Chapter 11 Flashcards
genetic material must be able to..
- contain the information to construct an organisms - a cookbook with ALL the recipes
- pass from parent to offspring and from cell to cell during cell division (transmission)
- be accurately copied (replication)
- account for the known variation between organisms
what is the genetic material?
-how is material passed from generation to generation?
what is the genetic material?
- late 1800’s scientist postulated a biochemical basis
- researchers convinced chromosomes- carry genetic information
- chromosomes= proteins + nucleotides
- protein= more complex
- expected to be the genetic material
Griffiths bacterial transformations
- late 1920’s Frederick griffith work with Streptococcus pneumoniae
- strains that secrete a capsule look smooth and are fatal in mice
- strains that do not secrete capsules look rough; not fatal
Griffiths experiment
- injected living type S bacteria into mouse- mouse died. Type S cells are virulent
- inject living type R bacteria into mouse. Type R cells are benign
- inject heat-killed S into mouse- benign
- inject heat-killed S bacteria into mouse and living R bacteria- virulent type S strain in dead mouse’s blood. Living R cells transformed into virulent S cells by a substance from the heat-killed type S cells
Griffiths bacterial transformations results
- genetic material from heat-killed type S bacteria was transferred to living type R bacteria
- R-bacteria were transformed
- Griffith did not know the biochemical basis of his “transforming principle”
Griffith’s bacterial transformation; “transforming principle”
- met requirements for genetic material
- variation- some bacteria make a capsule, some do not
- the R strain acquired the information to make capsule (a trait)
- the transformed R cells replicated this information and transmitted it to new cells during cell division to cause an infection
Avery, Macleod and McCarty
- what substance is being transferred from the dead type S bacteria to the live type R?
- used purification methods to purify different macromolecules (DNA,RNA and proteins)
- only purified DNA from type S could transform type R bacteria
What happens just before a cell divides?
cells exactly double their amount of DNA
Chargaff; DNA position
- he looked at the composition of DNA
- 3 components- a pentose sugar, phosphate group and a nitrogenous base
Chargaffs Rule
- amounts of the 4 bases were not equal
- however the amount of A=T and amount of G=C
- Chargaffs Rule
- double helix counts for this
Structure of DNA
- too small to see through microscope at the time
- portions of DNA’s structure could be inferred through a technique called X-ray diffraction
- purified DNA bombarded with X rays
- takes years to obtain a well made X-ray crystallograph
- Rosalind Franklin did this
Watson and Crick; DNA structure
- Wilkins gave x-ray graph to Watson and Crick
- associated the graph to the double helix
- found ball and stick model consistent with data
- double helix
- a purine w/ a pyrimidine
- correct width of helix
- fits with Chargaff’s A=T and G=C
5 levels of DNA structure
- nucleotides
- strand
- double helix
- chromsomes
- genome
Nucleotides
- DNA structure
- the building blocks of DNA and RNA
Strand
- DNA structure
- a linear polymer strand of DNA or RNA
double helix
- DNA structure
- the two strands of DNA
chromosomes
- DNA structure
- DNA associated with an array of different proteins into a complex structure
genome
- DNA structure
- the complete complement of genetic material in an organism
nucleotides: DNA
- Phosphate grouo
- Pentose sugar- deoxyribose
- Nitrogenous base
- Purines (A, G)
- Pyrimidines (C,T)
nucleotides: RNA
- Phosphate group
- Pentose sugar
- oxyribose - Nitrogenous base
- Purines (A,G)
- Pyrimidines (C, U)
Conventional numbering system
- sugar carbons 1’ to 5’
- base attached to 1’
- phosphate attached to 5’
strand structure
- sugar-phosphate backbone
- bases on the inside
- stabilized by H bonding
- specific base pairing
- major and minor grooves
Chargaff’s rule; strand structure
- A pairs with T via 2 H bonds
- G pairs with C via 3 H bonds
- keeps width consistent
- 10 base pairs per turn
Phosphodiester bond
-nucleotides covalently bonded via phosphodiester bond- phosphate group links 2 sugars
complementary; antiparallel
- 2 DNA strands are complementary
- 5’- GCGGATTT-3’
- 3’- CGCCTAAA- 5’
-2 strands are antiparallel
one stand 5’ to 3’ others 3’ to 5’
- conservative (DNA replication)
- the parental double helix stays together, the new double helix is completely new
- Dispersive (DNA replication)
each daughter double helix is a random mosaic of parental DNA and newly synthesized DNA
- semiconservative (DNA replication)
-each daughter double helix contains one parental strand and one new strand
how does DNA replicate?
- the double helix must open
- strands separate
- weak H bonds allow this
- sequence on one strand is a template for the other strand
- new nucleotides must obey the AT/GC rule
- the new strand is the “complementary sequence”
- end up with 2 identical DNA molecules
Origin of replication
- starting point
- opens up to a replication bubble
- produces 2 replication forks
- bidirectional replication
- bacteria have a signal origin
- eukaryotes have multiple points of origin- speeds up process
DNA replication in the cell (in vivo)
- chromosomes are BIG, millions of bases
- many proteins needed to copy a chromosome
- copying occurs at many sites
- or cell will take too long to divide
DNA helicase (protein in replication)
-binds DNA and travels 5’ to 3’ using ATP to separate strand and move fork forward
DNA topoisomerase (protein in replication)
relieves additional coiling ahead of replication fork
Single-strand binding proteins (replication)
keep parental strands open to act as templates
DNA polymerase (protein in replication)
-synthesizes new DNA strand- requires an RNA primer (short stretch of nucleotides to “grow off” of)
DNA polymerase uses dNTPS
- free nucleotides with 3 phosphate groups
- breaking covalent bond to release 2 phosphate groups provides energy to connect adjacent nucleotides
DNA polymerase- 2 enzymatic features
- can’t begin DNA synthesis on a bare template strand
- DNA primase must make a short RNA primer
- primer later removed and replaced with DNA - DNA polymerase can only work 5’ to 3’
leading strand
- DNA synthesized as one long continuous moelcule
- DNA primase makes one RNA primer
- DNA polymerase attaches in 5’ to 3’ direction
lagging strand
-DNA synthesized 5’ to 3’ as “Okazaki fragments”
leading and lagging strands
- DNA polymerase “hopes” back to replicate DNA 5’ to 3’
- DNA ligase fills gaps
DNA replication and errors
- error rate about 1/billion bases- really good!
- approx. 3 errors every time a human cell copies its DNA
- error rate of nucleotide addition by DNA polymerase about 1/100,000
- H-bonds between A&T/ G&C more stable than mismatches
DNA replication & Errors (mutations)
- DNA polymerase proofreads the sequence as it adds bases
- other DNA repair enzymes remove/fix errors as well
Telomeres
- no place for upstream primer, so DNA polymerase cannot copy the tip of the DNA strand with a 3’ end
- if this replication problem were not solved, linear chromosomes would become progressively shorter
Telomerase
- enzyme attaches many copies of DNA repeat sequence to the ends of chromosomes known as telomeres
- E.g GGGTTA(humans)
- telomere at 3’ does not have a complementary strand and is called a 3’ overhang
- complementary strand is embedded in telomerase
- prevents chromosome shortening
- provides upstream site for RNA primer
Telomeres and aging
- body cells have a predetermined life span
- skin cell sample grown in a dish will double a finite number of times
- infants, about 80 times
- older person, 10to20 times
- inserting a highly active telomerase gene into cells causes them to continue to divide
Werner Syndrome
- telomeres and aging
- DNA helicase gene mutation
- leads to lack of DNA repair and shortening of telomeres
- DNA replication impaired
- significant premature aging
telomeres and cancer
- in 99% of all types of human cancers. telomerase is found at high levels
- prevents telomere shortening and may play a role in continued growth of cancer cells
- mechanism is unknown
eukaryotic chromosome structure
- typical eukaryotic chromosome may be hundreds of millions of base pairs long
- length would be 2 meters
- must fit in cell 10-100um
- chromosomes composed of chromatin
- DNA + protein
Level 1 of DNA packaging; DNA wrapping
- DNA wrapped around histone protein complexes to form nucleosome
- “beads on a string”
- shortens length of DNA moelcule 7-fold
Level 2 of DNA packaging; 30nm-fiber
current model suggests asymmetric, 3D zigzag of nucleosomes
-shortens length another 7-fold (49-fold..)
Level 3 of DNA packaging; Radial loop domains
-interaction between 30nm fibers and nuclear matrix
3 levels of DNA packaging
- each chromosome located in discrete territory
- level of compaction of chromosomes not uniform
- Euchromatin vs. Heterechromatin
Cell Division
-when cells prepare to divide, chromosomes become even more compacted
