DNA Organization Flashcards
- Circular chromosomes
- Double stranded (dsDNA)
- May also contain plasmids
- Found in the nucleoid
Organization of DNA in most prokaryotes
Back bone has been nicked.
-relaxes from supercoiling
Relaxed circular DNA
Allows DNA to fit inside the cell
-Amount and type of supercoiling depends on the starting point
Supercoiled Circular DNA
Formation from contortion of supercoiling and twisting.
2 supercoiled DNA forms
Untwisting the DNA
-Compacting DNA
Negative supercoiling
Twisting
- introducing more twisting causes DNA to contort.
- Compacting DNA
Positive supercoiling
Amount and type of supercoiling controlled by:? -highly conserved enzymes -relieve twisting by two types: I. Make a nick in one strand II. Cuts both strands
Topoisomerases
2 DNA linked like a chain
Catenated DNA
How doe bacteria package their DNA?
a. using histones
b. by supercoiling the DNA
c. by acetylating the DNA
d. by methylating the DNA
e. all of the above
b. by supercoiling the DNA
-Nucleus, mitochondria, choloroplasts
Organization of DNA in Eukaryotes
Amount of haploid DNA in an organism in base pairs
C value
Increase in DNA doesn’t always mean and increase in complexity of the organism.
C value paradox
1n
haploid
2n
diploid
Complete set of chromosomes
karyotype
Group that gives DNA its (-) charge
Phosphate
DNA is positively or negatively charged?
Negatively
DNA and associated protein in the nucleus
Chromatin
Histones
Nonhistones
Proteins associated with DNA
Help pack chromatin into the nucleus
- (+) charged because DNA is (-) charged
- consists of H1, H2A, H2B, H3, H4
- very highly conserved
Histones
Various types:
- DNA replication
- DNA repair
- Transcription
- Recombination
Nonhistones
DNA wound around a histone core
Nucleosomes
DNA connecting two nucleosomes together
linker DNA
- Double helix is wound around histones to form nucleosomes
- 10nm fiber is condensed and wound into 30 nm fiber
- 30nm fiber is attached to protein scaffold.
Eukaryotic DNA packing
Can unpack and/or repack depending on gene transcription and other events
ex. during the cell cycle
Dynamic Packing
Eukaryotic DNA packing
Double helix wound around histones to form nucleosomes
10 nm fiber
Packing is less uniform during interphase (depends on the gene)
- dynamic upon gene expression
- organized to support proper gene expression
DNA packing
- Chromatin that exhibits normal condensation and decondensation during the cell cycle.
- genes are actively being transcribed and are thus loosely packed
- typically devoid of repetitive sequences
Euchromatin
Two types:
- Constitutive Chromatin
- Facultative Chromatin
- Packed tightly
Heterochromatin
- Type of heterochromatin
- present in all cell in the same location on every chromosome
ex. centromere
Constitutive Chromatin
- Type of heterochromatin
- can change during development
ex. Barr body on X chromosome - DNA condenses or decondenses depending upon the status of the cell. (permanently silencing genes)
- Euchromatin–>Heterochromatin
Facultatvie Chromatin
Occurring once or only a few times in the genome
-ex. protein coding
Unique-sequence DNA
Two types:
- Unique-sequence DNA
- Repetitive DNA
Distribution of sequences
Can occur a few times up to a million times, depending on the sequence.
ex. telomeres and rRNA(ribosomal RNA)
Repetitive DNA
Two types:
- Dispersed repeated sequences
- Tandemly repeated DNA
Repetitive sequence DNA
Distributed at irregular intervals on the genome
ex. transposable elements
Dispersed Repeated Sequences
Sequence that repeats itself many times in a row
ex. telomeres, or genes
Tandemly Repeated DNA
- Highly repetitive DNA
- a fraction of a eukaryotic organism’s DNA that differs in density from most of its DNA as determined by centrifugation, that consists of short repetitive nucleotide sequences, that does not undergo transcription, and that is often found in centromeric regions
Satellite DNA
- Consists of heterochromatin
- tandemly repeated sequences (specifically satellite DNA)
- form a constrictive region because of tight packing
Centromere
- Tandemly repeated DNA at the ends of linear chromosomes.
ex. TTAGGGTTAGGG… - Protect from exonucleases
- Protect from DNA repair enzymes that form free ended DNA to other free ended DNA
Telomeres
- Defense mechanism against foreign DNA
- enzymes that chew up the chromosome
exonuclease
