Transcript Replication, Transcription, and Translation

DNA Replication Review
Three main steps:
Helicase unzips/unwinds the DNA molecule
DNA Polymerase brings in new nucleotides
Ligase zips the new DNA back together
Why is DNA Replication important?
 The important idea is that an exact duplication of the DNA message is required,
so that each new cell in the body has the same set of genetic
instructions as the cells that preceded it.
 This also insures that every new generation of individuals has the same
genetic information as his/her parents.
DNA carries information that can be used to construct the
proteins which form structures and regulate the body’s
activities.
 Protein synthesis involves two processes: transcription and
translation.
 In transcription the DNA message is converted into an RNA
molecule.
 In translation the RNA message is used to assemble amino acids
into a protein chain.
Central Dogma
First, let’s learn about RNA
 RNA is also a nucleic acid, called ribonucleic acid
 It only has one strand (DNA has two)
 It contains the sugar ribose instead of deoxyribose
 It has the nitrogenous base URACIL (U) instead of
thymine (T)
There are THREE type of RNA
(wait for next slide for notes)
 Messenger RNA (mRNA)
 Long strands of RNA nucleotides that are formed complementary to
one strand of DNA
 Ribosomal RNA (rRNA)

Associates with proteins to form ribosomes in the cytoplasm
 Transfer RNA (tRNA)

Smaller segments of RNA nucleotides that transport amino
acids to the ribosome where proteins are made by adding 1 a.a.
at a time
Messenger, ribosomal, transfer RNA
TRANSCRIPTION (the first step in protein synthesis)
 Through transcription, the
DNA code is transferred
to mRNA in the nucleus.
 DNA is unzipped in the
nucleus and RNA polymerase
binds to a specific section
where a mRNA will be
synthesized
Transcription does not happen all the time
 Operon – the “switch” to
turn on/off transcription
 Promoter – DNA site
that promotes RNA
polymerase to bind
 Repressor – molecule
that binds to DNA to
block transcription
 Inducer – molecule
that takes repressor
away
 Transcription animation
Changing nucleic acids into amino acids
 The three-base code in DNA or mRNA is called
a codon.
 They are always coded in threes
 Each triplet code corresponds with one amino
acid
 This is where TRANSLATION begins
TRANSLATION (the last step in protein synthesis)
 Translation begins when
mRNA binds to the
RIBOSOME in the cell.
 In translation, tRNA
molecules act as the
interpreters of the mRNA
codon sequence.
 At the middle of the folded
strand, there is a threebase coding sequence in
the tRNA called the
anticodon.
 Each anticodon is
complementary to a codon
on the mRNA.
 Translation animation
Cleaning up the Message
 When the genetic message is
copied to make mRNA, the
message contains unwanted
base sequences.
 The ‘junk’ sequences (called
introns) are removed from the
message and the remaining
sequences (exons) are linked
together to produce a sequence
of codons that will translate
into a polypeptide.
 This process occurs before the
message leaves the nucleus.
The role of Ribosomes
 The third type of RNA is ribosomal
RNA (rRNA). Ribosomes are made
of RNA and PROTEIN.
 Ribosomes are the ‘decoding’ units
of the cell. (Sites of protein
synthesis)
 Ribosomes consist of two major
components — the small ribosomal
subunit which reads the RNA, and
the large subunit which joins amino
acids to form a polypeptide chain.
 Ribosomes have binding sites for
both tRNA and mRNA molecules.
 Crash Course Biology
 (14:08)