Genetics AP Biology

The Discovery of DNA Structure Rosalind Franklin: x-ray diffraction photographs of DNA Rosalind Franklin: x-ray diffraction photographs of DNA Watson & Crick built model based on x-ray diffraction photos Watson & Crick built model based on x-ray diffraction photos

DNA Structure Deoxyribose sugar backbone, alternating with phosphate groups Deoxyribose sugar backbone, alternating with phosphate groups Nitrogenous bases held together by hydrogen bonds: Adenine, Thymine, Cytosine and Guanine Nitrogenous bases held together by hydrogen bonds: Adenine, Thymine, Cytosine and Guanine Arranged in a double helix Arranged in a double helix Chargaff’s base pairing rules: A-T; G-C Chargaff’s base pairing rules: A-T; G-C

Anti-parallel nature of DNA 5’ 3’ 3’ 5’

DNA Replication Makes an exact copy Origins of Replication Double helix separates at Origins of Replication Each strand serves as the template for a new strand Semi-conservative replication Semi-conservative replication

DNA polymerase builds new strand 5’ to 3’ (adds nucleotides onto 3’ end (OH)

DNA replication occurs in a 5’ to 3’ direction: Leading strand Okasaki fragments Other side is constructed in 5’ to 3’ fragments called Okasaki fragments : Lagging strand

From Gene to Protein: DNA contains information of amino acid sequence in proteins

Transcription Synthesis of RNA using the DNA as a template: contains gene’s protein building instructions RNA Structure: Contains ribose sugar, Base thymine is replaced with uracil, single stranded Types of RNA: Ribosomal RNA (rRNA) – manufactured in nucleolus Transfer RNA (tRNA) Messenger RNA (mRNA)

Single stranded RNA molecules fold into secondary structures Characteristic secondary structure of tRNA Uracil

RNA Editing RNA molecule made by transcription – premRNA RNA molecule made by transcription – premRNA Contains stretches of bases – “introns” that do not code for proteins Contains stretches of bases – “introns” that do not code for proteins Introns are removed and coding sequences “exons” are spliced together Introns are removed and coding sequences “exons” are spliced together

Role of RNA Splicing “One gene, One polypeptide” hypothesis: a gene of DNA codes for one protein. “One gene, One polypeptide” hypothesis: a gene of DNA codes for one protein. Several proteins can be manufactured from a single gene due to alternative splicing Several proteins can be manufactured from a single gene due to alternative splicing

Translation tRNAs bring amino acids to the ribosome to assemble proteins

Translation – the specifics Amino acids joined to tRNAs by a specific Amino acids joined to tRNAs by a specific aminoacyl-tRNA synthetase Ribosomal Structure: Ribosomal Structure: P site – holds tRNA carrying amino acid the growing polypetide chain P site – holds tRNA carrying amino acid the growing polypetide chain A site – holds the tRNA carrying the next amino acid to be added to the chain A site – holds the tRNA carrying the next amino acid to be added to the chain E site – discharged tRNAs leave through this site E site – discharged tRNAs leave through this site

Translation – the specifics continued : mRNA, initiator tRNA, ribosomal subunits assemble. Initiator tRNA sits in P site and A site is vacant Initiation: mRNA, initiator tRNA, ribosomal subunits assemble. Initiator tRNA sits in P site and A site is vacant Elongation: Elongation: Codon recognition: H bonding between tRNA and codon in A site. Requires elongation factors & GTP Peptide bond formation: an rRNA molecule catalyzes formation of peptide bond growing chain from P site to the new amino acid in the A site Translocation: the tRNA in the A site (now has polypetide chain) moves to P site. Blank tRNA in P site moved to E site. Termination: stop codon – release factor binds to A site & causes the addition of water – hydrolyzes tRNA from protein chain.

DNA Mutations - Effects on Translation Point mutations Point mutations Insertions and Deletions Insertions and Deletions Frameshift mutations Frameshift mutations Base pair substitution Base pair substitution Missense – still codes for amino acid Missense – still codes for amino acid Nonsense – prematurely codes for a stop codon Nonsense – prematurely codes for a stop codon

Control of Transcription in Eukarytoic cells Transcription is most often the result of a chemical signal Transcription is most often the result of a chemical signal Some genes are constitutively active – meaning they are always turned on in a cell Some genes are constitutively active – meaning they are always turned on in a cell Chemical signals often in the form of hormones – either steroidal or peptide Chemical signals often in the form of hormones – either steroidal or peptide Peptide hormones cannot diffuse into the cell and bind hormone receptors on the cell membrane Peptide hormones cannot diffuse into the cell and bind hormone receptors on the cell membrane Steroid hormones are able to diffuse easily into the nucleus where they bind steroid hormone receptors that function as transcription factors (transcription factors “turn on/off” the transcription of a gene Steroid hormones are able to diffuse easily into the nucleus where they bind steroid hormone receptors that function as transcription factors (transcription factors “turn on/off” the transcription of a gene

Peptide hormones Binding of peptide hormone to cell membrane receptor Activation of cellular signaling molecules that carry signal to the nucleus Change in transcription of a gene Examples of peptide hormones: insulin, adrenaline, vasopressin, etc.

Steroid hormone signaling Examples of steroid hormones: estrogen, testosterone, glucocorticoids

Intronic RNA Once thought to be completely nonfunctional Once thought to be completely nonfunctional Within the last 5-7 years: microRNAs Within the last 5-7 years: microRNAs Now found that microRNAs can bind to newly transcribed mRNA and target them for degradation Now found that microRNAs can bind to newly transcribed mRNA and target them for degradation Another way that the cell controls what proteins are made Another way that the cell controls what proteins are made

Can be synthesized in the laboratory and can be used to shut down production of a protein – possible therapeutic uses