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)
Exons can be selectively removed to form different proteins
from the same gene (this is called alternative splicing)
Topic 7.2: TRAnSCRIPTIOn!
Sections of a Gene
A gene is a sequence of DNA which is transcribed into RNA
• Most genes encode proteins, but some do not (e.g. tRNA)
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)
1
2
4 5
1
2 3
5
Alternative
Splicing
Protein A
Exon 1 Exon 2 Exon 3 Exon 4
Exon 5
Protein B
Gene Expression
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)
The presence of regulatory proteins may be tissue-specific
or may be influenced by chemical signals (e.g. hormones)
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
Basal factors + Activator + Repressor
Normal levels
of transcription
High levels
of transcription
Low levels of
transcription
3-year-old
50-year-old
Twin
A
Twin
B
0
10
20
30
40
50
60
70
80
90
3 years 50 years
Number of differences
Twin
A
Twin
B
Methylation Patterns in Twins Over Time
