Lecture 1 Flashcards
The Central Dogma of molecular biology describes the directional flow of information;
DNA -> RNA -> Protein
- DNA replication
- DNA
- RNA synthesis (Transcription)
- RNA
- Protein synthesis (Translation)
- Protein
What is Genetic Material?
DNA (and sometimes RNA) as genetic material
Genetic material must have certain characteristics….
What are the Key Characteristics of genetic material? 4
- Must contain COMPLEX information
- Must REPLICATE faithfully
- Must encode the PHENOTYPE
- Must have the capacity to VARY
How do we know that DNA is the genetic material?
– the Hershey-Chase Experiment (1952)
- T2 bacteriophage (DNA/protein) – infect bacteria
- how was phage genetic material transmitted to bacteria (protein or DNA?)
- Grew E.coli infected with T2 phage in either 32P or 35S, -> phage with radiolabelled DNA (32P) and proteins (35S).
- Used these phage to infect unlabelled
E.coli. - Then isolated the cells… only the 32P labelled phage produced progeny phage that were radiolabelled, showing DNA not protein was passed on
RNA can also be genetic material
– Fraenkel-Conrat & Singer (1956)
- Used tobacco mosaic virus (TMV) uses RNA.
- Mixed RNA and protein from different TMV strains
- The viral progeny were identical to the strain of the RNA donor, not the protein donor.
- Proved that the RNA, not the protein, is the genetic material in TMV
Information flow in biological systems – exceptions to the central dogma = 2
- Major information pathways
- DNA
- DNA REPLICATION - DNA POLYMERASE
- Information is transferred from one DNA molecule to another
- TRANSCRIPTION - RNA POLYMERASE
- information is transferred from DNA to an RNA molecule
-TRANSLATION - RIBOSOME
- information is transferred from RNA to a protein through a code that specifies the amino acid sequence. - SPECIAL INFORMATION PATHWAYS
DNA - RNA - PROTEIN
- RNA DEPENDENT RNA polymerase; …or to another RNA molecule
- REVERSE TRANSCRIPTION
in some viruses, information is transferred from RNA to DNA.
DNA STRUCTURE: Primary, Secondary, Tertiary
- Primary: nucleotide structure
- Secondary: DNA’s stable three-dimensional configuration
- Tertiary: packing of double-stranded DNA in chromosomes
DNA and RNA are composed of polynucleotide strands - what are they in a nucleotide?
Nucleotide:
- Deoxyribose
(DNA) or Ribose (RNA) sugar
- Phosphate group
- Nitrogenous base (purine: A and G, pyramidine: C and U/T)
DNA polynucleotide strands vs RNA polynucleotide strand
DNA polynucleotide strands
1. T-A have 2 HYDROGEN BONDS
- PHOSPHODIESTER linkage connects the 5’-phosphate group and the 3’-OH group of adjoining nucleotides.
- C-G pairs have 3 HYDROGEN BONDS
- The strands run in opposite direction; they are antiparallel
- DNA has deoxyribose sugar (no oxygen here).
RNA Nucleotide strand
- In RNA, uracil (U) replaces THYMINE (T)
- RNA has ribose sugar (an OH group here)
The 3D shape of the double stranded DNA molecules are?
What is the most common one?
- The 3- dimensional shape of the double stranded DNA molecules can vary
- The B-DNA structure is the predominant structure in the cell
- B-DNA helix = right-handed helix with approximately 10 BASES PER TURN.
Special Secondary Structures Can Form in DNA and RNA - What are they?
- Hairpin; loop + stem
- a hairpin consisting of a region of paired bases (with form the stem) and a region of unpaired bases between the complementary sequences (which form a loop at the end of the stem) - Stem = a stem with no loop
- Complex secondary structure; secondary structure of the RNA component of RNase P of E.coli.
- RNA molecules often have complex secondary structures.
DNA molecules are very long.
How are they packed into a tiny cell? 3.
Overrotated, Underrotated, Topoisomerases
Supercoiling results from strain produced when rotations are added to a relaxed DNA molecule or removed from it.
- Overrotated: positive supercoiling;
underrotated: negative supercoiling
- Overrotated: positive supercoiling;
- *Topoisomerases ADD/REMOVE ROTATIONS by BREAKING the NUCLEOTIDE STRANDS
- Most DNA FOUND in cells is NEGATIVELY SUPERCOILED
- SEPARATION of the TWO STRANDS of DNA easier during REPLICATION and TRANSCRIPTION
- SUPERCOILED DNA can be PACKED INTO a SMALLER SPACE THAN RELAXED DNA
Positive Supercoil vs Negative supercoil DNA process
- Relaxed circular DNA
***Supercoild DNA is overwound or underwound, causing it to twist on itself.
a.) Add two turns (OVERROTATE)
a.) Positive supercoil -
a.) Positive supercoiling occurs when DNA is overrotated; the helix twists on itself
b.) Remove tow turns (UNDERROTATE)
b.)Negative supercoil
b.)negative supercoiling occurs when DNA is underrotated; the helix twists on itself in the opposite direction
Within a cell, DNA is associated with specialised proteins that organise it and give it structure
PROKARYOTIC CELL
- DNA, Wrapped around histone protein, chromosome,
nucleotide: (irregularly-shaped region within the cell of a prokaryote that contains all or most of the genetic material.)
- plasmid:( is a small, circular, double-stranded DNA molecule that is distinct from a cell’s chromosomal DNA.)
EUKARYOTIC CELL
- Chloroplast DNA
- Nuclear DNA
- Mitochondrial DNA
Chromatin is a highly complex structure with several levels of organisation - WHAT ARE THEY?
7.
- At the simplest level, chromatin is a DOUBLE-STRANDED HELICAL STRUCTURE OF DNA
- DNA is complexed with HISTONES to form NUCLEOSOMES
- Each nucleosome consists of 8 HISTONE proteins around which the DNA wraps 1.65 times
- The nucleosomes fold TO PRODUCE A 30-NM FIBER…
- ..that FORMS LOOPS averaging 300 nm in LENGTH
- The 300-nm LOOPS are COMPRESSED and FOLDED to PRODUCE a 250-nm-wide fibre.
- TIGHT COILING of the 250-nm fibre produces the CHROMATID of a CHROMOSOME.
