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GENETICS LECTURES - Coggle Diagram
GENETICS LECTURES
LECTURE 1
- Genetics is a branch of biology that examines heredity and variation
MOLECULAR COMPOSITION OF DNA
- linear molecule
- made up of repeating units (nucleotide)
- 4 nucleotides in DNA (ADENESINE, CYTOSINE, THYMINE, GUAMINE)
- Nucleotides are composed of 3 components
- pentose (5- carbon) sugar
- one or more phosphate groups
- nitrogenous base
KEY FEATURES OF DOUBLE HELIX
- Sugar- phosphate backbone
- each DNA strand has polarity
- DNA strands are anti-parallel to each other
- all nucleotide pairs are complementary base paired
RNA
- contains the sugar ribose
- has the nitrogenous base uracil.
- Smaller
- single stranded
TYPES OF RNA
- mRNA: produced during transcription and carries info from DNA to ribosome
- tRNA: transfers a single amino acid to ribosome during protein synthesis
- rRNA: provides mechanism for decoding mRNA into amino acids
- snRNA: involved in RNA splicing, regulation of transcription factors and maintaining telomeres
- ncRNA: RNA that isn't translated into protein
4 BASES IN DNA
- PURINES: DOUBLE RING
- Adenine (A)
- Guanine (G)
- PYRIMIDINES: SINGLE RINGS
- Thymine (T)
- Cytosine (C)
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HYDROGEN BONDS + BASE STACKING
- C + G has 3 hydrogen bonds
- A + T has 2 hydrogen bonds
- Hydrogen bonds are relatively weak
- BASE STACKING contributes as well to DNA stability
- the non polar, flat surfaces of bases stack as tightly as possible with each other, then group together away from water molecules
PHOSPHODIESTER BONDS: two hydroxyl groups in phosphoric acid react with hydroxyl groups on other molecules to form two ester bonds,
LOCATION OF GENETIC MATERIAL IN EUKARYOTES
- Nucleus
- Mitochondria (chloroplasts in plants)
HUMAN NUCLEAR GENOME
- 23 pairs of chromosomes
- each cell containing 2 complete sets of chromosomes
HUMAN MITOCHONDRIAL GENOME
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LECTURE 2
- During replication the 2 strands of parental DNA separate
- Each parental strand serves as templates for the synthesis of new daughter strands
- 3 hypothetical models for replication:
semiconservative consisting of one parent strand and one daughter strand
conservative two new synthesised daughter strands, leaving parental duplex intact
dispersive New DNA duplexes consisting of interspersed sections of parental and daughter strands
- end result: 2 new doubles helices with same base sequence as original
PROTEINS INVOLVED IN REPLICATION
1. DNA Topoisomerase travels slightly ahead of replication fork to remove knots caused by helicase
2. DNA helicase travels along one DNA strand in the 5' to 3' direction to separate DNA strands
3. single-strand binding proteins coat DNA strands to prevent them from reforming a double helix
ACTION OF DNA POLYMERASE
- Covalently links nucleotides from a primer made by DNA primase
PROOFREADING
- each incoming nucleotide, is temporarily held in place by Hydrogen bonds that form between base and new nucleotide and base in template strand.
- improper hydrogen bonds can form, with a result that an incorrect nucleotide is attached to new DNA strand
- if error is detected DNA polymerase activates a cleavage function, removes incorrect nucleotide and inserts correct one
- proofreading reduced frequency of errors
TELOMERES
- each end of eukaryotic chromosome is capped by a repeating sequence called telomere
- series of short nucleotides sequences at repeated at ends of chromosomes
- telomere at 3' end doesn;t have a complementary strand - this is 3' overhang
-telomere is slightly shortened each round of replication, but is restored by enzyme telomerase
TELOMERASE
- telomerase binds to DNA repeat sequence
- Telomerase synthesises a 6-nucleotide repeat sequence
- Telomerase moves the 6 nucleotides to the right and begins to make another repeat
- primase makes RNA primer near end of telomere and the DNA polymerase synthesises a complementary strand in the 5' to 3' direction until RNA primer is eventually removed
REPLICATION OCCURS IN 5' TO 3'
- Replication is catalysed by DNA polymerase
- all DNA polymerases synthesise a new DNA strand from existing templates
- also correct replication mistakes
- the 3'OH of growing strand attacks the high energy phosphate bond of incoming nucleotide, providing energy to drive the reaction
CONTINUOUS/DISCONTINUOUS REPLICATION
- DNA strands separate at origin (creating 2 replication forks)
- primers needed to initiate DNA synthesis**
- synthesis begins with leading strand in direction of replication fork
- Lagging strand has 1st OKAZAKI FRAGMENT is made in opposite direction
- leading strand elongates and 2nd OKAZAKI FRAGMENT is made
- Leading strand continues to elongate and a 3rd okazaki is made while 1st and 2nd connect together
DNA SYNTHESIS AND RNA PRIMERS
- DNA polymerase cannot begin new strands on it's own; requires exisitng piece of DNA/RNA to elongate
- DNA primase synthesises a short piece of RNA complementary of DNA templateRNA PRIMER
- DNA polymerase elongates primer, by adding successive DNA nucleotides to 3' end of growing strand
- note all new DNA strands have a short stretch of RNA at their 5' end
- when growing fragments come into contact with a primer, another DNA polymerase complex REMOVES RNA primer, replacing it with DNA nucleotides
- once replacement is complete DNA ligase joins fragments together
MOLECULAR STRUCTURE OF EUKARYOTIC CHROMOSOMES 1. FORMATION OF NUCLEOSOMES
- DNA wraps around histones to form nucleosome
-forms zig-zag shape
- shortens length of DNA 7-fold
2. ARRANGEMENT OF NUCLEOSOME INTO 30-NM FIBER
- shortens length another 7-fold
3. RADIAL LOOP DOMAINS
- interaction between 30nm fibers and nuclear matrix
- each chromosome located in discrete territory
- compaction in chromosomes is not uniform
- when preparing to divide, chromosomes become more compacted
- metaphase chromosomes highly compacted
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LECTURE 3
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- DNA -> transcription 2. RNA -> translation 3. PROTEIN
TRANSCRIPTION
- produces transcript (RNA copy) of gene
- mRNA specifies amino acid sequence of polypeptide
- occurs in nucleus
TRANSLATION
- process of synthesising specific polypeptide on a ribosome using mRNA template
- occurs in ribosome
RNA PROCESSING
- intervening step
- pre mRNA is processed into mRNA
PRIMARY TRANSCRIPT
- INTRONS: transcribed but not translated
- EXONS: sequence found in mature mRNA
RNA PROCESSING STEPS
- addition of 5' CAP
- Addition of poly-A tail
- splicing - removal of introns
CAPPING
- 5' cap needed for mRNA to exit nucleus and bind ribosome
- protects mRNA from degradation
POLY A TAIL
- 100-200 adenine nucleotides added 3' end
- increases stability and lifespan in cytosol
RNA SPLICING
- SPLLCEOSOME : removes introns precisely
- recognises specific sequences in introns
- 5' splice site donor
- branch site
- 3' splice site acceptor
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3 STAGES OF TRANSCRIPTION
- INITIATION.
- sigma factor enables RNA polymerase
- this recognises promoter region
- stage completed when DNA strands separate near promoter to form open complex
- ELONGATION
- sigma factor is released
- RNA polymerase slides along DNA in open complex to synthesise RNA (5' - 3' direction)
- Uracil substituted for thymine
- behind open complex, DNA rewinds back to helix
3.TERMINATION
- RNA polymerase reaches terminator sequence
- causes both RNA polymerase and RNA transcript to dissociate from DNA
- 2 phosphates of incoming ribonucleotides are released as pyrophosphates
3 FORMS OF RNA POLYMERASE
- RNA POLYMERASE I & II : transcribes genes for rRNA, tRNA and other small RNA genes
- RNA POLYMERASE II: transcribes mRNA
- requires several general transcription factors to initiate transcription
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TRANSCRIPTION SIGNALS IN PROTEIN CODING
- PROMOTER: signals beginning of transcription
- TERMINATOR: signals end of transcription
- REGULATORY SEQUENCE: site for binding of regulatory proteins which influence rate of transcription
- TRANSCRIBED REGION part of region contains information that specifies amino acid sequence
LECTURE 4
PROTEIN
- each protein has a unique order and number of amino acids in a polypeptide chain
LEVELS OF PROTEIN STRUCTURE - PRIMARY STRUCTURE is sequence of amino acids
- SECONDARY STRUCTURE results from interactions of nearby amino acids
- TERTIARY STRUCTURE 3D shape of protein
- QUATERNARY STRUCTURE results from interactions of protein subunits
ALPHA HELI
- secondary structures results from hydrogen bonding in backbone
- in alpha helix poly peptide backbone is twisted tightly in right handed coil (3.6 amino acids per turn)
- helix stabilised by hydrogen bonds
- chemical properties of side chains determine where alpha helix is positioned in folded protein
BETA SHEET
- Polypeptide folds back and forth on itself, forming pleated sheet that is stabilised between carbonyl groups in one chain and amide groups in another chain by hydrogen bonds
- adjacent sstrands can run parallel or anti-parallel
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GENETIC CODE
- 64 codons (3- ribonucleotide code words)
- 1 start codon (AUG) specifies methionine, signifies start of protein encoding sequence in mRNA
- there are 3 stop codons (UGA, UAA, UAG) signals endd of a protein coding sequence
Aminoacyl tRNA synthetase enzymes catalyse attachment of amino acid to tRNA resulting in aminoacyl tRNA
RIBOSOMES
- discrete sites for tRNA binding and polypeptide synthesis
- A SITE: aminoacyl site
- P SITE: peptidyl site
- E SITE: exit site
PROCESS OF TRANSLATION
- Initiation: first tRNA and ribosomal subunits assemble
- elongation: synthesis from start codon to stop codon
- Termination: complex disassembles at stop codon releasing completed polypeptide