7.3 Transcription in prokaryotes

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

Nucleotides are added in the form of ribonucleoside triphosphates

7.3.2. Distinguish between the sense and antisense strands of DNA.

Sense strand Anti-sense strand

Does it get transcribed?

No Yes

Is its base sequence the same as mRNA?

The sense strand has the same base sequence as the mRNA molecule, except that it contains thymine instead of uracil.

The anti-sense strand has a complementary base sequence to the mRNA molecule

A section of DNA on the antisense strand has the sequence

ATT TAG GCG CTA TGC TAG

What is the complementary DNA sequence?

What is the sequence on the codons of mRNA?

What is the sequence on the anti-codons of t-RNA

What is the sequence of amino acids?

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

Promoter and Terminator

3’

3’5’

5’

DNA in the nucleus of a eukaryotic cell or DNA of the nucleoid region of a prokaryotic cell

Transcription: Going from the DNA code to the code of mRNA

RNA polymerase has two functions

The promoter is a region of the DNA of a specific sequence that signals the RNA polymerase where to start transcription

RNA polymerase unwinds the DNA and breaks the hydrogen bonds between the complementary bases

Free floating nucleotides are added in a 5’ to 3’ direction. They are also added as ribonucleoside triphosphate (not show in the drawing)

RNA nucleotides are added forming a mRNA transcript

RNA polymerase also adds complementary RNA nucleotides to the antisense strand of DNA. Nucleotides are added in a 5’ to 3’ direction.

The terminator is a region of DNA that signals to the RNA polymerase where to stop transcription

At the terminator the RNA polymerase falls off and a mRNA transcript has been formed

IB Question: Explain the process of transcription in eukaryotes. [8]N08/4/BIOLO/HP2/ENG/TZ0/XX+

IB Question: Explain the process of transcription in eukaryotes. [8]N08/4/BIOLO/HP2/ENG/TZ0/XX+

DNA (sequence) is copied to (m)RNA; DNA separated; RNA polymerases separate the strands; only one strand is copied/transcribed / antisense strand is transcribed; RNA polymerase binds to promoter/initiation region (of template strand); ATP provides energy for attachment; nucleotides exist as nucleotide triphosphates; removal of phosphates (2) from nucleotide triphosphates provides energy for linkage; RNA polymerase catalyses the formation of the polymer / helps the nucleotides join; (m)RNA lengthens in 5' to 3' direction; RNA polymerase reaches terminator on DNA to stop transcription process; mRNA is separated from DNA; [8 max]

IB Question: Explain the process of transcription leading to the formation of mRNA. [8]N10/4/BIOLO/HP2/ENG/TZ0/XX

IB Question: Explain the process of transcription leading to the formation of mRNA. [8]N10/4/BIOLO/HP2/ENG/TZ0/XX

RNA polymerase; (polymerase number is not required)binds to a promoter on the DNA;unwinding the DNA strands;binding nucleoside triphosphates;to the antisense strand of DNA;as it moves along in a 5'→3' direction;using complementary pairing/A-U and C-G;losing two phosphates to gain the required energy;until a terminator signal is reached (in prokaryotes);RNA detaches from the template and DNA rewinds;RNA polymerase detaches from the DNA;many RNA polymerases can follow each other;introns have to be removed in eukaryotes to form mature mRNA; [8 max]

One gene may code for multiple polypeptides due to alternative splicing

7.3.4: State that eukaryotic RNA needs the removal of introns to form mature mRNA.

How many different mRNA codons can we make with adenine (A), uracil (U), guanine (G) and cytosine (C)?

AAAAAU….….

mRNA codon table

Three mRNA codons are stop codons(UAA, UAG and UGA) , so they do not code for any amino acid.

So how many mRNA codons code for an amino acid?

tRNA molecule about to fold

7.4 : Translation

tRNA moleculeOnce in the cytoplasm, mRNA is translated from the nucleic acid language to the protein language by transfer RNA (tRNA) which acts as an interpreter. Transfer RNA does two things: 1) it recognizes the appropriate codons in mRNA and 2) it picks up the appropriate amino acids.

In the process of translation the mRNA codons bind to the anti-codons of tRNA molecules.

There are twenty amino acids.

There is a specific tRNA activating enzyme for each amino acid. So there are 20 different tRNA activating enzymes.

Each tRNA activating enzyme recognizes all the different anti- codons on tRNA molecules for a particular amino acid and will attach the correct amino acid to the tRNA molecule. For example the tRNA activating enzyme for phenylalanine will recognize both the anticodons on the tRNA molecules for phenylalanine

t-RNA activating enzyme

20 different t-RNA activating enzymes; one for each of the 20 amino acids

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

Each amino acid has a specific tRNA activating enzyme, and some amino acids have more than one tRNA.

The type of amino acid which can attach to a tRNA molecule depends on the sequence of the anticodon. A tRNA activating enzyme recognizes the anticodon and uses ATP to bind the appropriate amino acid to the 3' end.

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

IB Question: Outline the structure of a ribosome. [4] M10/4/BIOLO/HP2/ENG/TZ2/XX+

IB Question: Outline the structure of a ribosome. [4] M10/4/BIOLO/HP2/ENG/TZ2/XX+

small subunit and large subunit; mRNA binding site on small subunit; three tRNA binding sites / A, P and E tRNA binding sites; protein and RNA composition (in both subunits);

IB Question: Outline the structure of ribosomes. [6]

IB Question: Outline the structure of ribosomes. [6]

made of protein;made of rRNA;large subunit and small subunit;three tRNA binding sites;Aminacyl/A, Peptidyl/P and Exit/E;mRNA binding site (on small subunit);70S in prokaryotes / 80S in eukaryotes;can be free / bound to RER (in eukaryotes);

7.4.3: STATE: Translation consists of initiation, elongation, translocation and termination

7.4.4: State that translation occurs in a5’→ 3’ direction.

7.4.5: Draw and label a diagram showing the structure of a peptide bondbetween two amino acids.

7.4.6: Explain the process of translation, including ribosomes, polysomes, startcodons and stop codons.

A mRNA transcript has the codon sequence of basesUUG GCA AUG CUC UUU

What is the complementary anticodon sequence?

Transfer RNA is a small molecule made up of only about 80 nucleotides. It is shaped like a cloverleaf. One of the loops on tRNA contains a base triplet called the anticodon. The anticodon is complementary to a codon triplet on the mRNA. At the other end of the tRNA molecule is a specific sequence of three nucleotides (CCA). The last nucleotide is the (A) and an amino acid can attach to it.

Translation: the process of going from the code of mRNA to making a polypeptide

Initiation: tRNA with complementary anticodon to the mRNA start codon bind together and small ribosomal subunit attaches to mRNA and then large ribosomal subunit attaches. Occurs at the P-site of the ribosomeAUG is always the start codon one

the mRNA

Initiation is completed

The three sites of a ribosome

Second tRNA molecule carrying the next amino with its anti-codon complementary to the second codon of the mRNA bind together at the A-site

First amino attaches breaks away from its tRNA molecule and bonds with the second amino acid forming a dipeptide

Ribosome translocates a distance of one codon. First tRNA molecule is now in the E-site and second tRNA molecule is in the P-site. A-site is now vacant

tRNA molecule in the E-site exits the ribosome and can now have another amino acid attached to it.

Next tRNA molecule with an anti-codon complementary to the next codon on the mRNA bind together at the A-site

Ribosome translocates one codon and a peptide bond is formed between the amino acids forming a polypeptide consisting of three amino acids. Elongation is the process of forming the polypeptide. The ribosome reads the mRNA from a 5’ to 3’ direction.

tRNA in E-site exits the ribosome

Elongation continues until one of three stop codons on the mRNA is reached.

At the stop codon the small and large robosomal subunits fall of the mRNA and the polypeptide is completed

IB Question: Translation occurs in living cells. Explain how translation is carried out, from theinitiation stage onwards. 8M10/4/BIOLO/HP2/ENG/TZ1/XX

IB Question: Translation occurs in living cells. Explain how translation is carried out, from theinitiation stage onwards. 8M10/4/BIOLO/HP2/ENG/TZ1/XX

translation involves initiation, elongation/translocation and termination; mRNA binds to the small sub-unit of the ribosome; ribosome slides along mRNA to the start codon; anticodon of tRNA pairs with codon on mRNA: complementary base pairing (between codon and anticodon); (anticodon of) tRNA with methionine pairs with start codon / AUG is the start codon; second tRNA pairs with next codon; peptide bond forms between amino acids; ribosome moves along the mRNA by one codon; movement in to direction; 5 3 tRNA that has lost its amino acid detaches; another tRNA pairs with the next codon/moves into A site; tRNA activating enzymes; link amino acids to specific tRNA; stop codon (eventually) reached; [9 max]

IB Question: The diagram below shows the structure of a ribosome during protein synthesis.

(b)State the names of the structures labelled above.(c) State the name of a structure shown on the diagram that has an anticodon. [1](d) Explain why the process used during protein synthesis in cells is called translation. [2]

M07/4/BIOLO/HP2/ENG/TZ1/XX+

IB Question: The diagram below shows the structure of a ribosome during protein synthesis.

(b)State the names of the structures labelled above.(c) State the name of a structure shown on the diagram that has an anticodon. [1](d) Explain why the process used during protein synthesis in cells is called translation. [2]

M07/4/BIOLO/HP2/ENG/TZ1/XX+

Award [2] for four correct and [1] for three or two correct.I: small (sub)unit (of ribosome);II: large (sub)unit (of ribosome);III: transfer RNA/tRNA;IV: messenger RNA/mRNA; [2 max](c) transfer RNA/tRNA [1](d) codon/triplet of bases to amino acid;nucleic acid / base sequence / (m)RNA to polypeptide / protein / amino acid sequence;genetic code has to be translated; [2 max]

IB Question: Explain briefly how termination of translation occurs. [2] M07/4/BIOLO/HP2/ENG/TZ1/XX+

IB Question: Explain briefly how termination of translation occurs. [2] M07/4/BIOLO/HP2/ENG/TZ1/XX+

stop/terminator / nonsense codon (is reached);polypeptide is released;mRNA detaches from ribosome;subunits of ribosome separate; [2 max]Ignore references to specific codons.

7.4.7: STATE: Free ribosomes synthesize proteins for use primarily within the cell, and that bound ribosomes synthesize protein primarily for secretion or for lysosomes.

mRNA codon table

The sense strand of DNA has the sequence TTA GAT GCGC TGG TGCwhat would be the sequence of amino acids in the polypeptide?

The anti-sense strand of DNA has the sequence AGA GAG TAT ATA what would be the sequence of amino acids in the polypeptide?

sense strand of DNA AAC CGC GTA TGC TGA TTTanti-sense sense strand of DNA

mRNA codons

tRNA anti-codons

amino acids

IB Question: Explain the significance of complementary base pairing for replication, transcription and translation.

IB Question: Explain the significance of complementary base pairing for replication, transcription and translation.