Unit 4 - Transcription and Translation AO's

Terms in this set (16)

mRNA is produced during transcription. In prokaryotes, RNA polymerase recognises a specific sequence of DNA called the promoter. The promoter basically "tells" the RNA polymerase where to start the transcription process. Transcription is initiated with the binding of RNA polymerase to the promoter site. The RNA polymerase then uncoils the DNA and separates the two strands. One of the strands is used as the template strand for transcription. The RNA polymerase will then use free nucleoside triphosphates to build the mRNA in a 5'→3' direction. These nucleoside triphosphates bond to their complementary base pairs on the template strand. As they bind they become nucleotides by losing two phosphate groups to release energy. Since RNA does not contain thymine, uracil pairs up with adenine instead. RNA polymerase forms covalent bonds between these nucleotides. It moves along the DNA to keep elongating the sequence of mRNA until it reaches a sequence of DNA called the terminator. This sequence of DNA "tells" the RNA polymerase to stop transcription. The RNA polymerase is then released from the DNA and the newly created mRNA separates from the template DNA strand. Finally, the DNA rewinds back to its original double helical structure.

Summary:

RNA polymerase binds to the promoter region

This initiates transcription

RNA polymerase uncoils the DNA

Only one strand is used, the template strand

Free nucleoside triphosphates bond to their complementary bases on the template strand

Adenine binds to uracil instead of thymine

As the nucleoside triphosphates bind they become nucleotides and release energy by losing two phosphate groups

The mRNA is built in a 5'→3' direction

RNA polymerase forms covalent bonds between the nucleotides and keeps moving along the DNA until it reaches the terminator

The terminator signals the RNA polymerase to stop transcription

RNA polymerase is released and mRNA separates from the DNA

The DNA rewinds
There are many different types of tRNA and each tRNA is recognised by a tRNA-activating enzyme. This enzyme binds a specific amino acid to the tRNA by using ATP as an energy source. The tRNA molecule has a specific structure. It contains double stranded sections (due to base pairing via hydrogen bonds) and loops. It has an anticodon loop which contains the anticodon and two other loops. The nucleotide sequence CCA is found at the 3' end of the tRNA and allows attachment for an amino acid. Each type of tRNA has slightly different chemical properties and three dimensional structure which allows the tRNA-activating enzyme to attach the correct amino acid to the 3' end of the tRNA. There are 20 different tRNA-activating enzymes as there are 20 different amino acids. Each enzyme will attach a specific amino acid to the tRNA which has the matching anticodon for that amino acid. When the amino acid binds to the tRNA molecule a high energy bond is created. The energy from this bond is used later on to bind the amino acids to the growing polypeptide chain during translation.

Summary:

Each tRNA activating enzyme recognises a specific tRNA molecule

The tRNA molecule is made up of double stranded sections and loops

At the 3' end of the tRNA there is the nucleotide sequence CCA to which the amino acid attaches to

The different chemical properties and three dimensional structure of each tRNA allows the tRNA-activating enzymes to recognise their specific tRNA

Each tRNA enzyme binds a specific amino acid to the tRNA molecule

The tRNA-activating enzyme will bind the amino acid to the tRNA with the matching anticodon

Energy from ATP is needed during this process
Translation occurs in the cytoplasm. It starts off with the tRNA containing the matching anticodon for the start codon AUG binding to the small subunit of the ribosome. This tRNA carries the amino acid methionine and is always the first tRNA to bind to the P site. The small subunit of the ribosome then binds to the 5' end of the mRNA. This is because translation occurs in a 5'→3' direction. The small subunit will move along the mRNA until it reaches the start codon AUG. The large subunit of the ribosome can then binds to the small subunit. The next tRNA with the matching anticodon to the second codon on the mRNA binds to the A site of small subunit of the ribosome. The amino acids on the two tRNA molecules then form a peptide bond. Once this is done, the large subunit of the ribosome moves forward over the smaller one.The smaller subunit moves forward to join the larger subunit and as it does so the ribosome moves 3 nucleotides along the mRNA and the first tRNA is moved to the E site to be released. The second tRNA is now at the P site so that another tRNA with the matching anticodon can then bind to the A site. As this process continues the polypeptide is elongated. Once the ribosome reaches the stop codon on the mRNA translation will end as no tRNA will have a matching anticodon to the stop codon. The polypeptide is then released. Many ribosomes can translate the same mRNA at the same time. They will all move along the mRNA in a 5'→3' direction. These groups of ribosomes on a single mRNA are called polysomes.

Summary:

The tRNA containing the matching anticodon to the start codon binds to P site of the small subunit of the ribosome

The small subunit binds to the 5' end of the mRNA and moves along in a 5'→3' direction until it reaches the start codon

The large subunit then binds to the smaller one

The next tRNA with the matching anticodon to the next codon on the mRNA binds to the A site

The amino acids on the two tRNA molecules form a peptide bond

The larger subunit moves forward over the smaller one

The smaller subunit rejoins the larger one, this moves the ribosome 3 nucleotides along the mRNA and moves the first tRNA to the E site to be released

The second tRNA is now at the P site so that another tRNA with the matching anticodon to the codon on the mRNA can bind to the A site

As this process continues, the polypeptide is elongated

Once the ribosome reaches the stop codon on the mRNA translation ends and the polypeptide is released

Many ribosomes can translate a single mRNA at the same time, these groups of ribosomes are called polysomes
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