Lecture B6: DNA Replication, Transcription and Translation
Learning Outcomes
DNA and genetics
DNA replication
DNA replication is semiconservative
DNA replication proceeds in two directions at many sites simultaneously
Enzymes involved in replication
Gene expression
Gene expression
The flow of information is from DNA to RNA to protein
Genes control characteristics through the production of proteins
Transcription and Translation
Transcription
Transcription produces mRNA
The transcription of a gene
Post-transcriptional modification
Eukaryotic RNA processing
Production of eukaryotic mRNA
Translation
Information written in DNA is translated into proteins
The genetic code dictates how codons are translated into amino acids
Dictionary of the genetic code
Characteristics of the genetic code
Translation of the genetic message
Ribosomes build polypeptides
Translation produces polypeptides
An initiation codon marks the start of the mRNA message
The two stages of initiation in translation
Ribosomes with unoccupied and occupied binding sites
Elongation adds amino acids to the polypeptide chain
Translation- Elongation adds amino acids to the polypeptide chain
Elongation adds amino acids to the polypeptide chain until a stop codon terminates translation
Summary
Animation: Transcription
24.38M
Категория: БиологияБиология

Lecture B6: DNA Replication, Transcription and Translation

1. Lecture B6: DNA Replication, Transcription and Translation

Foundation Year Program
Lecture B6:
DNA Replication,
Transcription and Translation
Introduction to Biology
2019-20

2. Learning Outcomes

Foundation Year Program
Learning Outcomes
At the end of the lecture students should be able to:
• Describe the process of DNA replication
• Explain the relationship between the processes of
DNA transcription, RNA processing and protein
synthesis
• Text reference: Campbell Concepts, 10.4-10.15
Introduction to Biology
2019-20

3. DNA and genetics

Foundation Year Program
DNA and genetics
• Genetics is the study of inheritance – how
characteristics are passed from parents to
offspring
• The hereditary information is encoded in DNA
and passed from one generation to the next
by precise copying
• Because of this DNA is frequently referred to
as the “genetic molecule”
Introduction to Biology
2019-20

4. DNA replication

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DNA replication
• DNA replication is the biological process of
producing two identical copies (replicas) of DNA
from one original DNA molecule
• Necessary precursor to cell division (next lecture)
Introduction to Biology
2019-20

5.

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DNA replication is semiconservative
Introduction to Biology
2019-20

6. DNA replication is semiconservative

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DNA replication
Introduction to Biology
2019-20

7.

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DNA replication proceeds in two
directions at many sites simultaneously
• Replication of a DNA molecule begins at sites
called origins of replication, short stretches of
DNA that have a specific sequence of
nucleotides.
• Proteins that initiate DNA replication attach to
the DNA at the origin of replication and
separate the two strands of the double helix
• Replication then proceeds in both directions,
creating replication “bubbles.”
Introduction to Biology
2019-20

8. DNA replication proceeds in two directions at many sites simultaneously

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Introduction to Biology
2019-20

9.

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Enzymes involved in replication
• DNA polymerases add
nucleotides to the
growing strands
• DNA ligase ties short
DNA fragments
together
• DNA polymerases and
DNA ligase also repair
DNA damaged by
harmful radiation and
toxic chemicals
Introduction to Biology
2019-20

10. Enzymes involved in replication

Foundation Year Program
• DNA replication ensures that all the somatic
cells in a multicellular organism carry the
same genetic information
• If the process is completed without errors,
two daughter cells identical to the original will
form.
• However, mistakes may occur during this
complicated process – these can result in
mutations
Introduction to Biology
2019-20

11.

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Gene expression
Introduction to Biology
2019-20

12. Gene expression

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Gene expression
• The expression of the information encoded in
DNA is a complicated, multi-step process
• The DNA program ultimately directs the
development of biochemical, anatomical and
physiological traits of the cell and individual
Introduction to Biology
2019-20

13. Gene expression

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The flow of information is from
DNA to RNA to protein
• DNA specifies traits by dictating protein
synthesis.
• The molecular chain of command is from DNA
in the nucleus to RNA and RNA in the
cytoplasm to protein.
Introduction to Biology
2019-20

14. The flow of information is from DNA to RNA to protein

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Genes control characteristics
through the production of proteins
• Transcription is
the synthesis of
messenger RNA
(mRNA) using DNA
as a template.
• Translation is the
synthesis of
proteins under the
direction of
mRNA.
Introduction to Biology
2019-20

15. Genes control characteristics through the production of proteins

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Transcription and Translation
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2019-20

16. Transcription and Translation

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Transcription
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17. Transcription

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Transcription produces mRNA
• Transcription of a gene occurs in three main steps:
1. Initiation: RNA polymerase attaches to a DNA region
called the promoter and starts RNA synthesis
2. Elongation: The newly formed RNA strand grows
3. Termination: The RNA polymerase reaches the
terminator DNA and detaches from both the newly
made RNA transcript and the DNA
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18. Transcription produces mRNA

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The transcription of a gene
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19. The transcription of a gene

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Post-transcriptional modification
• In prokaryotes, the RNA transcript is ready for
immediate translation
• Eukaryotic mRNA is more complex than
prokaryotic
– Contains introns (interrupting sequences)
that separate exons (the coding regions)
• It is processed in the nucleus and then
exported for translation
Introduction to Biology
2019-20

20. Post-transcriptional modification

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Eukaryotic RNA processing
• First there is RNA splicing
– Introns are removed and the exons are joined to
produce a continuous coding sequence.
• Then a cap and tail of extra nucleotides are added to the
ends of the mRNA to:
– Help the export of the mRNA from the nucleus
– Protect the mRNA from degradation by cellular enzymes
– Help ribosomes bind to the mRNA
• The cap and tail are not translated into protein.
Introduction to Biology
2019-20

21. Eukaryotic RNA processing

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Production
of
eukaryotic
mRNA
Introduction to Biology
2019-20

22. Production of eukaryotic mRNA

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Translation
Introduction to Biology
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23. Translation

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Information written in DNA is
translated into proteins
• The sequence of nucleotides in DNA provides a
code for constructing a protein
– This requires a conversion of a nucleotide
sequence to an amino acid sequence
• The flow of information from gene to protein is
based on a triplet code – three-base “words”
called codons
Introduction to Biology
2019-20

24. Information written in DNA is translated into proteins

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The genetic code dictates how codons are
translated into amino acids
• The genetic code directs the amino acid
translation of each of the nucleotide triplets.
– Three nucleotides specify one amino acid.
– Of the possible 64 codons, 61 code for amino
acids and 3 codons signal the end of translation.
– AUG codes for methionine and signals the start of
translation.
– UAA, UGA and UAG are the stop codons.
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25. The genetic code dictates how codons are translated into amino acids

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Dictionary of the
genetic code
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26. Dictionary of the genetic code

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Characteristics of the genetic code
• The genetic code is
– Redundant: some amino acids have more than one
codon
– Unambiguous: each codon codes for only one amino
acid
– (Nearly) universal: the genetic code is shared by
organisms from the simplest bacteria to the most
complex plants and animals
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2019-20

27. Characteristics of the genetic code

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Translation of the genetic message
• Translation is performed by transfer RNA
(tRNA) molecules
• Transfer RNA molecules do this by
– picking up the appropriate amino acid
– using a special triplet of bases, called an
anticodon, to recognize the appropriate codons in
the mRNA.
Introduction to Biology
2019-20

28. Translation of the genetic message

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A simplified
representation of a
tRNA
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2019-20

29.

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Ribosomes build polypeptides
• Translation occurs on the surface of the ribosome.
– Ribosomes coordinate the interaction of mRNA and
tRNA and, through this, the synthesis of polypeptides.
• Ribosomes have two subunits: small and large.
• Each subunit is composed of ribosomal RNAs (rRNA) and
proteins.
• Ribosomal subunits come together during translation.
• Ribosomes have binding sites for mRNA and tRNAs.
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30. Ribosomes build polypeptides

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Translation produces polypeptides
• Translation can be divided into the same three
phases as transcription:
1. Initiation
2. Elongation
3. Termination
Introduction to Biology
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31. Translation produces polypeptides

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An initiation codon marks the start
of the mRNA message
• Initiation brings together
– mRNA, a tRNA bearing the first amino acid, and the
two subunits of a ribosome.
– Initiation establishes where translation will begin.
Cap
End
Start of genetic
message
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Tail
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32. An initiation codon marks the start of the mRNA message

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The two stages of initiation in
translation
2. A large ribosomal subunit joins
1. mRNA binds to a small
the small subunit, allowing the
ribosomal subunit, and an
initiator tRNA binds to mRNA at ribosome to function.
the start codon that reads AUG • The first tRNA occupies the P site
and codes for methionine (first (growing polypeptide).
tRNA has the anticodon UAC). • The A site (next amino-acidbearing tRNA).
Introduction to Biology
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33. The two stages of initiation in translation

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Ribosomes with
unoccupied and
occupied binding
sites
Introduction to Biology
2019-20

34. Ribosomes with unoccupied and occupied binding sites

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Elongation adds amino acids to the
polypeptide chain
• Once initiation is complete, amino acids are added one
by one to the first amino acid (elongation process).
• This occurs in three steps:
1. The anticodon of an incoming tRNA molecule, carrying its
amino acid, pairs with the mRNA codon in the A site of the
ribosome.
2. The polypeptide separates from the tRNA in the P site and
attaches by a new peptide bond to the amino acid carried
by the tRNA in the A site.
3. The P site tRNA (now lacking an amino acid) leaves the
ribosome, and the ribosome translocates (moves) the
remaining tRNA (which has the growing polypeptide) from
the A site to the P site.
Introduction to Biology
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35. Elongation adds amino acids to the polypeptide chain

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TranslationElongation adds
amino acids to
the polypeptide
chain
Reminder:
A-site: Amino acid
P site: polypeptide
Introduction to Biology
2019-20

36. Translation- Elongation adds amino acids to the polypeptide chain

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Elongation adds amino acids to the polypeptide
chain until a stop codon terminates translation
Release
factor
Free
polypeptide
5
3
5
5
Stop codon
(UAG, UAA, or UGA)
3
2
3
GTP
2 GDP
Campbell Biology, 9th ed.
Stop codon comes into A site
Release factor binds
Energy input
The ribosome splits back into its separate subunits
New protein is released
Introduction to Biology
2019-20

37. Elongation adds amino acids to the polypeptide chain until a stop codon terminates translation

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Summary
• Genes are expressed when DNA directs protein
synthesis
• During gene expression, DNA is transcribed to
mRNA, which is then translated to protein
• Transcription in eukaryotes happens in the nucleus
• Translation is carried out by the ribosomes
Introduction to Biology
2019-20

38. Summary

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Animation: Transcription
Introduction to Biology
2019-20

39. Animation: Transcription

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Animation: Translation
Introduction to Biology
2019-20
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