Genetics Basic Genome: All the genetic information (DNA) in a cell (includes genes and non-gene regions) Chromosome(s): Cellular structure(s) of DNA Gene: A segment of DNA found on a chromosome (or plasmid) that codes for a protein or RNA molecule Genotype: A cell's entire collection of genes Phenotype: A cell's entire collection of proteins (the products of expressed genes)
DNA (Deoxyribonucleic acid) 1. Long strands made up of 4 nucleotides Adenine (A) Thymine (T) Guanine (G) Cytosine (C) 2. 3. 4. 5. Sugars (deoxyribose) and phosphates form the "backbones" of the DNA strands Two DNA stands pair and twist into characteristic double helix structure Nucleotides are paired within the helix DNA pairing rules: AT CG
DNA Replication: Flow of Genetic Information from One Generation of Cells to the Next Generation Information in DNA sequence (existing molecule) Information in DNA sequence (new molecule) 1. 2. The order of nucleotides in DNA constitutes the cell's genetic information Genetic information, the chromosome(s), must be copied before cells divide to ensure all genetic information is present in all cells
Process of DNA replication 1
1.
Enzymes gradually pull apart the two DNA strands of the double helix and each strand is precisely copied using the nucleotide pairing rules (A-T; G-C) Ex: A-G-C-G-C-A-T T-C-G-C-G-T-A
2. 3. 4. 5. 6.
DNA polymerase enzymes do the copying and then proofread to check for and correct mistakes All together, at least 30 enzymes and other proteins are involved Speed of copying is approximately 750 nucleotides per second (1 million nucleotides copied in 1333 sec (22 min)) Origin of replication: Specific place on a chromosome where DNA replication begins (this is NOT random) On a circular bacterial chromosome, replication moves in both directions improving speed
Transcription and Translation: Flow of Genetic Information within a Cell Genetic information is used to create proteins for the cell in the processes of transcription and translation (together called gene expression) Information in DNA sequence Information in mRNA sequence Protein molecule RNA (Ribonucleic acid) 1. Long strands made up of 4 nucleotides Adenine (A) Uracil (U) Guanine (G) Cytosine (C) 2. 3. 4. Sugars (ribose) and phosphates form the "backbones" of the RNA strands RNA is usually single stranded (not a double helix) RNA serves several roles in the cell: mRNA (messenger RNA): A copy of DNA sequence information made by the process of transcription that interacts with the ribosome during translation rRNA (ribosomal RNA): Structural component of ribosomes (forms a scaffold for the ribosomal proteins) tRNA (transfer RNA): Delivers amino acids to ribosomes during the process of translation 2
Transcription Information in DNA sequence (a gene) Information in messenger RNA (mRNA) sequence 1. RNA polymerase enzymes read the DNA sequence and copy (transcribe) that information into mRNA using specific pairing rules: G in DNA C in mRNA C in DNA G in mRNA T in DNA A in mRNA A in DNA U in mRNA (T is not found in RNA, U takes the place of T) Ex: DNA: T-G-C-C-A-A-T mRNA: A-C-G-G-U-U-A
How does RNA polymerase know where genes start and stop on a chromosome? RNA polymerase binds to special DNA sequences called promoters to begin transcription RNA polymerase stops transcribing and releases the completed mRNA when strand reaching special DNA sequences called terminators
Translation Information in mRNA sequence Protein molecule (polymer of amino acids) 1. 2. 3. 4. 5. The "language" of nucleotides found in mRNA is "translated" by ribosomes and transfer RNA (tRNA) into the "language" of amino acids (protein) When an mRNA strand is bound to a ribosome, tRNA molecules match groups of 3 mRNA nucleotides called codons to their corresponding amino acids Genetic code: The list of all possible 3 nucleotide sequences matched to the corresponding amino acids As amino acids are matched to codons, the ribosome joins the amino acids together into a chain The amino acid chain is a protein
3
How does a ribosome know where to start and stop translation? Start codon: AUG (methionine codon) almost always signals the start Stop codons: UAA, UAG, or UGA (nonsense codons) signals the stop Ex: mRNA: AUG-CCC-GUA-GGU-UAA Protein: Met-Pro-Val-Gly
Regulation of Bacterial Gene Expression A cell conserves energy by making only the proteins it needs at a given time Repression: Regulatory mechanism that keeps a gene from being transcribed (gene is "turned off") Induction: Regulatory mechanism that turns on transcription of a gene
Alteration of Bacterial Genetic Material Mutation: A permanent change in an organism's DNA sequence
Alteration of genetic material can occur by 1. Spontaneous mutation A. B. C. D. 2. Mistake by DNA polymerase (rate varies, but average is approximately 1 mistake per 1 billion nucleotides copied) Mutagens such as ultraviolet (UV) light or certain chemicals can increase the rate of mutation A change in DNA sequence Change in mRNA Sometimes change in amino acid sequence Altered proteins can be good, bad, or neutral for the cell
Recombination New DNA segment acquired by cell chromosome is broken DNA segments taken out and/or inserted Strands rejoined New DNA sequence 4
Bacteria can acquire DNA from other bacteria by 1. Transformation A. B. 2. Dead "donor" bacteria release DNA into their surroundings This DNA is taken up by "recipient" bacteria
Conjugation A. B. DNA is transferred directly from a living "donor" bacterium to a "recipient" bacterium Requires cell-to-cell contact (pili or sticky surface proteins bring cells together)
3.
Transduction A. B. Bacteriophages (viruses that infect bacteria) sometimes mistakenly package bacterial "donor" DNA into their capsids Bacterial "donor" DNA is transferred when the bacteriophage infects another "recipient" bacterial cell
Mobile DNA: Plasmids and Transposons Plasmids A. B. C. Small circular DNA molecules (independent of chromosome) Plasmids often carry bacterial toxin or antibiotic resistance genes Plasmids transfer from donor cell to recipient cell by conjugation, transformation, or transduction
Transposons 1. 2. Small linear segments of DNA with ability to recombine out of a DNA strand and recombine into another ("jumping genes") Transposons often carry bacterial toxin or antibiotic resistance genes
5
3.
Transposons can transfer from donor cell to recipient cell: Transposon jumps from donor chromosome to plasmid Plasmid is transferred via conjugation, transformation, or transduction Transposon jumps from plasmid to recipient chromosome
6