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Introduction to Molecular Genetics (Lectures 35-37), :pencil2: No…
Introduction to Molecular Genetics
(Lectures 35-37)
The building block:
Nucleotides
Phosphate Group
Sugar
Deoxyribose (in DNA)
H on 2' Carbon
Ribose (in RNA)
OH on 2' Carbon
Nitrogenous Base
Purines (double ring)
Adenine (A)
Guanine (G)
Pyrimidine (single ring)
Thymine (T) (in DNA)
Uracil (U) (in RNA)
Cytosine (C)
Chain of linked nucleotides =
Nucleic Acid
(DNA or RNA)
DNA
(Deoxyribonucleic Acid)
Planar
Double-stranded
Sugar-phosphate backbones
Linked by Hydrogen-bonded nucleotides
AT has 2 hydrogen bonds
CG has 3 hydrogen bonds
Strands are anti-parallel
One is oriented 5' to 3'
DNA can ONLY be synthesized in the 5' to 3' direction
Need 3' OH group to power the synthesis
One is oriented 3' to 5'
DNA is wrapped around
Histones
to form
Chromatin
Euchromatin
consists of loosely packed "string with beads" DNA wrapped around histones
Accessible to transcription :fire:
Genes
"A specific sequence of DNA that has a specific start and stop sequence, and/or encodes for a specific RNA or polypeptide"
A gene requires a
promoter
prior to its start point in order to be trasnscribed
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Eukaryotes: 1 gene = 1 protein (monocistronic)
Prokaryotes: 1 gene = multiple proteins (polycistronic)
Gene mutations
Different from chromosomal abberations, which affect large areas of DNA
Many have little or no effect due to our diploid genome
"wild-type" versus "mutant" - wild-type is the most common form of a gene in the population
Causes
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Types
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:pencil2:DNA Repair System
s
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Gene Regulation
Constitutive/housekeeping genes
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Regulated genes (most genes)
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Local regulation: like Lactose on the Lac Operon (see to the left)
Global Regulation: like how cells always want to use Glucose first
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DNA
transcribed
into RNA
tRNA, rRNA, regRNA, hnRNA, mature mRNA
RNA does have a 2' structure that helps it function
RNA Polymerases (important for cell replication)
Have no proof-reading function, hence makes more errors (thus RNA viruses mutate more)
Drugs that target transcription
Actinomycin D is a cancer treatment that disrupts RNA Pol function in pro/eukaryotes by inserting itself in the DNA helix
"death cap mushrooms" bind RNA Pol II and are hepatotoxic
Rifampin (antibiotic) targets RNA polymerase of bacteria and is the main treatment for TB
Transcription starts when RNA pol recognizes the promoter
Prokaryotes: RNA pol alph (x2), beta 1, beta', and sigma
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Eurkaryotes: (1) Each RNA polymerase (remember there are 3) has
transcription factors
that recognize the promoter
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Post-transcriptional modifications
5' capping
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3' Poly-A tail
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Exonal splicing
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Translation/Protein Synthesis
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Histones consists of H2A/B, H3 (x2), and H4 (X2) cores
Heterochromatin
consists of VERY tightly packed histones
Inaccessible to transcription :red_cross:
DNA/Chromatin modifications
DNA Methylation
Mismatch repair
Repeated GATC sequences, methyl group added to A
As DNA replicates, the new strand is not methylated completely
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CpG islands
300-3,000 CG's usually near promoter, methylation silences these islands
Histone Modifications
Methylation
Makes chromatin TIGHT, suppressing transcription
Acetylation
Makes Chromatin LOOSE, allowing transcription
Chromatin is condensed in dividing cells to form
Chromosomes
DNA
Replication
Semi-conservative
Resulting strands consist of new strand and old strand
6 steps
DNA replication starts at a specific sequence called the
Ori site
DNA Helicase
unwinds the DNA by breaking the hydrogen bonds between nitrogenous bases
Single-stranded binding proteins (SSBPs)
keep helix open
RNA Primase
adds a small primer to the existing DNA since DNA Polymerase is unable to start making DNA without a primer
In EUKARYOTES, there is an additional
initiator DNA
added to the primer by
DNA polymerase alpha
DNA Polymerase III
adds deoxyribonucleotides complementary to the template strand (in PROKARYOTES)
One strand is synthesized continuously, called the
leading strand
One strand is synthesized discontinuously, called the
lagging strand
Chunks that the lagging strand is synthesized in are called
Okazaki fragments
DNA Pol III has proof-reading ability
Rotates the lagging strand 180 degrees so that it is 5' to 3'
DNA Polymerase I
then removes the RNA primer and replaces it with DNA
Thus, each DNA sequence is a little shorter by the length of the primer after each replication
Thus, after 50-100 divisions the sequence is too short and the cell dies (in
somatic cells
)
Germ cells
express
Telomerase
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DNA ligase
seals the gap by building a bond between the phosphate and sugar groups
DNA Polymerase
Epsilon
adds to leading strand and DNA plymerase
Delta
adds to lagging strand (in Eukaryotes)
DNA replication can be targeted by medications
Quinolone Antibiotics
Ciprofloxacin/nalidixic acid
Target
DNA gyrase
in Prokaryotes
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The Cell Cycle
Regulated by CYCLIN proteins
P53 (tumor-suppressor) protein regulates G1/S checkpoint
Mutated in 80% of human tumors
If DNA is intact - signals cell to move into S phase
If DNA if damaged (but fixable) - signals for cell to repair damage
If DNA damage is irreparable, signals cell to apoptose
Phases
G1
: growth and synthesis of proteins - where cells spends most of life (3 weeks in skin cells to 20 years in B cells)
S
: Synthesis phase where DNA is replicated
G2
: enzymes and other proteins are synthesized and distributed through the cell, sets groundwork for cell division
M
Cell division! Either mitosis (somatic cells) or meiosis (sex cells)
Mitosis (somatic cells)
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Meiosis (Ova and sperm)
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G0
: for cells (such as neurons) that never enter S phase and do not divide. Not technically IN the cell cycle, so they are considered to be in G0 forever
:pencil2: No phosphate group = NulceoSIDE
:warning: Yellow path connects main concepts