7.3.1-7.4.7 Transcription and Translation

Outline the structure of ribosomes, including protein and RNA composition, large and small subunits, three tRNA binding sites and mRNA binding sites.

Ribosomes have a particular structure. They are made up of proteins and ribosomal RNA. They have two subunits, one large the other small. On the surface of the ribosome there are three sites to which tRNA can bind to. However not more than two tRNA molecules can bind to the ribosome at one time. Also there is a site on the surface of the ribosome to which mRNA can bind to.

Distinguish between the sense and antisense strands of DNA.

The antisense strand is the template DNA strand which is transcribed. The sense strand on the other hand is the DNA strand which has the same base sequence as the mRNA with thymine instead or uracil.

In what direction does Translation occur?

State that transcription is carried out in a 5? → 3? direction.

In what direction does translation occur?

State that translation occurs in a 5? → 3? direction.

Explain that each tRNA molecule is recognized by a tRNA-activating enzyme that binds a specific amino acid to the tRNA, using ATP for energy.

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

Explain the process of translation, including ribosomes, polysomes, start codons and stop codons.

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

Draw and label a diagram showing the structure of a peptide bond between two amino acids.

http://www.old-ib.bioninja.com.au/_Media/dipeptide-formation_med.jpeg

Explain the process of transcription in prokaryotes, including the role of the promoter region, RNA polymerase, nucleoside triphosphates and the terminator.

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