or decrease in protein production because gene is being regulated by different regulatory sequence)
- Creating hybrid gene (if two genes are joined in frame)(hybrid protein- 2 proteins joined together)
Can chromosomes be broken?
Chromosomes are not indestructible!!
Chromosomes can be broken by DNA damage (from ionizing radiation, oxidative damage, etc.) and when repaired, they may
get scrambled:
there are ‘fragile’ spots in chromosomes where breakage is more likely to occur. Also… if this occurs it usually happens during mitosis, not during interphase.
Describe what can cause leukemia
a chromosomal translocation can cause leukemia
The c-abl gene encodes a kinase that regulates cell proliferation (cell cycle); the translocation alters the control of this gene, placing it under the control of another gene’s (bcr) promoter
C-abl protein only gets made when it should
C-abl protein gets made all the time, driving cell division -> cancer
Reciprocal translocation between chromosome 9 & 22, producing extra long chromosome 9 and a short little chromosome 22.
Describe the structure and function of the centromere
are the constricted (central) portion of each chromosome,
• the DNA contains a-satellite DNA(repetitive sequence of DNA); made of non-transcribed(non structural DNA) 171 base repetitive sequences
• Repeated many thousands of times
• Constitutive heterochromatin
• Attach to the kinetochores during M phase
Describe the structure and function of the telomeres
– non-coding regions at the ends of the chromosomes.
– short repeated sequence: repeated 500-5000X
– include specialized proteins
– form a “capped” end structure
– Human repeat: TTAGGGTTAGGGTTAGGG
protect the chromosome end
This sequence is conserved in vertebrates, and similar in most organisms… so its function must be important.
We’re showing mitotic chromosomes here… but remember that these structures are still present in interphase chromosomes! Some of the proteins associated with chromosomes during interphase are different though.
Chromosome numbers can….
vary in closely-related species
Humans have 46 chromosomes, other great apes have 48
• The genes in humans and chimps differ by <1% at the DNA nucleotide level
• Looking at human and chimp chromosomes, we share all, except the human chromosome 2 is a fusion of two ape chromosomes (there are differences in the DNA sequence, of course)
This little pond critter Oxytricha trifallax has a whopping 15000 chromosomes!
so like total genome size, there isn’t much correlation between # chromosomes and complexity in different organisms!
Telomere functions:
• protect the ends of chromosomes from nucleases (DNA degrading
enzymes, many of these degrade from the ends of DNA)
- allow cells to distinguish chromosome ends from broken DNA
- prevent chromosomes from fusing with each other
- attachment to of DNA to nuclear scaffold(helps attach DNA to overall chromosome structure)
What is the problem with telomeres?
when chromosomes are replicated in most cells, the telomeres get progressively shorter!
Describe the effects of telomere shortening on cells and organisms
The steady loss of telomeres results in a
limited life-span for most cells
- in culture, most of our cells cycle a set number of times, then die through apoptosis (programmed cell death)
- same seems to happen in most eukaryotic organisms
- due to lack of telomerase enzyme in our cells
Describe why telomeres shorten
each round of DNA replication leaves 50-200 bp DNA unreplicated at the 3’ end
• Average-aged cells with telomeres that are 10-12 kb in length divide 50-60 times (knows as the ‘Hayflick limit’ after the dude who first figured this out)
• cellular senescence is when cells will no longer divide and will die by triggering apoptosis, this occurs when telomeres are about 4-6 kb long
• This is also why you can’t grow ‘normal’ cells in tissue culture forever
What about cells that need to divide a lot?
Telomerase is the solution!
Telomerase
an RNA-containing enzyme that adds more nucleotides to the 3’ end of the telomere DNA strands
. List the cell types that do and do not express telomerase
Telomerase is not present in all cells
- Absent in cultured cells from normal tissue
- Present in many cancer cells - unlimited
divisions - Present in one celled organisms -unlimited divisions
- Present in primordial germ cells (that
make gametes) -> unlimited divisions
AND telomeres are full length in progeny - Present in tissue (or ‘adult’) stem cells (although these cells do show shortened telomeres with age, just more slowly)’
- If we experimentally add telomerase
expression to normal cells, more cycles are observed
Describe why poor Dolly died prematurely.
was the first cloned mammal from an adult cell
She was made by taking a nucleus from the udder of an adult ewe, and transplanting it into a de-nucleated embryo
Dolly died at age 6 (most sheep live to about 12 years)
be her telomeres were 20% shorter than normal sheep by age 3 she started developing serious arthritis at age 5 premature aging occurred.
Questions (to which we still don’t have answers to!):
1. How do shortened telomeres cause aging?
2. Why didn’t the de-nucleated embryonic cell restore the length
of the telomeres in the 6 year-old nucleus?
Could loss of telomeres cause the
aging process?
They’re likely involved, but there are other factors too…
• Loss of telomeres certainly provide a maximal limit to the number of cell divisions
• Could we provide telomerase therapy to restore telomere length?
What are some factors in cell aging?
Telomere shortening
• Accumulated DNA mutations
• Chronic risk exposures such as oxidants, UV
• Glycation – sugar binding DNA, proteins, lipids
i
ome anticancer therapies are directed at
There’s a trade-off between longevity and the risk of cancer developing.
- Many cancer cells are mutated forms of adult
stem cells
- The longer an organism lives, the more cell
divisions their stem cells need to do
- Over time, stem cells can accumulate
enough mutations to fully escape cell cycle
control
- We now know that ‘cancer stem cells’ are a
handful of cells in a tumor that do all the dividing
telomerase is the only protein that functions at telomeres. true or false
false
Telomere repeat sequences
at the ends of chromosomes
Alpha-satellite DNA repeat sequences
at centromeres
In the chromosome arms:
- genes (made of exons and introns) encoding mRNAs which code for proteins
- ‘pseudogenes’ which are copies of normal genes that are non-functional (usually with null mutations in them)
- Genes encoding other RNAs: rRNAs, tRNAs, microRNAs,
long non-coding RNAs, etc.
