3 models proposed
Conservative: the two parental DNA strands are back together after replication. One daughter molecule contains both parental DNA strands, and the other daughter molecule contains DNA strands of all newly-synthesized material.
Semiconservative: the two parental DNA strands separate and each of those strands then serves as a template for the synthesis of a new DNA strand. The result is two DNA double helices, both of which consist of one parental and one new strand.
Dispersive: the parental double helix is broken into double-stranded DNA segments which act as templates for the synthesis of new double helix molecules. The segments then reassemble into complete DNA double helices, each with parental and progeny DNA segments interspersed.
When?
DNA replication takes
place in the S phase of interphase
3 stages of DNA replication
1. Initiation
A portion of the DNA double helix is unwound
Expose the bases for new base pairing
2. Elongation
Two new strands of DNA are made using parent DNA as a template
The new DNA molecules – each composed of one strand of parent DNA and one strand of daughter DNA – re-form into double helices
3. Termination
The replication process is completed
The two new DNA molecules separate from each other and re-form the double helix.
Replication machine is dismantled
Correction of DNA....After the DNA strand is copied, DNA Polymerase II runs the length of the strand looking for “mismatched” nucleotides (“proofreading”)
If a mismatch is found, DNA polymerase II removes the incorrect nucleotide and inserts the correct one. Ligase seals the bond. (“correction”)
Telomeres: Sections of highly repetitive sequences of “non coding” DNA at the ends of our chromosomes (protective cap)
3 main stages
Intiation: RNA Polymerase, binds to promoter region of DNA to be transcribed (TATAA box). RNA Polymerase opens double helix and transcription begins. This occurs both in prokaryotes and eukaryotes
Elongation: For each gene, only one strand of DNA is transcribed
The strand that is transcribed is called the antisense strand or template strand. RNA polymerase complex works its way along the DNA molecule, synthesizing a strand of mRNA that is complementary to the template strand of DNA
RNA polymerase can only add nucleotides to the 3' hydroxyl end of the previous nucleotide. As soon as the RNA polymerase complex starts moving along the DNA, a second RNA polymerase complex can bind to the promoter region and start synthesizing another mRNA
Termination: mRNA is synthesized until the end of the gene is reached
Terminator Sequence - a specific nucleotide sequence in the template strand serves as a signal to stop transcription RNA Polymerase complex reaches this sequence and detaches from the DNA strand
The newly synthesized mRNA is released
DNA helix re-forms
Post-transccriptional modifications
1. 5' cap (7-methylguanosine) is added
2. Poly-A polymerase enzyme adds a string of ~ 200 adenine ribonucleotides to the 3' end of primary transcript (Poly-A tail)
3. Removal of Introns whihc are the non coding areas
3 main stages
Intiation: mRNA binds to the mRNA binding site on the small subunit of a ribosome
The ribosome recognizes and binds to the 5' cap, on the mRNA
The initiator “met-tRNA” (carrying amino acid, methionine) binds to the START codon (AUG) on the mRNA
The large subunit joins and creates an active ribosome
Elongation consists of three steps that are repeated:
1. Addition of aa-tRNA to A site – complementary to next codon on mRNA
2. Formation of a peptide bond between amino acids – polypeptide passed to tRNA in A site
3. Ribosome complex shifts along the mRNA by one codon, in the 51 to 31 direction (translocation)
The ribosome will eventually reach a STOP codon on the mRNA
The three stop codons are: UAG, UGA, UAA
Genetic code
1) The genetic code is redundant
2) The genetic code is continuous
3) The genetic code is nearly universal
Post-translational modification: Most proteins go through an extensive series of processing steps, collectively called post-translational modification, before they are ready to go to work in a cell.
Molecular chaperones speed folding of the protein. Folding determines a protein's shape and therefore its function.
GENE EXPRESSION:
the transfer of genetic information from DNA to RNA to protein
3 key players
rRNA : Located in the cytoplasm
Involved in protein synthesis (translation)
Associated with proteins in the ribosomes
Directs the translation of mRNA into proteins
The place where mRNA’s and tRNA’s interact to assemble amino acids into polypeptide chains
tRNA: Located in the cytoplasm
Involved in protein synthesis (translation)
Links the codons on mRNA to the corresponding amino acid for protein synthesis
mRNA: Transcript of DNA
a single strand that is complementary to the DNA strand however EACH “T” is substituted with a “U”
Involved in protein synthesis (transcription and translation)
Carries genetic information from the nucleus to the cytoplasm
Provides the information that determines the amino acid sequence of a protein
About DNA:
Deoxyribonucleic acid and double stranded
DNA is a polymer, made up of subunits (monomer) called nucleotides
Nucleotides are made of:
*use these to identify nucleotides
Phosphate group
5-carbon sugar
Nitrogenous base (A,C,T,G)
Watson and Crick discovered the modern model of DNA: double helix
Each strand in the double helix is made up of a:
-Backbone with alternating phosphate and sugar groups bonded together by phosphodiester bonds
-Centre of the helix is made of nitrogen bases (A,T,C,G) bonded together by weak hydrogen bonds
Complimentary base pairings: Purines only pair with Pyrimidines (A pairs w/ T, G pairs w/ C) Hydrogen bonds within complementary base pairings has a difference…the picture shows this:
Three hydrogen bonds required to bond G & C
Two hydrogen bonds are required to bond A & T
About strands
The two strands of DNA are antiparallel...
The strands run in opposite directions, with each end of a DNA molecule containing the 3’ end of one strand and the 5’ end of the other strand
Prokaryote vs eukaryotes
DNA of Prokaryotic Cells:
DNA is circular and double stranded - called a plasmid
Chromosomal DNA packed tightly near the center of the cell in a specific region called the nucleoid
DNA of Eukaryotic Cells:
DNA is linear and double stranded
DNA contained within the nucleus
chromosomal mutations
Deletion
One or more genes are removed from the chromosome
Duplication
One or more genes is duplicated in a chromosome
Inversion
A segment of a chromosome is broken and then re-inserted in the opposite direction
Reciprocal Translocation
Section of one chromosome breaking and fusing to another chromosome
single gene mutation
Substitution: Replaces a base pair with a different base pair of nucleotides
Silent Mutations
Change in nucleotide sequence has no effect on amino acid sequence of a protein
Mutation does not change amino acids
Mis-Sense Mutations
Mutation changes the amino acid sequence of a protein
Non-Sense Mutation
Mutation that shortens a protein by introducing a stop codon
Framshift: Reading frame is changed and the resulting amino acid sequence of a protein
Deletion
one or more nucleotides are removed from a piece of DNA
Insertion
one or more nucleotides are added to a piece of DNA
Mutations
Permanent change in the nucleotide sequence of a cell’s DNA
All mutations are copied during replication and passed onto daughter cells
mutation can be beneficial, harmful or neutral
Genetic disease: Huntiington's disease, Crohn's disease, cystic Fibrosis
lac operon
Lactose if present
-Allolactose binds to LacI repressor protein
-LacI changes conformation, unable to bind lac operator
-RNA polymerase can bind to lac promoter
-Transcription occurs
Lactose if ABSENT
-LacI repressor protein binds to lac operator
-Prevents RNA polymerase from binding to the promoter
-No transcription
trp operon
Trp absent
-Repressor does not bind to operator
-Transcription occurs
Trp present
-When tryptophan reaches a certain level, tryptophan binds to a repressor protein
-Changes conformation of repressor and allows repressor to bind to the trp operator
-No transcription