topic 4: gene expression Flashcards
what is genotype and phenotype?
genotype: the DNA sequence of an organism
phenotype: the observable physical characteristics
what is the central dogma of genetic information flow?
DNA (transcription) —> RNA (translation) —> protein
what is phenotype the result of?
protein production or gene expression
what is gene expression? how many stages does it include?
- it is DNA-directed protein synthesis
- includes 2 stages:
(1) transcription
(2) translation
what molecule acts as an intermediate between genes (genotype) and the proteins which they code for (phenotype)?
RNA
what is transcription?
- the DNA-directed synthesis of RNA
- transcription produces messenger RNA (mRNA)
what is translation?
- the synthesis of a polypeptide which occurs under the direction of mRNA
- translation occurs on ribosomes
what are the differences in gene expression between prokaryotes and eukaryotes?
prokaryotes:
- no nuclear envelope to separate transcription and translation: both take place in the cytoplasm
- mRNA produced by transcription is immediately translated without further processing (no RNA processing)
- translation can start before the end of transcription
eukaryotes:
- the nuclear envelope separates transcription from translation
- transcription: takes place in the nucleus
- RNA processing: (takes place in the nucleus) eukaryotic DNA transcripts are modified to yield the mRNA (final product of transcription)
- translation: takes place in the nucleus
what is pre-mRNA?
also known as primary transcript, is the initial RNA transcript
[only present in eukaryotes]
information flow from gene to protein is based on?
- a triplet code consisting of 3 consecutive nucleotides known as codon
- each gene is transcribed in a complementary mRNA containing the nucleotide triplets (codons)
- each codon specifies the addition of a specific amino acid
- the gene is translated into an amino acid chain forming a polypeptide chain
which DNA strand of the 2 is used as a template for the synthesis of the complementary RNA strand?
the 3’ to 5’ DNA strand (known as the template strand)
in what direction is RNA synthesized?
RNA polymerase synthesizes RNA in a 5’ - 3’ direction using the template DNA strand
in what direction are codons on the mRNA read during translation?
they are read in the 5’ - 3’ direction by the ribosomes
if you’re given the double stranded DNA figure unlabeled, which strand is used as a template for the mRNA strand synthesis?
the 3’ - 5’ strand
why is the DNA coding strand named so?
it is called the coding strand because it has the exact same sequence as the mRNA strand produced (with T replaced with U)
mRNA IS ALWAYS PRODUCED IN THE _______ DIRECTION
5’ - 3’
how many codons are there?
- 64 codons
- 61 code for amino acids
- 3 are “stop” signals to end translation
what is the start codon?
AUG (methionine)
what are the characteristics of the genetic code?
- it is redundant: more than 1 codon can encode for a specific amino acid
- it is not ambiguous: each codon can specify only 1 amino acid
list the names of the 20 amino acids
1. ala
2. arg
3. asn
4. asp
5. cys
6. gln
7. glu
8. gly
9. his
10. ile
11. leu
12. lys
13. met
14. phe
15. pro
16. ser
17. thr
18. trp
19. tyr
20. val
- alanine
- arginine
- asparagine
- aspartic acid
- cysteine
- glutamine
- glutamic acid
- glycine
- histidine
- isoleucine
- leucine
- lysine
- methionine
- phenylalanine
- proline
- serine
- threonine
- tryptophan
- tyrosine
- valine
give an example to how genes can be transcribed and translated after being transplanted from one species to another
- genetically modified bacteria
- bacteria can be programmed by the insertion of human genes to produce certain human proteins for medical use
- example: INSULIN
what do ribonucleotides (RNA nucleotides) consist of?
- a nitrogenous base (A, G, C, U)
- a pentose sugar (ribose)
- a phosphate
what are the differences between DNA and RNA?
- DNA is double-stranded, RNA is single-stranded
- the sugar in DNA is deoxyribose, the sugar is RNA is ribose
- the base URACIL is present in RNA to replace thymine
what are the major types of RNA and their functions?
- mRNA (messenger): is translated into proteins
- rRNA (ribosomal): part of ribosome structure
- tRNA (transfer): transfers amino acids to the growing polypeptide chain during translation
where does transcription and translation take place in prokaryotes and in eukaryotes?
- prokaryotes: both in the cytoplasm (no nuclear envelope to separate)
- eukaryotes: transcription in the nucleus, and translation in the cytoplasm
describe transcription
- DNA-directed synthesis of RNA
- the first stage of gene expression
- catalyzed by RNA polymerase
- RNA synthesis similar to DNA replication, except that uracil substitutes thymine
- starts with binding of RNA polymerase to DNA region called promoter
- no primer is needed in this case, unlike DNA replication
what are the molecular components of transcription?
- promoter: DNA sequence where RNA polymerase attaches, located at the 5’ end of the gene (upstream)
- terminator: the sequence signaling the end of transcription in bacteria (different in eukaryotes)
- transcription unit: the stretch of DNA that is transcribed
how many types of RNA polymerase do bacteria have, and how many do eukaryotes have?
- bacteria: 1 RNA polymerase
- eukaryotes: 3 types of RNA polymerase
which RNA polymerase is involved in transcription in eukaryotes?
SOS
RNA polymerase II
what is the function of RNA polymerase?
- synthesizes RNA from the template DNA strand
- adds nucleoside triphosphate (NTP) only in 5’ - 3’ direction
- does not require the presence of an RNA primer for adding NTPs
what is the function of each type of eukaryotic RNA polymerases?
SOS
- RNA polymerase I: rRNA synthesis
- RNA polymerase II: mRNA and snRNAs synthesis
- RNA polymerase III: tRNA and small RNAs
what are the stages of transcription?
- initiation: RNA polymerase II binds to the promoter
- elongation: adds NTPs in 5’ - 3’ direction
- termination: the RNA transcript is released from DNA when it reaches the termination of transcription
what type of direction is translation?
- transcription is unidirectional
- DNA template is read 3’ - 5’ always
- mRNA is produced 5’ - 3’ always
what are the additional features of eukaryotic promoters in initiation of transcription?
- general transcription factors: proteins that mediate the binding of RNA polymerase II to the promoter and the initiation of transcription
- TATA box: a specific sequence within a promoter responsible for the assembly of the initiation complex
what is the transcription initiation complex (in eukaryotes)?
transcription factors + RNA polymerase II, bound to a promoter
what are the transcription initiation stages in eukaryotes?
- general transcription factors bind to the TATA box within the promoter
- RNA polymerase II is recruited and binds to the promoter
describe termination of transcription in bacteria
- RNA polymerase stops transcription at the end of the terminator
- mRNA is translated without being processed
- translation can start before the termination of transcription
describe termination of transcription in eukaryotes
- RNA polymerase II transcribed a polyadenylation signal (AAUAAA)
- the RNA transcript is released 10-35 nucleotides after that signal
in prokaryotes, translation is immediately started after transcription termination, what occurs in eukaryotes?
- the primary RNA transcript or pre-mRNA is modified
- after processing, the mature mRNA is transferred to the cytoplasm for translation
what does RNA processing include? (3 modifications)
- addition of 5’ Cap : a modified guanine (G) nucleotide at the 5’ end
- addition of a poly-A tail at the 3’ end: (50-200 adenine residues)
- RNA splicing: removal of certain segments from the primary transcript
