Gene Expression and Control Flashcards
Genetic Information
- DNA carries all the genetic information needed to build and maintain an organism
- Genetic information consists of base sequences
- Genes are subunits of that sequence
The Central Dogma of Biology
-Gene expression involves transcription (DNA to RNA) and translation (mRNA to protein)
Transcription
- A cell transcribes the base sequence of a gene into messenger RNA (nRNA)
- mRNA carries a protein-building message
- Base-pairing rules in DNA replication also apply to RNA synthesis in transcription, but RNA uses uracil in place of thymine
RNA and DNA Nucleotides
- RNA has an extra oxygen on sugar
- RNA has Uracil, DNA has Thymine (both go with A)
The Process of Transcription
- In transcription, RNA polymerase binds to a promoter in the DNA near a gene
- RNA polymerase assembles a strand of RNA by linking RNA nucleotides in the order determined by the gene
- The new mRNA is a copy of the gene from which it was transcribed
- Many polymerases can transcribe a gene region at the same time
Translation
- Process by which an amino acid chain is assembled in the order specified by an mRNA
- Translation requires the participation of two more types of RNA
- tRNA (transfer RNA)
- rRNA (ribosomal RNA)
- rRNA acts as an enzyme: catalyzes the formation of peptide bonds
mRNA and the Genetic Code
- The information in mRNA consists of sets of three nucleotides (codons) that form code for specific amino acids
- Sixty-four codons, most of which specify amino acids, constitute the genetic code
- 20 amino acids in proteins, most have more than one codon
rRNA and rRNA
- the translators
- Ribosomes and transfer RNA (tRNA) interact to translate an mRNA into polypeptide
- Ribosomes consist of two subunits of rRNA and structural proteins
Ribosomes
- During tranlsation, one large and one small ribosomal subunit (rRNA) converge as a ribosome on an mRNA
- rRNA reads the mRNA and acts as an enzyme to form peptide bonds between amino acids, assembling them into a polypeptide chain
tRNA
- rRNAs deliver amino acids to ribosomes in the order specified by mRNA
- each tRNA has two attachment sites
- An anticodon that can base-pair with a codon
- A site that binds to the kind of amino acid specified by the codon
Polysomes
- In cells making a lot of protein, many ribosomes may simultaneously translate the same mRNA
- Polysome: a cluster of ribosomes that are simultaneously translating an mRNA
Mutated Genes and Their Products
- Mutations are permanent changes in the nucleotide sequence of DNA, which may alter a gene product
- A mutation that changes a gene’s product may have harmful effects
- ex. Mutations that affect the proteins in hemoglobin reduce blood’s ability to carry oxygen
Types of Mutations
- Substitution mutations: type of mutation in which a single base-pair changes possible consequences for amino acid sequence
- Insertions and Deletions (Indels): Mutation in which one or more base pairs are lost or inserted into the DNA
Silent (aka. synonymous)
No change to amino acid sequence
Missense
- Codon with mutation codes for a different amino acid
- May be neutral, advantageous, or deleterious-harmful
Nonsense
- Codon with mutation codes for a STOP
- Usually deleterious
Frameshift
-Usually deleterious, but may be neutral or advantageous
What Causes Mutations?
- Most mutations result from unrepaired DNA polymerase errors during DNA replication
- Some result from transposable element activity, or form exposure to radiation or chemicals
- Small segment of DNA that can spontaneously move to a new location in a chromosome
IonizingRadiation Damage
-Ionizing radiation (x-rays) breaks chromosomes and produces free radicals
Nonionizing Radiation Damage
-Nonionizing radiation (UV light) results in thymine dimmers, which lead to skin cancer
Environmental Damage
- Some natural and synthetic chemicals cause mutations in DNA
- ex. Cigarette smoke transfers small hydrocarbon groups to bases in DNA causing mispairing during replication
