lecture 9 Flashcards
if two daughter cells do not divide properly
you will have a piece of chromosome added onto another chromosome
how do we know that DNA carries genetic info?
Griffith experiment (1920s)
What did Griffith’s experiment find?
two strains of S. Pneumoniae
1. one is lethal (S strain)
2. one is harmless (R strain)
Heat-killed S strain does not kill mice
Mixing heat-killed red with yellow kills mice even though the red strain is dead!
CONCLUSION: transformation!!!!
the ability of the unknown factor to “transform” the harmless strain into a harmful one
Avery, MacLeod and McCarthy experiment (1944)
isolated individual components from the S strain cells and tested their ability to transform the R strain
conclusion: the molecule that contains the heritable info is DNA
Hershey and Chase (1952)
conclusion:
DNA enters bacteria and carries the genetic information
primary DNA structure
the nucleotide sequence
secondary DNA structure
any regular, stable structure taken up by some or all of the nucleotides
tertiary DNA structure
complex 3D folding of nucleic acids
(ex. into eukaryotic chromatin and bacterial nucleoids)
Nucleoside
base + sugar (ex. adenosine)
Nucleotide
base + sugar + phosphate (ex. adenosine monophosphate AMP)
phosphate linkages
5’ phosphate of one nucleotide unit is joined to the 3’ hydroxyl group of the next nucleotide, creating a PHOSPHODIESTER linkage
nucleotide net charge
negative
From Purines: you can make
Adenine and Guanine
(depending on the substrate)
From Pyrimidines: you can make
Cytosine, Thymine, and Uracil
pentose forms a covalent bond with the base called a
beta-glycosidic linkage
(bonds joining a sugar to another group)
DNA secondary structure: the double helix
the two strands of DNA are wound around the same axis to form a right-handed double helix
the strands run in opposite directions (anti-parallel, one 3’ to 5’ and the other 5’ to 3’)
Two DNA strands interact via
base pairing (H bonding) through complementary bases
Complementary bases in the double helix
-Adenine forms two H bonds w thymine (or uracil)
-Guanine forms three H bonds w cytosine
A=T (or A=U) and G≡C base pairs predominate in double stranded DNA and RNA
base stacking: which interactions?
van der waals interactions and some dipole-dipole interactions
bases are not directly ontop of eachother
base stacking between the hydrophobic bases minimizes their
contact with water and is very important for stabilizing the double helix
your DNA is highly — charged
negatively
DNA structure (shape, xray)
DNA produces an Xray diffraction pattern: discovered by Rosalind Franklin
DNA molecules are helical with two periodicities (regular repeating patterns)
DNA secondary structure: watson-crick base pairing
watson and crick determined the double helix structure of DNA
The specific pairing of bases (G≡C, A=T) permits the
duplication of genetic information
Chargaff’s rule
A=T and G=C
A+G = T+C
You have isolated the DNA from 2 species of bacteria. Thymine accounts for 10% of the bases in the first and 25% in the second species.
What is the relative proportion of adenine, guanine, and cytosine for the two species? (chargaff’s rule)
species 1:
T 10%, A 10%, G 40%, C 40%
species 2:
T 25%, A 25%, G 25%, C 25%
Watson-Crick (WP) paper published in 1953
DNA will be duplicated by opening the double stranded helix and using one of the strands to copy the info from
(nobel prize given to them from this! most cited paper ever?!)
DNA grooves
DNA contains two grooves where the bases are exposed known as the major groove and minor grooves
They are common locations for protein-DNA interactions
the alpha helix fits in the — perfectly!
major groove
Chromatin
complex of DNA and protein
Chromosomes
Chromatin in a compacted form
Why is compaction (or chromatin) necessary in chromosomes?
how much is DNA compacted?
each human cell contains 2m DNA packaged into a cell that is 5-8 micrometers in diameter.
DNA is compacted about 10,000 fold
Karyotype
an ordered display of the full set of chromosomes
Human chromosome characteristics
humans have 46 chromosomes, most are a homologous pair (one maternal, one paternal)
-can be used to detect genetic defects
Genome
the total genetic information carried by all the chromosomes in a cell
a lot of the DNA in our genome is called
junk DNA!
because its function is unknown
Is there a simple relationship between gene number, chromosome number, and total genome size?
no!
what are the two broad phases of the cell cycle?
Interphase and Mitosis
Interphase
when chromosomes are duplicated
-interphase chromosomes are extended and hard to see using a light microscope
Mitosis
when chromosomes are segregated to two daughter molecules
-mitotic chromosomes are extremely compact and easy to visualize
telomeres
repeated sequences at the ends of chromosomes. protects from degradation
if we did not have telomeres, what would happen to the chromosome?
each time replication occurred, the chromosome would shorten
Replication origin
sequence where replication begins
centromere
sequence that allows DNA to be separated during M phase. region of attachment to mitotic spindle
based on what you know about hydrogen bonding, what kind of nucleotides would you expect to be common at the replication origin?
A and T because it is easier to open (weaker H bond)
Interphase chromosomes: distribution
- NOT randomly distributed in the nucleus
- each chromosome occupies a specific region
- chromosomes may be attached to the nuclear envelope or to the protein meshwork that supports it
nuclear lamina
the protein meshwork that supports the chromosomes
