Gene Expression Flashcards
What did Archibald Garrod (1902) suggest?
Suggested that genes dictate phenotype through enzymes
Alkaptonuria
- hereditary disease
- caused by the absence of an oxidase enzyme that breaks down homogentistic acid (alkapton)
- no enzyme = build up of homogentistic acid,
which becomes oxidized and turns black
What does the phrase “One Gene - One Enzyme” Refined mean?
- Not all proteins are enzymes, yet their synthesis depends on specific genes
- Research showed that many proteins are composed of several polypeptide subunits, each of which has its own gene
- Idea restated as “one gene - one polypeptide” hypothesis. However we know that even this is limited
Gene expression includes what two processes and a brief decsription of each?
- Transcription: A DNA strand provides a template for the synthesis of a complementary RNA strand
mRNA, tRNA, rRNA, snRNA, other RNA types - Translation: mRNA is used to determine the amino acid sequence of a polypeptide
Translation occurs at ribosomes
What is the location of transcription?
Eukaryotes - Transcription occurs in the nucleus
Prokaryotes - Transcription occurs in the cytoplasm
What are the 4 key types of RNA in a cell and their roles?
- mRNA, messenger RNA: dictates the amino acid (AA) sequence of a polypeptide
- tRNA, transfer RNA: transport amino acids to the ribosomes during translation, and transfer the AA to the growing polypeptide chain
- rRNA, ribosomal RNA: Along with ribosomal proteins, compose ribosomes (which are location for translation)
- snRNA, small nuclear RNA: along with proteins, forms complexes that are used in eukaryotic RNA processing
What are some key features of transcription?
Transcription - Mechanism that converts messages encoded by DNA into a complementary RNA copy (i.e. mRNA)
- RNA polymerase synthesizes this RNA
- Only one of the two DNA strands is copied template (antisense) strand = transcribed non-template coding strand = not transcribed
- RNA polymerase adds ribonucleotides to the growing 3’ end of an mRNA chain
- Synthesis proceeds in 5’ to 3’ direction
- Thus the template is “read” 3’ to 5’ direction
The gene equals what two parts?
Gene = Promoter + Transcription unit
What % of DNA in humans codes for genes?
2% of DNA in humans codes for genes (thus over 98% is non-coding)
What are the 3 stages of transcription?
- Initiation
- Elongation
- Termination
Transcription initiates at what location while the terminator signal the end of transcription?
Promoter (TATA)
What are key aspects of transcription initiation?
- Genes are identified by promoters, specific nucleotide sequences
Promoter region includes TATA Box upstream of transcription start site - Transcription factors bind and attract RNA polymerase
RNA polymerase binds to promoter region - Bacteria contain a single type of RNA polymerase
- Eukaryotes contain three RNA polymerases (I, II, and III)
- DNA is unwound to expose template strand, TFs released
- RNA polymerase begins RNA synthesis at transcription start site
- RNA nucleotides are paired complementary to template in 5’ to 3’ direction
What are key aspects of transcription elongation?
- RNA polymerase continues moving along and unwinding template
- It continues to synthesize and elongate the mRNA by adding complementary RNA nucleotides in 5’ to 3’ direction
What are key aspects of transcription termination?
- At the terminator, RNA polymerase releases from the template
- Pre-mRNA transcript is released
- DNA rewinds to its double helix (more transcripts will be made)
- Only the transcription unit of the gene is copied to mRNA
True or False: DNA replication occurs in the nucleus, while transcription and translation occur in the cytoplasm?
FALSE: Transcription occurs in nucleus
A segment of DNA has the nucleotide sequence of 5’ GCATTAGAC 3’. What would be the sequence of its complementary mRNA ?
5’ GUCUAAUGC 3’
A = U
T = A
G = C
C = G
A template strand of DNA has the sequence 5’ AATACGG 3’, which mRNA sequence will be transcribed?
5‘ CCGUAUU 3’
True or False: DNA Pol III synthesizes the leading and lagging strands in DNA replication, while RNA polymerase synthesizes RNA in transcription
True
What is the role of the 5’ Cap end?
- 5’ Cap - Modified form of guanine is added, next to 5’ UTR
- Helps protect mRNA from hydrolytic enzymes and assists in ribosome attachment
What is the role of the 3’ end (Poly (A) Tail)?
- Poly(A) Tail - 50 to 250 adenine nucleotides added
- Inhibits hydrolysis of mRNA end
- Directs export of mRNA from the nucleus
What are the two types of RNA splicing and a brief description?
- Introns: “Intervening Sequences”; NONCODING sections
- Lie between coding regions
REMOVED - Exons: “Expressed Sequences”; CODING regions
- Spliced together: single uninterrupted message
- Translated into amino acid sequences plus the 5’ and 3’ untranslated (UTR) regions
Why do genes have introns?
Several possible functions:
- Some introns regulate gene activity
Alternative RNA splicing:
- More than one polypeptide can be synthesized from the same gene, depending on how the pre-mRNA introns are spliced out
Exon Shuffling:
- Exons control particular protein domains (area of particular shape) – new combinations for evolution
Genetic codes consist of series of information in blocks called?
Codons
What is a triplet code?
Triplet code - 4^3 = 64 codons
- Unambiguous: each codon can only code for one amino acid
What is the open reading frame (ORF)?
Refers to the continuous sequence of nucleotides in DNA and RNA molecules that can be translated into a protein
