Unit 5, Chapter 16 AP BIO

Central Dogma - DNA is transcribed into mRNA, which is translated into polypeptides(proteins)

• Gene - segment of DNA that codes for a protein or RNA

• Small RNAs can regulate gene expression

Annoying terminology!

• Either DNA strand can be used for transcription - depends on where the gene is.

• Template Strand = “non-coding strand” or “antisense strand” -used for transcription

  • Look for 3’ TAC 5’

  • RNA polymerase enzyme binds to this region and starts to transcribe by adding complementary RNA nucleotides (in a 5’ to 3’ direction)

• 3’ TAC(translated into AUG) 5’ is transcribed into 5’ _ 3’ - this is the “Start” Codon that corresponds to the amino acid Methionine (Met).

• Coding Strand = “non-template strand” or “sense strand” - not used. Looks just like the mRNA except it has T instead of U!

1. Transcription factors - bind to promoter region (specific DNA sequence)

2. This allows RNA Polymerase to bind to template strand and start adding complementary RNA nucleotides (5’ to 3’)

3. Enzyme moves along the template strand until it reaches terminator sequence and is released!

4. End result? mRNA = messenger RNA that will bind to a ribosome for translation!

Difference between proks & euks:

Prokaryotic cells - mRNA can bind to ribosome right away

Eukaryotic cells - mRNA has to be Processed (spliced and Capcised)

RNA Processing - “Cut & Cap” the pre-mRNA

1. Nucleotides added to both ends

5’ end gets a G nucleotide cap

3’ end gets a poly-A tail (multiple A nucleotides)

WHY add cap & tail?

- Allows mRNA to leave the nucleus

- Protects mRNA from hydrolytic enzyme

- Allows mRNA to bind to the ribosome

1. Splicing - spliceosome (complex of proteins and small RNAs)

   

   1. cut out non-coding sequences (introns)

   2. put coding sequences (exons = expressed) back together

_Alternate splicing_- can put exons together in different order

• This would increase the diversity of proteins!

1. mRNA binds to small subunit of ribosome.

2. Transfer RNA (tRNA) carrying Met binds to start codon on mRNA (AUG)

3. Large subunit binds to small subunit

4. Ribosome moves along mRNA in 5’ to 3’ direction as tRNAs bring correct amino acids

Triplet code - RNA is read in 3-letter sequences (codon)

1 amino acid = 1 codon = 3 nucleotides

1. Ribosome forms peptide bonds between amino acids to form polypeptide

2. When the ribosome reaches “stop codon”, it detaches.

3. Polypeptide is processed and folded into a protein. Initial Met is usually removed.
   

   

Practice!

1. Replicate! Highlight template strand! Transcribe & Translate!

   

DNA 5’ATGCCCATTACTCGGTAA 3’

3’TACGGGTAATGAGCCATT 5’
5’AUGCCCAUUACUCGGUAA 3’

Met, Ala, His, Tyr, Pro

1. What’s the minimum # of DNA nucleotides (gene length) required to make a polypeptide that’s 900 amino acids long?
   
   54,000 nucleotides.