Download 21.5 RNA and Transcription

21.5 RNA and Transcription
A typical ribosome
consists of a small
subunit and a large
subunit. The subunit
shapes shown
contain both protein
and rRNA.
Learning Goal Identify the different types of RNA; describe
the synthesis of mRNA.
General, Organic, and Biological Chemistry: Structures of Life, 5/e
Karen C. Timberlake
© 2016 Pearson Education, Inc.
RNA and DNA
RNA
• makes up most of the nucleic acid found in the cell.
• transmits genetic information from DNA to operate the cell.
• has several types as shown below.
• differs from DNA in the following ways:
• The sugar in RNA is ribose; the sugar in DNA is
deoxyribose.
• The base uracil replaces thymine.
• RNA molecules are single stranded; DNA molecules
are double stranded.
• RNA molecules are much smaller than DNA molecules.
General, Organic, and Biological Chemistry: Structures of Life, 5/e
Karen C. Timberlake
© 2016 Pearson Education, Inc.
Types of RNA
There are three major types of RNA:
• 5% of RNA is messenger RNA (mRNA), which carries
genetic information from DNA to the ribosomes.
• 15% of RNA is transfer RNA (tRNA), which translates
the genetic information in mRNA into the amino acid
sequence for the protein.
• 80% of RNA is ribosomal RNA (rRNA), which is the
most abundant type of RNA; it is combined with
proteins to form ribosomes.
General, Organic, and Biological Chemistry: Structures of Life, 5/e
Karen C. Timberlake
© 2016 Pearson Education, Inc.
tRNA
A typical tRNA molecule
• can be drawn as a two-dimensional cloverleaf.
• shows more twists that illustrate the L-shape of tRNA in
the three-dimensional model.
• has an acceptor stem at the 3′ end with the nucleotide
sequence ACC where an enzyme attaches an amino
by forming an ester bond with the free — OH group.
• contains an anticodon, which is a series of three bases
that complements three bases on mRNA.
General, Organic, and Biological Chemistry: Structures of Life, 5/e
Karen C. Timberlake
© 2016 Pearson Education, Inc.
tRNA Structure
(a) A two-dimensional model of tRNA is represented as a
cloverleaf. (b) The three-dimensional model of RNA has more
twists that shows its L-shape.
General, Organic, and Biological Chemistry: Structures of Life, 5/e
Karen C. Timberlake
© 2016 Pearson Education, Inc.
RNA and Protein Synthesis
In the nucleus, genetic information for protein synthesis
• is copied from a gene in DNA.
• makes mRNA in a process called transcription.
The mRNA molecules move out of the nucleus into the
cytosol, where they bind with the ribosomes.
The tRNA molecules convert the information in the mRNA
into amino acids in a process called translation.
General, Organic, and Biological Chemistry: Structures of Life, 5/e
Karen C. Timberlake
© 2016 Pearson Education, Inc.
RNA and Protein Synthesis
The genetic information in DNA is replicated in cell
division and used to produce messenger RNA that
codes for amino acids used in protein synthesis at the
ribosomes.
General, Organic, and Biological Chemistry: Structures of Life, 5/e
Karen C. Timberlake
© 2016 Pearson Education, Inc.
Transcription: Synthesis of mRNA
In transcription, a section of DNA containing the gene
unwinds.
• RNA polymerase uses the DNA template strand to
form the new mRNA using bases that are
complementary to the DNA template.
• An mRNA is synthesized using complementary base
pairing, with uracil (U) replacing thymine (T).
• The newly formed mRNA moves out of the nucleus to
ribosomes in the cytoplasm.
Core Chemistry Skill Writing the mRNA Segment for a
DNA Template
General, Organic, and Biological Chemistry: Structures of Life, 5/e
Karen C. Timberlake
© 2016 Pearson Education, Inc.
Transcription: Synthesis of mRNA
DNA undergoes transcription when RNA polymerase makes a
complementary copy of a gene using the 3′ to 5′ DNA template strand.
General, Organic, and Biological Chemistry: Structures of Life, 5/e
Karen C. Timberlake
© 2016 Pearson Education, Inc.
mRNA Processing: Exons, Introns
In eukaryotes,
• DNA contains exons that code for proteins and
introns that do not code for proteins.
• a pre-RNA is formed that includes the noncoding
introns.
• the noncoding introns are removed.
• the exons are joined to form mRNA, which goes
to the ribosomes with the information for the
synthesis of protein.
General, Organic, and Biological Chemistry: Structures of Life, 5/e
Karen C. Timberlake
© 2016 Pearson Education, Inc.
mRNA Processing: Exons, Introns
A pre-mRNA, containing copies of the exons and introns from the gene,
is processed to remove the introns to form a mature mRNA that codes
for a protein.
General, Organic, and Biological Chemistry: Structures of Life, 5/e
Karen C. Timberlake
© 2016 Pearson Education, Inc.
Regulation of Transcription
Regulation of mRNA syntheses is controlled at the
transcription level by a specific mRNA that is
synthesized when the cell requires a particular protein.
• The transcription of a gene requires the RNA
polymerase to bind to DNA in plants and animals.
• A group of protein complexes called a transcription
factor must recognize and bind to promoter
segments.
• Other proteins, called activators, bind with the
transcription factor complex to increase the rate of
RNA transcription.
General, Organic, and Biological Chemistry: Structures of Life, 5/e
Karen C. Timberlake
© 2016 Pearson Education, Inc.
Regulation of Transcription
Transcription factors at the promoter region bind
RNA polymerase to DNA, which activates the
transcription of a gene.
General, Organic, and Biological Chemistry: Structures of Life, 5/e
Karen C. Timberlake
© 2016 Pearson Education, Inc.
Study Check
What is the sequence of bases in mRNA produced from a
section of the template strand of DNA that has the sequence
of bases
3′ C T A A G G 5′?
1) 5′ G A T T C C 3′
2) 5′ G A U U C C 3′
3) 5′ C T A A G G 3′
General, Organic, and Biological Chemistry: Structures of Life, 5/e
Karen C. Timberlake
© 2016 Pearson Education, Inc.
Solution
What is the sequence of bases in mRNA produced from a
section of the template strand of DNA that has the sequence
of bases
3′ C T A A G G 5′?
3' C T A A G G 5'
2) 5' G A U U C C 3'
General, Organic, and Biological Chemistry: Structures of Life, 5/e
Karen C. Timberlake
© 2016 Pearson Education, Inc.