Molecular Genetics PDF

10/18/24 Molecular genetics (5.5, 16.1–16.3, 17.1-17.2 ) How is genetic information: 1. stored 2. replicated 3. translated 1 DNA (deoxyribonucleic acid) Characteristics: 1) carries a large amount of information (in the arrangement...

10/18/24 Molecular genetics (5.5, 16.1–16.3, 17.1-17.2 ) How is genetic information: 1. stored 2. replicated 3. translated 1 DNA (deoxyribonucleic acid) Characteristics: 1) carries a large amount of information (in the arrangement of monomers). 2) it can be accurately copied. 3) is stable under a range of environmental conditions. 4) the information it carries is used code for amino acid sequences. 2 Structure of DNA DNA is a type of Nucleic acid polymers made from nucleotides Nucleotides are composed of three parts: 1) 5 carbon sugar 2) Phosphate group 3) Nitrogen base (variable structure) 3 1  10/18/24 Adenine nucleotide NAD+ ATP 4 The sugar of one nucleotide bonds to the phosphate of another nucleotide. The sugar and phosphate always have the same chemical structure. Sugar phosphate backbone 5 In DNA there are 4 possible nitrogen bases: Adenine (A) Thymine (T) Cytosine (C) Guanine (G) The sequence of nitrogen bases in DNA is a source of information. 6 2  10/18/24 DNA normally exists as two separate strands linked by hydrogen bonds. Each strand runs in opposite directions. 5’ end (5 prime) 1’ (prime) 5’ 2’ 4’ 3’ 3’ end (3 prime) 7 Hydrogen bonds 5’ 3’ Phosphate is at the Five’ end 3’ 5’ 8 Hydrogen bonds form between the nitrogen bases of the 2 strands. Cytosine only bonds with guanine 1 ring 2 rings Thymine only bonds with adenine Complimentary base pairing 9 3  10/18/24 Knowing the sequence of nitrogen bases in one DNA strand lets you know the sequence in the other strand. 10 Double strands of DNA are twisted around one another into a double helix. 11 RNA (ribonucleic acid ) Another type of nucleic acid Structure Similar to DNA except: 1) The 5 carbon sugar is different 2) Thymine is not present. It is replaced by uracil (U). Uracil is complementary to adenine (A). In RNA: A-U, C-G 3) Does not necessarily form a double strand. 4) It can also act as an enzyme. It is therefore less stable than DNA. 12 4  10/18/24 DNA Replication One strand of DNA acts as a template for the construction of a new strand Semi conservative replication: each new set of double DNA strands contains an old strand 13 During DNA synthesis, individual nucleotides A T have a triple phosphate. C G The removal of two G C DNA polymerase phosphates provides energy for the addition T A of the nucleotide to the P P P P growing DNA strand. P C 2 P 14 DNA Replication 1. Double strands of DNA are pulled apart. 2. Complimentary nucleotides are synthesized into new DNA strands. 3. Hydrogen bonds re-form between nitrogen bases. 15 5  10/18/24 Enzymes are responsible for the 1st two stages of replication DNA helicase: breaks hydrogen bonds between double strands. DNA helicase hydrogen bonds 16 DNA helicase moves in one direction only. In eukaryotes, replication occurs at many locations on the chromosome at the same time. 17 DNA polymerase III: Joins nucleotides together in their proper sequence by reading the sequence of the original strand. Can only synthesize new strands in one direction: 5’ to 3’ 3’ DNA polymerase III 5’ 3’ 5’ 18 6  10/18/24 DNA polymerase III: Joins nucleotides together in their proper sequence by reading the sequence of the original strand. Can only synthesize new strands in one direction: 5’ to 3’ 3’ 5’ 5’ 3’ 3’ 5’ 19 DNA polymerase III requires a primer (short sequence of RNA nucleotides) to start a new DNA strand. 3’ TA GA T AA AT TA 5’ GG CC AT 20 Primers are synthesized at specific locations on the chromosome by primases 3’ TA Primase GA AU T AA CU AT U TA 5’ GG CC Primer (RNA) AT 21 7  10/18/24 Once the primer made, DNA polymerase III make a complimentary in a 5’ to 3’ directions 3’ DNA A AT U polymerase III AG UA T TA UC AA 5’ G GT CC Primer (RNA) AT 22 Once the primer made, DNA polymerase III make a complimentary in a 5’ to 3’ directions 3’ A AT U AG UA T TA UC A T TA AA 5’ GG TT CC Primer (RNA) AT Complimentary DNA 23 3’ 5’ 5’ 3’ RNA 3’ DNA 5’ One strand (the leading strand) is made continuously… 24 8  10/18/24 …the other strand is made in sections 3. DNA In the leading strand, polymerase I replaces the DNA polymerase III follows the helicase primer with DNA but doesn’t 1. As new connect the sections of DNA 2. At each primer, sugar/ are expose, DNA polymerase III phosphate to primase makes makes the complimentary previously 4. DNA ligase RNA primers DNA up to the next formed connects primer strand. sugar/ phosphates 25 Protein synthesis (How genetic information is used) DNA codes for amino acid sequences. 4 different nitrogen bases in DNA code for 20 different amino acids in proteins. Groups of three bases (codons) code for a single amino acid. 26 examples: TAC codes for the amino acid methionine AAA codes for the amino acid phenylalanine 27 9  10/18/24 There are 64 different possible combinations of 3 nucleotide bases Some nitrogen base combinations code for the same amino acid. Example: GCA GCT all code for arginine GCC GCG 28 2022-10-13, 9:45 AM Codon table of amino acids 3´ Gly Phe Leu Glu (G) (F) (L) Ser Asp (E) (S) C A G U C A G 1st mRNA nucleotide (D) A GU U C Tyr C A (Y) 2nd mRNA nucleotide Ala (A) G U G U G U 3rd mRNA nucleotide C A A C C A G U C A G U Cys (C) Amino acid Val (V) A C U U G U G C A G Trp (W) 3´ 5´ 3´ G U Arg (R) A G U C C U A C G A Leu (L) Ser (S) G A A C C U Lys (K) C C A A Asn U G A CU U G A C U G Pro (P) (N) G G A U His C U G A C Thr (H) (M) (T) Gln Arg (Q) Met Ile (I) (R) 3´ Start Stop 29 https://upload.wikimedia.org/wikipedia/commons/7/70/Aminoacids_table.svg Page 1 of 1 Overview of protein synthesis In eukaryotes TRANSCRIPTION RNA PROCESSING TRANSLATION 30 10  10/18/24 Stages of protein synthesis 1. Transcription Copying of one DNA strand into an RNA strand (messenger RNA). Occurs inside the nucleus. 2. RNA Processing (eukaryotes only) None coding sections of RNA are removed. 3. Translation Converting RNA sequence into an amino acid sequence. Occurs outside the nucleus. 31 Transcription (in prokaryotes) RNA polymerase terminator DNA promoter DNA Initiation 32 Elongation RNA Termination mRNA 33 11  10/18/24 Transcription Takes place in the nucleus. Promoter: DNA sequence where transcription starts. Terminator: DNA sequence where transcription ends. RNA polymerase: Enzyme that carries out transcription 34 Steps in Transcription 1. Initiation. RNA polymerase attaches to DNA at the promoter region. No primer is required. DNA strands are pulled apart by RNA polymerase. 2. Elongation. RNA nucleotides are bonded together based on the DNA sequence. 35 3. Termination. Transcription occurs until a terminator sequence is encountered. At the terminator, the RNA polymerase detaches from the DNA and releases the RNA strand. The resulting RNA strand is known as messenger RNA or mRNA. The nucleotide sequence is complimentary to the transcribed DNA strand. 36 12  10/18/24 Transcription in Eukaryotes RNA polymerase does not bind directly to the promoter region. A number of proteins (transcription factors) bind the promoter region. RNA polymerase then binds to the transcription factors 37 Start of gene DNA End of gene mRNA m detached mRNA 38 RNA processing DNA contains nucleotides that code for amino acids, mixed with DNA that does not. Coding and non coding nucleotides are transcribed. Introns: mRNA that does not code for amino acids Exons: mRNA that codes for amino acids (they are expressed). Spliceosomes can recognize introns and remove them from mRNA 39 13  10/18/24 Translation Occurs in the cytoplasm on ribosomes: made of RNAs and proteins. Aligns mRNA and transfer RNA Has binding sites for tRNA molecules. Amino acids (polypeptide) tRNA mRNA 40 transfer RNA (tRNA) Two attachment sites: 1. amino acid attachment site. 2. anticodon- A set of three nucleotides complimentary to mRNA nucleotides. The type of amino acid attached to a tRNA depends on its anticodon sequence. 41 Aminoacyl-tRNA-synthetases Join amino acids to tRNAs based on the anticodon ATP sequence of the tRNA. Requires ATP Every type of amino acid has a different version of the synthetase enzyme 42 14  10/18/24 Steps in translation 1. Ribosomes, mRNA and tRNA bind together. This always occurs at same nucleotide sequence (the start codon). Met AUG Met P GTP GDP 43 2. A 2nd tRNA molecule binds to the 6. The process is ribosome. Its repeated until a stop anticodon is codon sequence is complimentary to the reached. mRNA in the binding GTP site of the ribosome. GDP P 5. The second tRNA is GDP P shifted from the second to GTP the first binding site. 3. The amino acid from the first tRNA is removed from the 4. The first tRNA is released tRNA and bonded to from the ribosome. the amino acid on the second tRNA. 44 Termination of translation 7. When a stop codon 8. The release factor causes the amino acid enters the ribosome’s sequence to detach from the tRNA, the mRNA and tRNA binding site, a tRNA to detach from the ribosome and the 2 parts release factor moves of the ribosome to separate into the binding site. Release factor GTP Stop codon GDP P 45 15  10/18/24 46 Definition of a gene RNA polymerase terminator DNA promoter DNA Initiation 47 Genes can be considered sections of DNA. -they are always located in the same region of a chromosome Alleles make proteins that perform the same function but have a different DNA code. 48 16  10/18/24 In eukaryotes there are additional steps in transcription 49 In eukaryotes more than one protein can be made from a section of DNA by splicing different sections of mRNA together coding regions (exons) non coding regions (introns) Transcription RNA splicing Each mRNA will code for a different protein 50 So, genes aren’t necessarily a easily defined sections of DNA 51 17  10/18/24 Molecules that disrupt translation The structure of ribosomes in prokaryotes and eukaryotes is different. Common antibiotics (eg. streptomycin) prevent prokaryote ribosomal subunits from joining. Some plants produce a molecule (ricin) that prevents the subunits of eukaryotic ribosomes from joining. 52 18