General Genetics Chapter 9
there are 2 conformations of DNA: B DNA and Z DNA. What are the main structural features of B DNA?
-10 base pairs per turn -bases are aligned perpendicular to the central axis of the molecule
A-DNA
-11 bp/turn -23 A diameter -prevalent under high salt conditions -doubtful it occurs under biological conditions
features of double-helix
-2 strands twisted around a common axis -10 bp per complete twist -2 strands are anti-parallel (5'-3' and 3'-5') -right-handed (spirals away from you in clockwise direction) -sugar-phosphate backbone on outside -nitrogenous bases stacked on inside -2 chains held together by hydrogen bonds between complementary bases (A to T with 2 bonds and C to G with 3 bonds) -2 asymmetrical grooves on outside of helix (major 2.2nm and minor 1.2nm) -complete turn of helix = 34 A or 3.4nm -bases are spaced at 3.4 A or 0.34nm -diameter of helix = 20 A or 2nm
C-DNA
-9.3 bp.turn -19 A diameter -prevalent under great dehydration conditions -doubtful it occurs under biological conditions
What did Chargaff conclude from how observations?
-A interacts with T -C interacts with G
other forms of DNA
-B-DNA -A-DNA -C-DNA -Z-DNA
how can nucleotides found in DNA be discriminated from those found in RNA?
-DNA nucleotides may contain thymine -DNA nucleotides contain deoxyribose -DNA nucleotides never contain uracil
what is the chemical difference in RNA structure compared to DNA?
-RNA contains the sugar ribose instead of 2'-deoxyribose -RNA contains the base uracil instead of thymine
What are the main structural differences between RNA and DNA
-RNA contains uracil -RNA is usually single-stranded -RNA uses ribose as a sugar
How can nucleotides found in RNA be discriminated from those found in DNA?
-RNA nucleotides contain ribose -RNA nucleotides don't contain thymine -RNA nucleotides may contain uracil
Franklin and Wilkins experiment
-a DNA fiber is exposed to x-rays -the scatter of rays from the fiber is caught on a photographic film -the uniformity or irregularity of the scatter is indicative of the structure of DNA
adjacent nucleotides are linked by 5'-3' phosphodiester bonds
-at one end of the linear DNA chain, the nucleotide will have a free 5'-phosphate -at the other end, the nucleotide will have a free 3'-hydroxyl
Which 2 main structural features result in complementary sequences of DNA forming a double helix?
-bases stack -hydrogen bonding between bases
B-DNA
-biologically significant conformation -prevalent under aqueous, low-salt conditions
James Watson and Francis Crick
-did not perform any experiments -used other experiments and inferences to solve the structure of DNA
What are the features of the structure that forms when short regions of RNA form base pairs?
-double stranded -helical
steps of Chargaff's analysis of base composition
-extract the chromosomal material from each cell type to be tested -remove protein -hydrolyze the DNA to release the bases from the DNA strands -separate the bases by chromatography -extract bands from paper into solutions and determine the amounts of each base by spectroscopy -calculate and compare the base content in the DNA from different organisms
what is each nucleotide composed of?
-nitrogenous base -sugar -phosphate
Z-DNA
-prevalent in vitro in C-G containing DNA -left-handed, double-helix -12 bp/turn -18 A diameter -zig-zag conformation -may be involved in transcription and recombination in vivo
nitrogenous bases
-purines -pyrimidines
sugar
-ribose -deoxyribose
RNA double-helix
-right-handed -have the A form with 11 to 12 base pairs per turn
In RNA, what is the orientation of the bases in the single and double stranded parts of the molecule?
-single stranded outwards -double stranded inwards
what are the implications of the double-helix structure?
-suggests an obvious way to achieve accurate replication -suggests the means by which information can be stored through the sequence of nucleotides
DNA can adopt 2 different conformations. Which is the most abundant conformation seen in living cells?
B DNA
What 2 structures can a DNA double helix form?
B and Z DNA
nucleotides are used as the fundamental repeating structural unit of _____ and _____
DNA RNA
DNA sequences with a high proportion of base pairs between _____ and _____ are most stable
G C
Which area is devoted to the study of the structure of DNA and how it functions?
molecular genetics
What was the other strategy Crick and Watson used?
molecular modeling
A sugar, such as ribose or deoxyribose, attached to a base but not to a phosphate group is called a _____
nucleoside
base + sugar =
nucleoside
base + sugar + phosphate =
nucleotide
dCTP
nucleotide found in DNA with 3 phosphate groups
guanosine triphosphate
nucleotide used to make RNA that contains 3 phosphate groups
UMP
nucleotide with ribose sugar and a single phosphate group
ADP
nucleotide with ribose sugar with 2 phosphate groups
second level of DNA structure
nucleotides are linked together to form strands of nucleic acids
lowest level of DNA structure
nucleotides are the building blocks of nucleic acids
In a DNA double helix, where is the sugar phosphate-backbone located?
on the outside of the helix
The At/GC base pairing rule requires that the purines always base pair with the pyrimidines. Why is this?
pairing a purine with a pyrimidine ensures a consistent diameter of the helix
DNA is negatively charged because of the negative charge found on the _____ groups in the DNA backbone
phosphate
The difference between a nucleoside and a nucleotide is taht the nucleotide has 1 or more _____ groups attached to the sugar and the nucleoside does not
phosphate
DNA nucleotides have 3 main parts to their structure; atleast one _____ group; a nitrogenous _____; and a 5 carbon sugar called _____
phosphate base deoxyribose
What is the name for the type of bond connecting 2 sugar molecules via a phosphate group in the backbone of DNA and RNA?
phosphodiester linkage
nucleotide
repeating structure unit of DNA and RNA
The nucleotides used to make RNA and DNA are made from 3 main parts. One of those parts is a sugar molecule; in RNA the sugar molecule is called _____
ribose
B DNA can adopt the Z DNA conformation under certain physiological conditions. It is possible that Z DNA plays a role in gene expression; specifically, it could play a role in the _____ of particular genes
transcription
Crick and Watson assumed that the nucleotides in DNA were all linked together in linear chains using the same kind of bond
true
In a DNA double helix, the bases point towards the _____ of the helix
center
Which way does the double helix turn?
clockwise
When the bases in one strand of DNA can hydrogen bond exactly with the bases in another strand of DNA, then the 2 strands of DNA are said to be _____.
complementary
purines
composed of 2 fused hetero-cyclic rings
pyrimidines
composed of a single hetero-cyclic ring
DNA and RNA nucleotides have 3 main parts, one of which is a sugar. In DNA the sugar is _____
deoxyribose
When comparing the structural features of RNA with the structural features of DNA, DNA uses the sugar _____ and RNA uses the sugar _____
deoxyribose ribose
2 types of nucleic acids
deozyribosenucleic acid and ribonucleic acid
highest level of DNA structure
double helix forms complex structures
What structure forms when regions of RNA form base pairs with themselves?
helical double stranded structure
RNA length ranges from only a few _____ nucleotides to several ____ nucleotides long
hundred thousand
What is Chargaff's rule?
in double stranded DNA, A=T and C=G
Franklin and Wilkins results
indicated that DNA is a multiple-stranded fiber of 20 A thickness with groups spaced 3.4 A along the fiber and a repeating unit every 34 A
The wider groove in DNA is called _____ and the narrower grove is called _____
major minor
phosphates
1 or more phosphates could be attached to the C-5' of the sugar
what are the carbon each nucleotide is attached to?
1' = base 5' = phosphate group 3' = hydroxyl group
The structure of DNA that Crick and Watson proposed has _____ strands of DNA to form double helix. The DNA bases of each strand of DNA point towards the central axis of the molecule and are held together by _____
2 hydrogen
third level of DNA structure
2 strands of DNA can associate to form a double helix
What are the main structural features of the tRNA molecule?
3' acceptor site anticodon
If one strand of DNA has the sequence 5'-ATGCTACATA-3', what would be the sequence of the bases on the complementary strand?
3'-TACGATGTAT-5'
how long is a single turn of the DNA double helix?
3.4 nm
double stranded DNA with 1800 nucleotides with 600 deoxycytidine monophosphate nucleotides, how many deoxyguanosine monophosphate nucleotides would be present?
600
number of hydrogen bonds to form
A-T = 2 G-C = 3
What are the base pairings in RNA
A-U U-A G-C C-G T is not found in RNA
conformation of B and Z DNA
Z = left handed twist B = right handed twist
deoxyribose
base is always attached to C-1' of the sugar
Rosalind Franklin and Maurice Wilkins
analyzed fibers of DNA by X-ray diffraction
Erwin Chargaff
analyzed the base composition of DNA from various species and observed certain regular relationships -A + G = T + C -A = T and C = G -A + T almost never equals to C + G
When one strand of DNA in a double helix has the chemical polarity 5' to 3', and the other, running in the opposite direction, has the chemical polarity 3' to 5', then it would be said that the 2 strands are _____.
antiparallel
