Chapter 10: Molecular genetics (Gene regulation (How does a cell know when…
Chapter 10: Molecular genetics
The search for inheritable material
Bacterial transformation: Bacteria alter their genetic makeup by absorbing foreign DNA molecules.
Hershey and Chase
Proved that DNA carried genetic material and not proteins by tagging bacteriophages with radioactive isotopes 32P and 35S.
X-ray crystallography showed that DNA was a double helix.
Watson and Crick
Discovered a model of DNA as double helix.
Meselson and Stahl
Proved that DNA replicated semi-conservatively with N15 and N14.
Structure of DNA
Two complementary strands running in opposite directions (anti-parallel)
5 carbon sugar (deoxyribose), phosphate, nitrogenous base (A, T, C, G)
A bonds with T. C bonds with G.
Bases held together by hydrogen bonds
DNA replication in Eukaryotes
Semi-conservative replication - one new strand one old strand to create an identical copy of DNA.
, which also double checks the strand to ensure no mistakes are made.
DNA Helicase unzips the DNA strands by breaking H bonds.
Each unzipped strand becomes a template strand for replication, which later forms complementary base pairs.
Telomeres: Prevent the possible loss of genes at the ends of chromosomes.
Structure of RNA
Made of nucleotides.
Made of ribose, phosphate and nitrogenous bases (A, U, C, G).
3 types of RNA: mRNA (carries messages from DNA in the nucleus to cytoplasm), tRNA (Carries amino acids to ribosome), and rRNA (makes up a ribosome).
Codons: triplet of nucleotides. Anticodons: complementary to codons.
Process: DNA makes RNA.
Occurs in nucleus.
Triplet code in DNA is transcribed into a codon sequence in RNA (remember T becomes U).
Initial transcript is processed by enzymes.
Introns (non coding regions for protein) are removed by enzymes.
Exons (coding regions) are then pieced back together, forming final transcript.
mRNA that leaves the nucleus is hence shorter than the original RNA sequence.
mRNA sequence converted to amino acid sequence (polypeptide chain).
Occurs in ribosome.
tRNA carries amino acids in the cytoplasm to the ribosome.
These anticodons bind to codons in mRNA in triplets.
Some tRNA molecules can bind to two or more different codons.
How does a cell know when to turn a gene on and off?
Operon: Cluster of functional genes and the switches that turn them on and off.
Inducible operon: Usually off until triggered on by an environmental factor.
Repressible operon: Usually turned on unless actively turned off because it is temporarily unneeded.
Parts of the operon: Promoter, operator, TATA box.
Promoter: the binding site of RNA polymerase, and it must bind to DNA in order for transcription to take place. It's an 'on' switch.
Operator: the binding site for the repressor. It's an 'off' switch.
TATA box: Helps RNA polymerase bind to promoter.
Mutations: Changes in genetic material. They are spontaneous.
Base-pair substitution. One nucleotide converts to another.
It can result in severe damage of tissues, or lead to beneficial or no change at all because some triple codons can result in the same amino acid produced.
Insertion or deletion
Deletion: Loss of one letter. Insertion: Adding one letter to the DNA sequence.
Both result in a frameshift: Sequence becomes unreadable.
Are alterations in chromosome number/ sturcture.
Aneuploidy: Abnormal number of chromosomes. Leads to diseases like down syndrome.
Polyploidy: Having a complete extra set of chromosomes. (Mostly in plants).
Aneuploidy and polyploidy both result from nondisjunction: homologous fail to separate during meiosis.
The Human Genome (Organisms genetic material)
Made of 3 billion base pairs of DNA and 20 000 genes.
97% of DNA does NOT code for proteins. Other sequences control gene expression, interrupt genes, or never get transcribed.
Genetic engineering and recombinant DNA
Inserting functioning genes into non functioning ones.
Could cure cystic fibrosis and cell anemia.
Engineered microbes can degrade oil and harmful chemicals.
Genetically modified animals like salmon could be released and spread genes to wild species.
This could then adversely affect people who eat them.
Cut DNA at specific recognition sites to form restriction fragments.
Also known as molecular scissors.
For the analysis of DNA.
Separates large DNA molecules based on their rate of movement through agarose gel in an electric field.
Polymerase chain reaction
Rapidly copies and amplifies DNA for study.