Transcription and Gene Expression 2.7 & 7.2

Essential Idea: Genetic information in DNA can be accurately copied and can be translated to make the proteins needed by the cell. 2.7 DNA replication, transcription and translation Understandings: Transcription is the synthesis of mRNA copied from the DNA base sequences by RNA polymerase Skills: Deduce the DNA base sequence for the mRNA strand

Essential Idea: Information stored as a code in DNA is copied onto mRNA. 7.2 Transcription and Gene Expression Understandings: Transcription occurs in a 5’ to 3’ direction   Nucleosome help to regulate transcription in eukaryotes Eukaryotic cells modify mRNA after transcription Splicing of mRNA increases the number of different proteins an organism can produce Gene expression is regulated by proteins that bind to specific base sequences in DNA The environment of a cell and of an organism has an impact on gene expression Application: The promoter as an example of non-coding DNA with a function Skill: Analyze changes in the DNA methylation patterns

I. Central Dogma of Biology First stated by Crick – 1953 DNA is used to make RNA which is used to make proteins

II. Transcription Genes – sections of DNA that code for proteins Only 1 strand is transcribed 1. Sense strand – NOT transcribed - carries same sequence as resulting mRNA (but with Ts instead of Us) 2. Anti-sense strand – Transcribed - complimentary to sense strand and to mRNA

C. 3 segments of the gene 1. Promoter – short sequence of bases that is not transcribed, but indicates where transcription should start a. RNA Polymerase – unzips DNA and attaches at promoter, creating a transcription bubble

2. Transcription unit – section of the DNA that codes for RNA a 2. Transcription unit – section of the DNA that codes for RNA a. Free-floating RNA nucleotides pair up with complimentary DNA b. New nucleotides always add on to the 3’ end (build 5’ 3’) c. RNA Polymerase bonds RNA nucleotides together

3. Terminator – sequence of nucleotides that cause the mRNA to detach from the DNA a. In eukaryotes, transcription goes past the terminator for a while and then detaches – now have pre-mRNA

III. Post-transcription modification Sections of the pre-mRNA are removed Spliceosomes – composed of small nuclear RNAs (snurps) Results in introns being removed, and exons being joined to form mature mRNA

B. Alternative Splicing - different pieces may be introns at different times, and exons may be rearranged - means 1 gene can code for more than one protein

IV. Gene expression A. Methylation – methyl groups (CH3) packed around genes to block transcription 1. Example – Barr bodies in females (1 X chromosome inactivated)

Protein regulation 1. Transcription factors – help RNA polymerase bind to promoter 2. Transcription activators – loop DNA to bring activator closer to promoter

3. Repressors/Silencers – bind to DNA to block transcription

Environmental factors can enhance or suppress gene expression 1. Pollutants – stimulate expression of genes that cause asthma 2. Fur color in Himalayan rabbits 3. Hydrangea color and soil pH