DNA Flashcards
Transforming principle
2 strains streptococcus pneumonaie; S strain virulent encapsulated; R stain avirulent, unencapsulated; showed live R with heat killed S virulent and encapsulated concluding dead S could transform part of R
DNA As Hereditary Material experiments
Transforming principle, DNA is transforming, blender experiment
DNA is transforming
fractioned various components of S strain bacteria found that DNA not protein was transforming
Blender experiment
worked with bacteriophage composed of DNA and protein; radioactively labled DNA and protein; infected bacteria with radioactive phage and used blender to separate phage coat from infected cells; found only DNA entered bacteria and was in progeny; conclusively shoed DNA as hereditary material
Nucleotide
consists of sugar (for DNA deoxyribose), nitrogenous base (purine or pyrimidine), and a phosphate group
Purines
adenine and guanine
Pyrimadines
Thymine and cytosine
DNA
polymer of nucleotides with phosphodiester linkages
Double helix
two complementary strands held together by hydrogen bonds; strands are antiparallel; has sugar phosphate backbone running along sides of helix bases are in the center
Principles of base pairing
%A=%T; %G=%C; A with T C with G
Genetic information carried in
sequence of bases
Directionality of DNA
5’ -> 3’ for one strand 3’ ->5’ for its complement
Compacting of DNA with reference to human genome
human genome contains about 3 billion base pairs which have to fit in DNA nucleus which is accomplished by 10,000 fold compaction
Chromatin
DNA complex with proteins
Heterochromatin
highly condensed, typically devoid of genes, associated with low gene expression; 2 types facultative heterochromatin and constitutive heterochromatin
facultative heterochromatin
heterochromatic in some cells euchromatic in others
constitutive heterochromatin
always heterochromatic
euchromatin
less condensed associated with transcriptionally active genes
Most basic level of packaging
nucleosome; consists of 200 bp DNA and 2 molecules of each histone H2A, H2B, H3, H4 creating a bead on a string structure
packaging after nucleosome
nucleosomes further packaged into 30nm fiber through histone interaction and participation of histone H1
Levels of packaging
Nucleosome -> 30nm fiber -> 30nm fibers organized into looped domains -> condensed DNA in mitotic chromosomes; net result is each DNA molecules packaged into mitotic chromosome 10,000 fold shorter than its extended length
Genome components
unique sequence, repetitive sequence
Genes
unique sequence; regions of DNA that control discrete hereditary characteristics; usually corresponding to a single protein or RNA; can consist of introns, exons, and various regulatory sequences
exons
include all protein coding sequences (segment of eukaryotic gene that is transcribed into RNA and dictates that amino acid sequence of part of a protein)
intron
noncoding sequence within eukaryotic gene that is transcribed into an RNA molecule but is then excised by RNA splicing to produce mRNA
Repetitive sequences
- Retroviral-like elements
- Non-Retroviral (non-LTR) retrotransposons
- DNA transposons
- Simple sequence repeats
Retroviral-like elements
A form of retrotransposon (mobile DNA elements that move through an RNA intermediate) Have long terminal repeats (LTR) similar to retroviruses
non-retroviral (non LTR) transposons
many are transcriptionally acitve; often mutated or truncated; only a small subset are able to transpose;
LINE- Long interspersed nuclear elements eg L1 element aprox 7kb
SINE- short interspersed nuclear elements; eg Alu element aprox 100-500bp
DNA transposons
mobile DNA elements; no RNA intermediate
simple sequence repeate
Satalites: repeats of 20-200bp units; totaling 100kb-1Mb; eg Alpha satellite at centromere
Mini-satalites: repeats of 10-100 bp units; totaling 1kb-20kb; eg telomeric DNA repeats
Micro-satalites: repeats of 1-6bp units; usually totaling < 150bp eg CA repeat
DNA replication is semiconservative
each new DNA molecule contains 1 parents strand and 1 newly synthesized strand; shown in experiment where e
- grew bacteria in medium with heavy nitrogen
- transfered to medium with light nitrogen
- isolated newly replicated DNA and measured the density
- Results consistent with semiconservative model of DNA replication
DNA replication originates
originates at replication origins; proceeds in both directions (bidirectional); replication must initiate from a primer
DNA synthesis direction
occurs 5’ ->3’ meaning antiparallel strands are replicated in opposite directions
Leading strand
grows in direction of unwinding, produces one continuous new strand
Lagging strand
grows in opposite direction of unwinding ; as more template DNA is exposed more fragments are initiated;; produces small discontinuous segments of DNA (Okazaki fragments); these are ligated together to form a continuous strand
DNA replication factors
DNA polymerase, DNA helicase, single stranded DNA binding proteins, Primase, DNA ligase, Teleomerase
DNA polymerase
Catalyzes addition of new nucleotides at 3’ end of growing chain; multiple distance DNA polymerases mediate specialized functions in higher eukaryotes
DNA helicase
unwinds DNA in front of the replication fork to prevent tangling of strands
Single- stranded DNA binding proteins
Binds unwound single stranded DNA and prevents it from reanealing
Primase
DNA dependent RNA-primase generates a a short piece of RNA that base pairs with template DNA and serves as primer for DNA synthesis; will be removed and replaced with DNA
DNA ligase
Seals any gaps to produce continuous DNA strand (particularly gaps between Okazaki fragments)
Telomerase
Assists in replication of chromosome ends (telomers) where lagging strand synthesis is problematic
