University:
Felician University
Course:
BIO-405 | Genetics
Academic year:

2020
Views:

238

Pages:

1
Author:

Giselle A.

TRAnSCRIPTIOn Sections of a Gene Gene Expression A gene is a sequence of DNA which is transcribed into RNA • Most genes encode proteins, but some do not (e.g. tRNA) Transcription Factors Gene expression is regulated by proteins (transcription factors) that bind to specific sequences associated with a promoter • Activators bind enhancer sites (⬆︎︎ rate of transcription) • Repressors bind silencer sites (⬇︎︎ rate of transcription) A gene sequence has three main sections: • Promoter (transcription initiation site) • Coding sequence (the region transcribed) • Terminator (transcription termination site) As DNA is double stranded, only one strand is transcribed • The antisense strand is transcribed into RNA • The sense strand is not transcribed into RNA Transcription RNA polymerase binds to a promoter and unwinds DNA • It breaks the H bonds between complementary bases Nucleoside triphosphates bind to complementary bases • In RNA, uracil pairs with adenine instead of thymine RNA polymerase covalently joins the nucleotides together • The two extra phosphates are released (provides energy) Transcription occurs in a 5’ → 3’ direction (antisense strand) • At the terminator site, RNA polymerase is detached and the RNA sequence is released (and the DNA rewinds) Splicing Eukaryotic cells modify RNA after transcription has occurred • Modifications must occur to produce mature mRNA Non-coding regions within genes are removed (splicing) • Introns are non-coding regions in genes (intruding) • Exons are the coding regions of genes (expressing) 2 4 5 Exon 3 Alternative Splicing + Activator + Repressor Normal levels of transcription High levels of transcription Low levels of transcription Nucleosomes Nucleosomes also help regulate transcription in eukaryotes • Histones proteins have protruding tails that determine how tightly the DNA is packaged within nucleosomes Modifications to these tails alters the DNA packaging: • Acetylation makes DNA less tightly packed • Methylation makes DNA more tightly packed Cells package DNA differently according to genetic needs • Active genes remain unpackaged as euchromatin • Inactive genes are tightly packed as heterochromatin DNA Methylation DNA can also be directly methylated to change expression patterns of genes over time in response to external stimuli • Increased methylation = decreased transcription Twin Twin A B Twin Twin A B Exon 4 Exon 5 1 2 3 5 90 Number of differences 1 Exon 2 Basal factors Methylation Patterns in Twins Over Time Exons can be selectively removed to form different proteins from the same gene (this is called alternative splicing) Exon 1 The presence of regulatory proteins may be tissue-specific or may be influenced by chemical signals (e.g. hormones) 80 70 60 50 40 30 20 10 0 Protein A Protein B 3-year-old 50-year-old 3 years 50 years