Unit 3 Biology-Nucleic acid and gene code

1. Chapter 1.4
The genetic code
and protein
synthesis

2. 1.4: The genetic code and protein synthesis
Learning Intentions Success Criteria
• explain the features of the genetic
code: universal, triplet and
degenerate.
• describe protein synthesis and
gene expression.
• outline and apply the steps of
transcription.
• outline and apply the steps in
RNA processing.
• outline and apply the steps in
translation.

3. Chromosomes As Gene Carriers
DNA is a sequence of nucleotides.
A unique subset of nucleotides is
referred to as a “gene”.
● The specific location on a
chromosome is referred to as
a “gene locus”.
Gene: the unique sequence of DNA
which holds the recipe for a
specific trait.
E.g. brown eye gene → produces
the proteins to give a person
brown eyes

4. Chromosomes As Gene Carriers
DNA holds coded information to join amino acids to form proteins.
This information gets decoded to form the protein which gives us
our traits.
● Melanin gene → Melanin protein → skin color

5. Protein Synthesis
Protein synthesis is the process
of reading DNA and building a
protein based on its blueprint.
It consists of 3 stages:
1. Transcription
2. RNA processing
3. Translation

6. Step 1: Transcription
DNA cannot be directly made into a
protein because:
1. It contains many genes and we
only need the relevant one
2. It is too large to leave the
nucleus
3. If we use the DNA directly then
it will degrade in the cytoplasm
but the cell needs it again!
The specific gene which needs to be
made into a protein gets transcribed
(read/copied) into a molecule known
as mRNA in a process known as
transcription.

7. Transcription
1. Initiation: an enzyme known as RNA
polymerase attaches to a region of the
DNA known as a promoter region. RNA
polymerase causes the DNA to unwind.
1. Elongation: RNA polymerase reads the
template DNA strand (in a 3’ to 5’
direction) and builds the mRNA molecule,
adding complementary base pairs to the 3’
end.
1. Termination: the RNA polymerase
encounters a “stop sequence” and the pre-
mRNA detaches from the template strand.

9. Step 2: RNA processing
pre-mRNA undergoes a process
known as RNA processing (or
post-transcriptional modification)
to convert it into the molecule
mRNA.
This makes it smaller and more
stable.

10. RNA processing
1. Methlyguanosine cap (a modified
guanine) is added to the 5’ end of
the pre-mRNA. Protects from
degradation and assists in ribosome
binding.
2. A small region of the 3’ end is
snipped and up to 250 Adenines are
added to the mRNA (poly-A tail).
Protects from degradation + assists
in export from nucleus.
3. Introns are spliced and exons are
joined together

11. Splicing
Introns are spliced by an enzyme called
a spliceosome.
Spliceosomes read the pre-mRNA and
slice GU at the 5’ end and AG at the 3’
end.
This cuts out the introns and then the
exons are joined together.

12. Alternative Splicing
The human genome contains only about 21 000 genes, and this range is also typical of
other mammals.
However, one gene can be regulated in different ways so that it can produce more than
one protein.

13. Step 3: Translation
Translation is the process in
which the sequence mRNA is
decoded and translated into a
protein chain by the addition of
corresponding amino acids.
● Occurs in the cytoplasm on
the ribosomes

14. Structure of Ribosomes
Ribosomes are made of ribosomal
RNA (rRNA) and proteins making
up two sub-units.
They have mRNA binding sites
and tRNA binding sites (E,P,A).
Can be found free in cytoplasm
OR attached to endoplasmic
reticulum.

15. How the ribosome reads the mRNA
1. The mRNA attaches to the ribosome
at the mRNA binding site.
2. The ribosome reads the first three
nucleotides (the start codon - AUG)
3. A tRNA carries the corresponding
amino acid to the ribosome (AUG →
methionine [met])
4. The ribosome reads the next three
nucleotides and a tRNA brings the
corresponding amino acid. The amino
acids join to each other by peptide
bonds.
5. Continues until a stop codon
(UAA,UAG or UGA) signals the end
of translation.

16. Structure of tRNA
Codon - sequence of three
nucleotides on the mRNA
Anticodon- sequence of three
nucleotides on the tRNA
● The codon/anticodon are
complementary.
● Each type of tRNA carries a
specific amino acid at the
amino acid attachment site.

17. Codon table

18. The triplet code
One genetic instruction consists
of a group of three bases, such as
AAT, GCT and so on.
Because of this, the genetic code
is referred to as a triplet code.

19. The Degenerate Code
The triplet code is known as
degenerate or redundant.
● There are 4 bases (A,U,C,G).
So there are 64 (4^3)
possible combinations of 3
letters e.g. AUG, ACU etc.
● There are only 20 amino
acids. Therefore multiple
codons code for the SAME
amino acid.

20. The Universal Code
The same sequence of
nucleotides codes for the
same amino acid (for
example, CCA codes for
proline) in plants, animals and
bacteria.