Initiation

Central Dogma first step

The first step of the central dogma is the conversion from DNA into RNA, a process known as transcription.

Location of transcription in eukaryotes

In eukaryotes, transcription occurs in the nucleus because DNA does not leave the nucleus.

Location of transcription in prokaryotes

In prokaryotes, which lack a nucleus, transcription occurs in the cytoplasm.

Phases of transcription

Transcription has three phases: initiation, elongation, and termination. In eukaryotes, a fourth step (processing of pre-mRNA) must also occur before translation.

Pre-mRNA

The initial RNA transcript generated during transcription in eukaryotes, which must be processed into mature mRNA before translation.

Promoter region

A DNA sequence that signals the start of transcription. It is where transcription factors and RNA polymerase bind.

TATA box

A common promoter sequence found in over 25% of eukaryotic genes. It contains the thymine–adenine–thymine–adenine motif and is sometimes used interchangeably with the term "promoter."

Consensus sequence

Promoter sequences that are common across many genes. They are often recognized by transcription factors.

Transcription factors (TF)

Proteins that bind specific sequences in the DNA promoter.

They are usually named TFII + letter (e.g., TFIIH).

They help initiate transcription by several coming together to form the initiation complex.

Initiation complex

A complex of transcription factors bound to the promoter, which recruits and includes RNA polymerase.

RNA polymerase role in initiation

RNA polymerase

1. joins the initiation complex

2. unzips double-stranded DNA

3. forms a transcription bubble to begin transcription.

Transcription bubble

1. The unwound region of DNA created by RNA polymerase.

2. It exposes the template strand and coding strand for elongation.

Coding strand (sense strand, + strand)

The DNA strand whose sequence matches the RNA transcript (except RNA has U instead of T). It is also called the sense strand.

Template strand (antisense strand, – strand)

The DNA strand that RNA polymerase actually reads. RNA is complementary to this strand, but has the same sequence as the coding strand.

Big things to know about initiation

1. there is a promoter region

2. transcription factors come in and bind bc they recognize a sequence

3. transcription factors bind and form an initiation complex

4. RNA polymerase comes and incorporates into that complex

5. RNA polymerase unzips double-stranded DNA, forming bubble

Direction of RNA synthesis

RNA polymerase reads DNA from 3′ to 5′ on the template strand, producing RNA in the 5′ → 3′ direction.

Upstream and downstream regions

The coding region (downstream, closer to the 5′ end of RNA, so the 3’ end of the DNA) is transcribed into RNA. Upstream regions regulate transcription levels through activators, repressors, and enhancers.

Activator regions

DNA sequences near the promoter that, when bound by proteins, increase the likelihood of forming the initiation complex and enhance transcription.

Repressor regions

DNA sequences near the promoter that, when bound by proteins, decrease the likelihood of forming the initiation complex and repress transcription.

Enhancer regions

DNA sequences farther upstream from the promoter. Proteins binding to enhancers increase transcription by stabilizing or facilitating initiation complex formation.

Role of activators, repressors, and enhancers

These regulatory sequences control levels of gene expression, allowing the body to adjust how much of a protein is produced depending on needs.

Key event of initiation

The critical event in transcription initiation is the formation of the initiation complex at the promoter region. Once RNA polymerase binds, unwinds DNA, and forms the bubble, elongation can begin.

key thing about initiation

initiation complex must form at promoter region