Replication, Transcription, and

Translation

Information Flow from DNA to Protein

The Central Dogma of Molecular Biology

Replication is the copying of DNA in the course of cell division.

Transcription is the synthesis of RNA from DNA.

Translation is the synthesis of polypeptides through the combined efforts of rRNA, mRNA, and tRNA.

Approximately 2% of the nucleotides in DNA result in the formation of polypeptides. These regions in the DNA are called coding regions. The remaining 98% of the nucleotides do not appear to have any function and are called junk DNA.

Basic Model of Replication

Semiconservative ReplicationReplication proceeds simultaneously at thousands of replication bubbles along each DNA strand. Replication of an entire DNA strand requires approximately 10 minutes.

A single parent DNA duplex molecule is transformed into two daughter DNA duplexes, each containing one strand of the parent DNA and one newly synthesized DNA strand.

This form of DNA replication is termed semiconservative.

Replication at the

Molecular Level

Nucleoside triphosphates carrying each of the four bases move into place by forming hydrogen bonds with the bases exposed on the DNA template strand.

DNA polymerase catalyzes bond formation between the 5’ phosphate group of the arriving nucleoside triphosphate and the 3’ —OH at the end of the growing polynucleotide strand.

ReplicationHuman cell division is extremely complex and takes a little less than one day. Approximately one third of this time is involved in replicating DNA.

Replication of DNA is carried out with an error level of less than 1 wrong nucleotide per 10 billion. This extreme accuracy preserves the genetic integrity from generation to generation.

DNA replication involves:

A replication bubble, with two replication forks.

Several Enzymes, including:

Unwinding Enzyme

DNA polymerase for continuos replication

DNA ligase for discontinuous replication

Exo- and Endonucleases - to correct errors

Replication

Replication: The Big Picture

The template strand can only be read in the 3’ to 5’ direction, and the new DNA strand can grow only in the 5’ to 3’ direction.

Only the leading strand, is able to grow continuously; the DNA polymerase traveling in the 3’ to 5’ direction, is moving in the s a m e d i r e c t i o n a s t h e replication fork.

On the other strand, the DNA polymerase is moving in the opposite direction as the replication fork.

The lagging strand is replicated i n s h o r t s e g m e n t s c a l l e d Okazaki fragments. These short DNA segments are joined together by DNA ligase.

leading strand

lagging strand

TranscriptionTranscription is the synthesis of rRNA, tRNA, and mRNA, using the nucleotide sequence information from DNA.

The RNA is synthesized as a complementary copy of one of the two DNA single strands (template strand), in a process similar to DNA synthesis.

The template strand in a transcription bubble is read by RNA polymerase, starting at an initiation site and ending at a termination site.

The error rate of RNA polymerase is less than 1 base in 10,000, much higher than for DNA polymerase.

Transcription: The Big Picture

Transcription begins when RNA polymerase recognizes a control segment in DNA that precedes the nucleotides to be transcribed.

Transcription: The Big Picture

RNA polymerase moves down the DNA segment, adding complementary nucleotides to the growing RNA strand.

Transcription ends when the RNA polymerase reaches a termination sequence that signals the end of the sequence to be copied.

Introns and Exons

After its initial synthesis, an RNA molecule is called primary transcript RNA (ptRNA).

ptRNA is subsequently modified by posttranscriptional processing to form m-, r-, and t-RNA’s. Some posttranscription processing occurs in the nucleus and some in the cytoplasm.

An important part of posttranscriptional processing is the deletion of non-coding RNA segments (introns), and splicing together the coding segments (exons).

Repressors and Inducers

Only a fraction of the DNA in the coding regions of any one cell is actually expressed (~2%).

Repressor proteins turn off DNA synthesis coding for proteins not needed in a particular cell type.

Inducer proteins turn on DNA synthesis for required proteins.

Note: The binding of some inducer and repressor proteins to DNA is influenced through alteration of their three - dimensional structure by

interactions with hormone molecules.

Translation

Translation is the process of polypeptide synthesis, mediated by rRNA, mRNA, and tRNA.

Translation occurs in the cytoplasm at structures called ribosomes (which contain rRNA). mRNA binds to a ribosome and provides the genetic message which specifies a particular polypeptide sequence. tRNA molecules carrying attached α-amino acids supply the amino acids to be polymerized into the peptide.

Translation: The Big Picture

Ribosomes consist of two subunits, one large and one small, and are composed of rRNA (60%) and protein (mostly enzymes required for protein synthesis).

tRNA Structure

Each amino acid is attached to a tRNA molecule by an enzyme molecule called an aminoacyl-tRNA synthetase. This enzyme has recognition sites for both the amino acid and the corresponding tRNA.

Translation: tRNA synthesis

Translation: tRNA synthesis

Translation at the Molecular Level

Translation at the Molecular Level

Translation at the Molecular Level

Translation at the Molecular Level

Translation at the Molecular Level

Translation at the Molecular Level

Translation at the Molecular Level

Translation at Multiple Sites

Translation at Multiple Sites

The Genetic Code

The sequence in an mRNA is a coded sentence that spells out the order in which amino acid residues

should be joined to form a protein.

Codon A sequence of three ribonucleotides that codes for a specific amino acid or stops translation.

Genetic code The sequence of nucleotides, coded in triplets (codons) in mRNA, that determines the sequence of amino acids in protein synthesis.

The Genetic Code

Of the 64 possible three-base combinations in RNA, 61 code for specific amino acids and 3 code for chain

termination (the stop codons).

The “meaning” of each codon is the genetic code, and is universal to all but a few living organisms.

Most amino acids are specified by more than one codon.

Codons are always written in the 5’ to 3’ direction.

The Genetic Code

DNA informational strand:

5’ ATG CCA GTA GGC CAC TTG TCA 3’

DNA template strand:

3’ TAC GGT CAT CCG GTG AAC AGT 5’

mRNA:

5’ AUG CCA GUA GGC CAC UUG UCA 3’

Protein: Met Pro Val Gly His Leu Ser

Expansion of the Central Dogma

DNA-Directed-DNA-Polymerase

DNA-Directed-RNA-Polymerase

RNA-Directed-RNA-Polymerase

RNA-Directed-DNA-Polymerase