Lecture 23 & 24 From gene to protein (I)

Flashcards covering key vocabulary and concepts from the lecture notes.

Gene expression

The process by which DNA directs RNA synthesis (transcription ) and protein synthesis (translation)

Central dogma

Summarizes the flow of information in cells: DNA -> RNA -> proteins

Codons

A series of non-overlapping, three-nucleotide words that specify one amino acid.

Garrod’s hypothesis

Proposed that genes dictate phenotype through their control of enzyme production & catalysis of specific chemical reactions

Beadle and Tatum

Provided 1st evidence that genes encode enzymes using Neurospora crassa (bread mold) model system.

Beadle and Tatum’s conclusion

Each mutated gene must normally encode that enzyme.

Refinement of “One gene, one enzyme” hypothesis

Later research refined “one gene - one enzyme” hypothesis to “one gene - one polypeptide” because many proteins consist of more than one polypeptide, each encoded by a separate gene.

Codons

Units of 3 nucleotides. Different combinations of nucleotide triplets specify one of twenty different amino acids.

Genetic code

Genetic instructions for a polypeptide chain are written in DNA as a series of three nucleotide codon “words”

Codon reading

During translation, codons in mRNA are read in the 5’->3’ direction

Frameshift mutation

Insertion or deletion of a single base pair will cause a “frameshift” mutation that causes the genetic message to be misread.

Nirenberg and Matthaei

Determined sequence of first codon, UUU that coded for the amino acid phenylalanine.

Start codon (AUG)

Not only codes for the amino acid methionine but also indicates the start of translation.

Stop codons (UAA, UAG, UGA)

Do not code for amino acids but instead signal STOP (i.e. terminate translation).

Open reading frame (ORF)

A sequence of DNA that contains an ATG methionine start codon followed in frame by 33 or more codons before reaching a a stop codon. A predicted protein-encoding gene.

Promoter

The DNA sequence where RNA polymerase binds.

Template strand

RNA is transcribed from only one strand.

mRNA

Encode protein

tRNA, rRNA, snRNA

Structural RNA; functions during protein translation

Transcription vs Replication

DNA product (ATCG) versus RNA product (AUCG) (dNTP vs NTP)

Promoters

DNA sequences that define start site of transcription

Cis-acting elements

DNA promoter elements serve as attachment sites for DNA binding proteins that regulate initiation of transcription.

5’ mG cap

Modified form of guanine, the 5’ methyl guanosine cap, is added the 5’ end of the pre-mRNA molecule. Helps protect mRNA from exonucleases. Also functions as an “attach here” signal for ribosomes.

3’ poly A tail

At the 3’ end, an enzyme adds 50 to 250 adenine nucleotides. In addition to protecting from nucleases, also facilitates export of mRNA from the nucleus.

RNA splicing

The removal of noncoding segments, introns, which lie between coding segments, exons, and the subsequent splicing together of the exons.

Introns

Intervening sequences

Exons

Expressed sequences

Spliceosomes

Ribonucleoprotein complex. Protein + several RNAs (small nuclear RNA (snRNA))

Alternative splicing

Allows for a greater variety of proteins than could be predicted by just the number of genes in the genome.

Polycistronic mRNA

An mRNA that codes for more than one protein – commonly found in bacteria but not in eukaryotes.