Download Chapter 13 - Gene Function

Chapter 13 - Gene Function
What do our genes actually encode for?
PROTEINS
How do we make proteins from our genes?
Transcription – synthesis of RNA from a DNA template
Translation – synthesis of protein from a RNA template
Chapter 13 - Gene Function
Chapter 13 - Gene Function
Chapter 13 - Gene Function
Transcription
Why do we need to make RNA from DNA first?
Believe it or not it’s a great regulatory site
If we don’t need a particular protein, do our cells want to waste making it?
How does a cell know when to express a particular gene (DNA sequence) & how
does the cell control this expression?
The ability to form an intermediate coding molecule (RNA) is the answer to these
questions & a simple example of this control is depicted by the bacterial lac operon
Chapter 13 - Gene Function
Chapter 13 - Gene Function
Structure RNA
1.
Ribose – 5 carbon sugar – forms part of the backbone structure
2.
Nitrogenous base – Information is coded in the sequence of these bases
A. Purines – Double ring structure – Adenine (A) & Guanine (G)
B. Pyrimidines – Single ring structure – Uracil (U) & Cytosine (C)
3. Phosphate group – forms part of the backbone structure
RNA is single-stranded
Chapter 13 - Gene Function
Chapter 13 - Gene Function
Types of RNA
Ribosomal RNA (rRNA) – associates with proteins to form the ribosome, the structural
site for protein synthesis (protein synthesizing factory)
Transfer RNA (tRNA) – responsible for connecting & carrying amino acids to
ribosome for synthesis into a protein
Messenger RNA (mRNA) – the template for translation (protein synthesis) – a
sequence of nucleotides that will determine the sequence of amino acids in the final
protein – it is a copy of the DNA BLUEPRINT!
Chapter 13 - Gene Function
Chapter 13 - Gene Function
Transcription
Transcription factors regulate the initiation of transcription
They along with RNA polymerase are attracted to a sequence of DNA known as the
promoter
The DNA unwinds via the mechanism already discussed & RNA polymerase
facilitates the growth of the RNA molecule
1. No primer is needed, unlike DNA replication
2. Only one strand of the DNA will serve as the template
3. RNA transcription is directional: 5’ to 3’
4. Multiple RNA strands can be simultaneously transcribed
Chapter 13 - Gene Function
mRNA processing
Before the mRNA can be used for translation it must be processed into a final coding
sequence. It will undergo the following:
1. Capping – Short sequences of nucleotides added to the 5’ end
2. Tailing – Addition of adenine nucleotides to the 3’ end
3. Splicing – Removal of introns (non-coding sequences) & the joining of exons
(coding sequences)
Chapter 13 - Gene Function
Chapter 13 - Gene Function
Chapter 13 - Gene Function
Translation
How do we make a protein from RNA?
What do we need?
1. Strand of mRNA
2. Ribosomes: large & small subunits
3. Charged tRNAs – tRNAs with an attached amino acid
Translation is divided into 3 stages
1. Initiation
2. Elongation
3. Termination
Chapter 13 - Gene Function
How does the mRNA serve as the template for translation?
How is the correct sequence of amino acids obtained?
The mRNA is divided into 3 nucleotide sections called codons
Each codon encodes for an amino acid as depicted on the following table
Chapter 13 - Gene Function
Chapter 13 - Gene Function
How does tRNA transfer amino acids to a growing polypeptide chain?
tRNAs link up to the codons on the mRNA via their own anticodons – a three
nucleotide sequence that is complementary with the codon’s sequence
Each tRNA molecule carries a specific amino acid, so that when the anticodon & codon
match up within the ribosome, the amino acids are assembled in the proper sequence
Chapter 13 - Gene Function
Thus if we have a sequence of the following codons:
5’ – CCC – CAC – UUU – AAU –UAG 3”
What is the sequence of amino acids that is encoded for by this mRNA?
Proline – Histidine – Phenylalanine – Asparagine - Stop
Chapter 13 - Gene Function
Chapter 13 - Gene Function
Chapter 13 - Gene Function
Chapter 13 - Gene Function
Chapter 13 - Gene Function
Chapter 13 - Gene Function
Chapter 13 - Gene Function
Chapter 13 - Gene Function
Chapter 13 - Gene Function
Translation specifics
Translation proceeds in a 5’ to 3’ direction on the mRNA
The codon is a 3 nucleotide sequence on the mRNA
For each codon there exists an anticodon on a tRNA
The tRNA is responsible for carrying a specific amino acid to its codon on the mRNA
Peptide bonds are formed between the amino acids by enzymes located in the
ribosomal complex
Multiple proteins can be translated simultaneously like RNA transcription polyribosome
Chapter 13 - Gene Function
Mutations
How do mutations in our DNA lead to phenotypic changes?
Two categories of mutations
Point mutations – change in a single DNA base which leads to:
1. Missense mutation – change in amino acid due to the DNA base change
2. Nonsense mutation – DNA base change causes the appearance of a “stop”
codon
3. Silent mutation – DNA base change does not change the amino acid
sequence
Insertion / Deletion mutations – adding or removing DNA bases which leads to a:
1. Frameshift mutation – a disruption in the reading frame which can lead to
changes in all amino acids downstream of the insertion or deletion
Chapter 13 - Gene Function
Chapter 13 - Gene Function
Quick little example
Normal DNA
THE ONE BIG FLY HAD ONE RED EYE
Missense
THE ONE DIG FLY HAD ONE RED EYE
Nonsense
THE ONE BIG FLY
Frameshift
THE ONE DBI GFL YHA DON ERE DEY
Chapter 13 - Gene Function
Chapter 13 - Gene Function
Chapter 13 - Gene Function
PRACTICE QUESTIONS
1.
What enzymes are used during the steps of transcription? Translation?
2.
What are the 3 types of RNA?
3.
Define the following terms: codon, anticodon, semi-conservative replication,
polyribosome
4.
What direction does replication, transcription, & translation occur?
5.
Given that a template strand of DNA is 3’ TAC AAA ACC CCG GAG ATC 5’,
transcribe the appropriate mRNA (assuming no introns) & translate the appropriate
protein.
6.
What is the difference between a nonsense, missense, & silent mutation?
7.
What is a polyribosome? How is this structure similar to the transcriptional
structure?
8.
What is an exon? Intron?