central dogma of molecular biology. gene expression how does a cell control the timing and choice of...
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Gene Expression
• How does a cell control the timing and choice of synthesizing a particular protein?
• A gene is expressed when it is transcribed (a.k.a. turned on or activated)
Ch. 10, Sect. 4 – Protein Synthesis
• RNA – single stranded– Uracil instead of
thymine– Ribose instead of
deoxyribose– Shorter than DNA– Folds to form some
double-stranded regions (A-U, C-G)
Types of RNA
• Messenger RNA – carries DNA’s message
• Ribosomal RNA – makes up ribosomes
• Transfer RNA – transfers amino acids to a growing protein
Transcription – in the nucleus
1. Initiation – RNA polymerase binds to promoter on DNA; DNA unwinds
2. Elongation – RNA polymerase moves along the DNA, adding complementary free-floating nucleotides
3. Termination – RNA polymerase reaches STOP signal and falls off
Ch. 11, sect. 1: Turning genes on
• Promoter Region (on DNA) = RNA polymerase binding site (TATA box)
• Transcription Factors (proteins) – help to position RNA polymerase
• Transcription process results in a pre-mRNA strand complementary to gene sequence
Pre-mRNA RNA
• Pre-mRNA contains introns and exons– introns are not translated– exons are translated
• Spliceosomes cut out the introns and bond the exons together– ** exons are the parts that exit the nucleus
as mRNA **
Genetic Code
• Base sequence determines the protein made
• Each 3-nucleotide sequence in mRNA = codon– Codes for one particular amino acid (there are
20) or the STOP signal
• tRNA has a strategically placed anticodon (complementary to codon)– each tRNA holds one particular amino acid
Translation – in cytoplasm
• Initiation – tRNA+methionine and ribosome attach to mRNA
• Elongation – Ribosome moves 3 base pairs down the mRNA and docks the next tRNA+AA [peptide bond]
– polypeptide grows as the ribosome moves along mRNA
• Termination – ribosome reaches STOP– All components separate
From DNA to a Physical Trait
• Scientists mapped DNA
• Knowing location and sequence of gene can help with cure or treatment
Ch. 12 sect. Genetic Disorders
• Carcinogen = cancer-causing agent – chemical (components in tobacco smoke)
– radiation (UV rays, X-rays)
– viral (Human papilloma virus)
• Mutation = change in base sequence of DNA– germ [passed on to offspring], autosomal
[affects organism], lethal [death pre-birth], beneficial [phenotypic change helps organism]
Chromosomal Mutations – During Cell Division
• Deletion
• Inversion
• Translocation
• Nondisjunction
• Point Mutations – occur on a single gene– Substitution (wrong nucleotide) may or may
not change the corresponding amino acid
– Deletion or insertion results in a frameshift
• Codons are grouped incorrectly
Gene Mutations – During DNA Replication or transcription
Prenatal Genetic Testing
• Looks for gene markers • Amniocentesis – analyzes
cells from amniotic fluid
• Chorionic Villi – tests fetal cells
- Good genes put into a virus- Virus injected into fetus- Virus infects fetus, giving it the good gene