Chapter 18: Transcription and Translation: What’s in a Gene? Summary

Types of molecules encoded by DNA

The central dogma

• This is the information pathway from DNA to RNA to proteins.
• Messenger RNA is first derived from DNA to form a protein.
• Transcription is the process of copying DNA into a messenger RNA.
• The translation is the process of building a protein molecule from the messenger RNA (mRNA).

Transcription

• During transcription, the double strands of DNA are separated and one strand is used as a template to generate an mRNA molecule.
• The RNA synthesized molecules work for the cell directly or serve as important information copy.
• Promoters are sequences that identify the genes that need to undergo transcription process.
• The length of bacterial promoters is 50 40 to 50 bp upstream of the genes.

Types of promoter sequences

• The -10 box contains the TATATATT sequence which is located 10 bases away from the start site of transcription.
• The -35 box contains a TTGACA sequence 35bp from the transcription start site.

Transcription initiation process

1. RNA polymerase is the enzyme that reads and copies DNA into the mRNA.
2. RNA polymerase enzyme observes the base-pairing rules by matching an RNA base to a complementary DNA base.
3. RNA polymerase facilitates covalent bond formation between the RNA nucleotides.
4. RNA polymerase reads one strand of DNA called the template to build the new RNA sequence.
5. The RNA polymerase reads the template in the 3′ to 5′ direction to generate an mRNA in the 5’ to 3’ direction.
6. RNA polymerases use regulatory proteins to recognize promoters and regulate the process of transcription as well.
7. Sigma proteins are regulatory proteins in bacteria that attach to RNA polymerase.
8. Transcription factors in eukaryotes help polymerases bind to promoters.

Transcription termination

• Transcription terminators release RNA polymerase from DNA and stop the transcription process.
• A transcription terminator contains a hairpin loop that is formed when an RNA strand folds back on itself.
• This loop knocks off RNA polymerase from DNA ending the transcription process.

RNA processing

• The RNA sequence derived from transcription is called a pre-mRNA.
• The pre-mRNA contains introns and exons.
• To remove the introns, the pre-mRNA goes through the process of splicing.
• The splicing process occurs through the formation of a lariat.
• The pre-mRNA is also stabilized by the addition of a 5′ cap and 3′ cap.
• The 5′ cap involves the addition of a GTP while the 3′ cap involves the addition of a poly-A tail.

Translation

• The initiation of translation begins from initiation sequences.
• Bacterial initiation sequence contains:
  • The start codon (AUG, GUG, UUG)
  • The ribosomal binding site(10 bp upstream)
• Eukaryotic initiation sequence is similar to that in bacteria.
• The rRNA attaches to the cap and moves to the ribosomal binding site with the start codon.

Translation initiation

• The 30s ribosomal unit bind to the fMet-tRNAf and the ribosomal binding site on mRNA.
• This forms an initiation complex that binds on a large 50s rRNA subunit.

Ribosomes

• The ribosomes have several active sites:
  • A site is the acceptor site on which the incoming aminoacyl-tRNA bind.
  • P site is the site where the growing peptidyl-tRNA complex is held.
  • E site is the site where the deacylated tRNA leaves the ribosome.
• The aminoacyl-tRNA transfers an amino group to the ribosome at the A site.
• The polypeptide at P site moves to the A site where a peptide bond forms with the new amino acyl-tRNA.
• The former tRNA is deacylated and leaves the ribosome through the E site.
• After the translocation process, the ribosome moves one codon ahead to allow another amino acid to be added.

Translation termination

• Termination of translation occurs through stop codons like UAA, UGA, and UAG.
• Bacterial translation termination involves release factors such as:
  • RF-1 recognizes UAA and UAG.
  • RF-2 which recognizes USA and UAA codons.
  • RFs act at the ribosomal A-site and use peptidyl-tRNA in the P-site.
• In eukaryotes, the protein released from the ribosome is released to the endoplasmic reticulum.

Mutation

• Mutations are changes that occur in the DNA composition.
• Spontaneous mutations are the mutations that occur because of the way the enzyme’s function.
• Agents that increase the rate of mutation are called mutagens.
• Induced mutations are formed from the action of mutagens.

Mutations that affect proteins

• Missense mutations – Change in single base pair, which substitutes an amino acid
• Nonsense mutations – Results into a premature stop codon
• Frameshift mutations – Deletion or insertion of a nucleotide which results to a sequence that is not divisible by three.

Revision

What are the parts of DNA?
nucleotides, deoxyribo sugar,nitrogen bases

What are the parts of RNA?
nucleotides, ribos sugar, nitrogen bases, hydrogen bonds

What nitrogen bases match up in DNA?
Adenine & cytosine and Guanine & thymine

What nitrogen bases match up in RNA?
adenine & cytosine and guanine & uracil

What makes up a nucleotide?
phosphate group, (phosphorus in the middle and oxygen around the outside) ATP, pentose sugar, nitrogen base

What is a repeating nucleotide called?
DNA/RNA strand

What is a anticodon? Where is it found?
3-base sequence of the tRNAs. It’s found in the ribosome

What is a codon? Where is it found?
Every three letters in this mRNA. It’s found in mRNA

What is transcription?
process where RNA polymerase grabs onto the DNA, it then moves down the DNA creating a copy behind is called mRNA, Then termination signal will release the mRNA which will go to the nucleus inside the cytoplasm and will make a protein out of amino acids.

What is translation?
Process where a mRNA will grab on to a ribosome then it’s going to make proteins.

Where does translation take place?
cytoplasm

How many possible combinations of nucleotides are there when 3 are present?
64

What are polypeptides made up of?
string of amino acids

What is the building blocks of a protein?
amino acids

What is the backbone of DNA?
sugar attaches to a phosphate to a sugar to a phosphate to a sugar.

What is the number of nucleotides for a specific amino acid?
3

What are the main differences between RNA and DNA?

1. DNA has thymine, RNA has uricil
2. DNA has a double helix, RNA has a single helix
3. DNA is in the nucleus, RNA is found everywhere needed
4. DNA has deoxyribos, RNA has a ribose sugar

What are the enzyems for DNA replication? (3)
DNA ligase, DNA polymerase and DNA helicase

What are the enzymes for RNA transcription? (1)
RNA polymerase

Where does transcription take place?
nucleus

Where are proteins made?
ribosome

What are the building blocks of DNA and RNA?
nucleotides

What are the 3 steps of transcription?
initiation, elongation, termination

Which strand codes for the amino acid inside the ribosome?
mRNA

What is replication?
process when a new DNA molecule is made form another DNA molecule

What are the 3 forms of RNA?
mRNA, rRNA and rRNA

What does rRNA do?
combines with proteins to make ribosomes

What does mRNA do?
(single-strand) receives information from the DNA molecule and assembles a pattern of amino acids

What does tRNA do?
(hair-pin shaped) matches up with the mRNA in the ribosome to eventually make a protein