how DNA causes changes in the traits of organisms
DNA will code for protein which will influence the expression of a trait
How are genes expressed?
Some genes are only expressed by certain cells
Some genes are only expressed at certain times
How does DNA produces proteins?
DNA uses RNA to produce proteins
What is RNA?
RNA = ribonucleic acid
- Very similar to DNA
- Sugar phosphate backbone with nucleic acids attached
Differences with DNA
Has a ribose sugar, in DNA its deoxyribose (missing an oxygen)
- DNA is double stranded, RNA is single stranded
- Difference with the bases, no thymine, instead have a base called uracil that pairs with adenine
three types of RNA (different roles that are played)
- mRNA carries that protein blueprint from DNA to ribosomes
- tRNA brings the amino acids to the ribosomes to build proteins
o triplet of exposed bases at one end called the anticodon
o at the other end there is amino acid attachment site
o there are 61 different tRNA molecules
each tRNA has a specific anticodon, and carries a specific amino acid
- rRNA is used to actually build ribosomes - ribosomes are made of subunits
- large unit and small subunit
o these subunits are what is made in the nucleolus and are only assembled afterwards
How is RNA produced?
TRANSCRIPTION is what produced RNA
- Involves copying a sequence from DNA on RNA
- Copying a sequence of nucleic acids bases to a complementary strand of nucleic acid bases (in the same “language” of nucleic acid bases)
What is translation?
TRANSLATION is where we produce a chain of amino acids to form a
protein based on the sequence on the RNA strand
- Here you’re translating from the “language” of nucleic acid bases to the language of amino acids
Where does transcription occurs in eukaryotic cells?
In eukaryotic cells, transcription is occurring in the nucleus
mRNA and pre mRNA steps
mRNA goes through an extra step in the nucleus
o pre mRNA goes through an extra step of processing where it becomes mature mRNA where it can escape the nuclear pores and then perform translation in the cytoplasm
Initiation
- You have the promoter – a specific sequence of DNA that is identified by an enzyme that will initiate transcription
- RNA polymerase binds to the promoter and as it binds it pulls apart the two strands of DNA
- RNA nucleotides will be brought in if they are complementary to the bases in the DNA strand
where does elongation happens?
Elongation (happens across the transcription unit)
Elongation process
- RNA polymerase will work its way along the template strand and will add nucleotides as it goes
- Sequence on the RNA is complementary to the sequence on the DNA strand - Termination
- RNA polymerase continues until it reaches the termination sequence
How does transcription occur (3 parts)?
Initiation, transcription and termination
Termination
Sequence on the RNA is complementary to the sequence on the DNA strand - Termination
RNA polymerase continues until when?
RNA polymerase continues until it reaches the termination sequence
Differences between DNA replication and Transcription
DNA replication
Cell replicates in the S period of interphase
DNA replication occurs across all the DNA
Replicating both strands
DNA polymerase
Copying DNA to DNA
Original DNA strands stay apart
Transcription Happening more or less constantly
Only replicating what is between the promoter and the terminator
Only replicating one strand
RNA polymerase
Copying DNA to RNA Original DNA strands re-coil
How does translation occur (3 stages)?
Initiation, elongation and termination
Initiation in translation in eukaryotes
- When a ribosome forms from the large and small subunit
- Somewhere along the mRNA you have the start codon and its
at this point that a specific tRNA will come in with the specific anticodon needed - The tRNA with the anticodon will line up with the start codon
- The ribosome
Elongation in translation in eukaryotes
- During elongation, another tRNA can come in and continue along
the mRNA strand by bringing the next amino acid - At this point the whole ribosome is going to shift down the mRNA as the amino acids link up and form a chain
- tRNAs will take their turn and link into the ribosome to elongate the protein chain
Termination in translation in eukaryotes
- Elongation reaches until you reach a stop codon (will not code for a protein)
- A release factor will come in and stick to one of the stop codons and will cause the ribosome to come apart and effectively put an end to translation
What can mRNA have during termination at the same time?
Can have more than one ribosomal subunit on the mRNA at the same time
What can happen in the prokaryotic cell regarding translation?
In prokaryotic cells, since there is no barrier between the DNA and the surrounding cell, you can have translation starting before the RNA polymerase produces the mRNA from transcription
What is a major difference between this process in eukaryotic vs prokaryotic cells?
mRNA in eukaryotic cells (in the nucleus) goes through RNA processing
- this is a process by which there is an addition of a cap and tail to the ends of the pre-mRNA
- and then parts of the pre-mRNA (introns) are cut out of the sequence
- the cap and tail are there to help protect the mRNA from degradation
- the introns are specific parts of the mRNA that will not be expressed, in contrast to the parts that will be expressed (exons)
o the exons are then glues together into a shorter piece of mRNA
