Theme 1B Flashcards
Structure of DNA and Chromosomes
3 Components of DNA
- Pentose sugar - 5’C sugar
(Ribose RNA or Deoxyribose DNA) - Nitrogenous Base
(Purine G/A or Pyrimidine C/U/T) - Phosphate
Purine
nitrogenous base
guanine or adenine
Pyrimidine
nitrogenous base
cytosine, uracil, thymine
Pentose Sugars of DNA & RNA
Ribose - has OH attached to 2’C
2-Deoxyribose - has H attached to 2’C
(know structures - slide 3)
Nitrogenous bases of DNA & RNA
DNA = ACGT
RNA = ACGU
Nucleosides?
Deoxyribonucleosides?
- molecules of sugar & a base
- 2-deoxyribose sugar + purine/pyrimidine base
Base binds to C1’ of deoxyribose/ribose
4 base + sugars:
deoxyadenosine
deoxyguanosine
deoxythymidine
deoxycytidine
(What are ribose versions of these^ and what would they look like?)
- adding OH removes “deoxy”
Nucleotides
nucleoside molecule + phosphate
- phosphate group attached to carbon - 5’ of the ribose and deoxyribose sugars
4 nucleotides:
- deoxyadenosine triphosphate (dATP)
- deoxythymidine triphosphate (dTTP)
- deoxyguanosine triphosphate (dGTP)
- deoxycytidine triphosphate (dCTP)
(adding OH instead of an H to their structures removes “deoxy” and “d”)
DNA
polymer of deoxyribonucleotides
- nucleotide monomers polymerize via phosphodiester bonds (C-O-P-O-C)
- covalent bonds form between phosphate and C-3’ and C-5’ of two pentose sugars (pentose-phosphate backbone)
- polynucleotide has polarity with a C-5 PO4 end and C-3’ OH end
- type of nucleic acid depends on sugar (DNA=deoxyribose & RNA=ribose in P-P backbone)
What is the charge of a DNA molecule
negative because of the phosphate group
What is an identifying characteristic of DNA? RNA?
has hydrogen group
has hydroxyl group
Erwin Chargaff
analyzed overall quantities of four nitrogenous bases in various organisms
Chargaff’s Rule
% A = % T
% C = % G
% Purines (A+G) = % Pyrimidine (C+T)
How did scientists know that the info coding for life is in the nitrogenous base?
sequence of nucleotides changes; everything else remains the same
hence, ACGT sequence gives the unique genotype
X-Ray diffraction studies - Rosalind Franklin & James Watson
DNA molecules were ________.
0.34 nm periodicity suggested that bases were ___________.
X-shaped pattern indicated ________.
contributed to determining 3-D structure of DNA
-cylindrical & about 2 nm in diameter
- stacked like pennies on top of each other
- helical structure
Watson proposed definitive model
(Watson & Crick)
Two strands of the phosphate-pentose backbone spiral as a ____________ about a common axis.
The two strands run _________.
Purine on one strand is always base-paired with a ________.
double helix
anti-parallel
pyrimidine
Base pairing is _________, which allows…
complimentary; determining sequence of one strand using another
What type of bonding occurs between two strands?
hydrogen bonding
Nucleic Acid Hybridization
annealing of single strands of DNA/RNA by forming H-bonds
- highly specific (strands must be complimentary)
Nucleic Acid Hybridization is _______ & ___________.
temperature-driven; concentration-dependent
DNA-RNA hybrids are found in _________, ____________, & _______.
DNA replication; transcription; reproduction of some RNA viruses.
Complementary base pairing allows _____ to act as templates for DNA replication of new strands.
Parental strands can unwind by _______________ between bases.
_____________ is where the double helix will contain a parental strand and a newly synthesized strand.
parental strands; breaking hydrogen bonds; semi-conservative replication.
How is DNA organized in prokaryotes and eukaryotes?
-DNA is associated with proteins and organized into structures called chromosomes
Chromatin
given region of DNA with its associated proteins on a chromosme
Chromosomes can be ________ & ___________
can be linear or circular
- prokaryotes have one and other small independent circular DNA called plasmids in cytoplasm
- eukaryotic chromsomes are linear and enclosed in nucleus
Plasmids
not essential for life but can potentially help bacteria to compete
- can also replicate
Origins of Replication
DNA sequences along chromosome which initiate DNA replication
Telomere
DNA sequences located at the ends of the chromosome that prevent degradation and allow proper replication of chromosomal ends
Centromere
DNA sequences required for correct segregation of chromosomes by directing formation of the kinetochore in which mitotic spindle attaches
What happens to segregation if the chromosome did not have a centromere or two centromeres in two different locations?
would not split properly/evenly
Majority of eukaryotic cells are _______.
diploid
Only sexually-reproductive cells have a _____ genome
haploid
3 Reasons for DNA organized as chromosomes
- Chromosomes compact DNA so it will fit in cell/nucleus
- Chromosomal structure protects DNA from damage
- Chromosomes can be easily separated and transmitted to each daughter cell during cell division
Histones
basic positively charged proteins that DNA wind around
- prokaryotes do not have histones
- degree of compaction with histones can regulate gene expression and to what degree
Nucleosomes
histone octamer with approx. 147 bp of DNA wrapped around it
Histone H1 does what?
bind linker DNA and to nucleosomes to form 30 nm chromatin fibre
Euchromatin regions have
lower DNA compaction and genes are actively expressed
Heterochromatin regions
chromosomal regions of high DNA compaction where gene expression is silenced because enzymes involved in transcription are big and do not have access to genes as there is not enough space
Constitutive heterochromatin
DNA always highly compacted (centromeres and sub-telomeric regions).
Facutative heterochromatin
can switch to euchromatin depending on cell type and during development
- the correct genes need to be turned on at right time and place
Why aren’t nucleosides incorporated into DNA?
there are no phosphates to make phosphodiester bonds because nucleosides are base of sugar (no phosphate)
