Transcription

Central Dogma of Biology• Information flows from DNA RNA Proteins• Think of it as exchanging money in a different

country: proteins cannot be built directly from DNA

• Three main processes involved:– Replication (copies DNA)– Transcription: DNA RNA– Translation: RNA Protein

RNA vs. DNA

• 3 main differences:– Sugar is called

ribose (DNA has deoxyribose)–Uracil (U) instead

of thymine• pairs with

adenine (A)– Single-stranded

RNA Molecules

• 3 major types– Messenger RNA (mRNA): codes for proteins– Ribosomal RNA (rRNA): part of the ribosome– Transfer RNA (tRNA): helps read the mRNA code

to build proteins during the process of translation

What is Transcription?

• Transcription is a process that uses a portion of DNA to make a complementary RNA strand– Similar to replication

• Just one gene at a time is transcribed, not the whole DNA molecule

Transcription Process

• 3 basic steps, just as in replication:– Part of DNA double helix unwinds (initiation)– Complementary RNA bases are assembled

(elongation)– Completed RNA strand detaches from DNA

(termination)

Initiation

• RNA polymerase finds the beginning of a gene• A transcription complex made of RNA

polymerase and other enzymes begins to unwind part of the DNA strand

Elongation

• RNA polymerase uses the DNA strand as a pattern to build a complementary RNA strand– C pairs with G– A pairs with U

• RNA strand hangs off the DNA strand, and DNA helix zips back together

Termination

• After the whole gene has been transcribed, the RNA strand detaches completely from the DNA.

• The transcription complex made of RNA polymerase and other proteins falls apart.

Replication vs. TranscriptionWhere does the process take place?

Enzymes involved Step 1 Step 2 Step 3

Replication

Transcription

Replication vs. TranscriptionWhere does the process take place?

Enzymes involved Step 1 Step 2 Step 3

Replication Nucleus

Transcription Nucleus

Replication vs. TranscriptionWhere does the process take place?

Enzymes involved Step 1 Step 2 Step 3

Replication Nucleus

DNA helicase,DNA polymerase

Transcription Nucleus

RNA polymerase

Replication vs. TranscriptionWhere does the process take place?

Enzymes involved Step 1 Step 2 Step 3

Replication Nucleus

DNA helicase,DNA polymerase

DNA strand is unzipped

Transcription Nucleus

RNA polymerase

DNA strand is unzipped (initiation)

Replication vs. TranscriptionWhere does the process take place?

Enzymes involved Step 1 Step 2 Step 3

Replication Nucleus

DNA helicase,DNA polymerase

DNA strand is unzipped

Complementary bases attached; whole DNA strand

Transcription Nucleus

RNA polymerase

DNA strand is unzipped (initiation)

Complementary RNA bases attached; only one gene (elongation)

Replication vs. TranscriptionWhere does the process take place?

Enzymes involved Step 1 Step 2 Step 3

Replication Nucleus

DNA helicase,DNA polymerase

DNA strand is unzipped

Complementary bases attached; whole DNA strand

Two identical DNA molecules

Transcription Nucleus

RNA polymerase

DNA strand is unzipped (initiation)

Complementary RNA bases attached; only one gene (elongation)

mRNA strand floating free from DNA (termination)

Transcription Practice• List 3 differences between DNA and RNA.• Write the following DNA sequence: GTTCACTAG• Write the complementary DNA strand.• Circle one DNA strand. Simulate transcription by writing its

complementary RNA sequence.• Does the RNA strand more closely resemble the DNA strand from

which it was transcribed or the complementary strand that wasn’t used? Why?

• What is the main enzyme involved in transcription? What is its function?

• List 3 types of RNA and briefly describe each one.