Unit 4

Biological Information Flow

Information Flow

REPLICATION - The Players

• Leading strand DNA• Lagging strand DNA• DNA Polymerase III• Helicase• RNA Primase and

RNA Primers• Okazaki Fragments• Ligase

REPLICATION (DNA Synthesis)

• DNA Helicase partially unwinds the double helix at an area known as the replication fork.

• As the two DNA strands separate and the bases are exposed, the enzyme DNA polymerase III moves into position at the point where synthesis will begin.

• Primase synthesizes RNA primers (why are there more on lagging strand?)

REPLICATION cont.

• Leading strand is synthesized contiguously but lagging strand is synthesized discontiguously….WHY?

• After DNA is synthesized, another DNA Polymerase removes RNA primers and replaces them with DNA

• The gaps in sugar-phosphate backbone are then sealed with ligase resulting in 2 exact copies of DNA!

Replication cont.

TRANSCRIPTION = making mRNA from DNA

DNA = STORING GENETIC INFORMATIONRNA = three different functions:

• Ribosomal RNAs – (rRNA) makes us the ribosome. Each ribosome consists of about 60% ribosomal RNA and 40% protein. • Messenger RNAs – (mRNA) "record" information from DNA in the cell nucleus and carry it to the ribosomes. • Transfer RNAs- (tRNA) delivers amino acids one by one to protein chains growing at ribosomes

TRANSCRIPTION cont.

• Not all DNA is transcribed, only transcription units (genes).

• Each transcription unit encodes an RNA transcript or family of transcripts.

• These transcripts are – processed – transported to the cytoplasm– translated (mRNA)

Note: mRNA is made in the NUCLEUS!

Transcription Unit

(Left to right, 3'' to 5'' on copied [template] strand)

Transcription

1. Carried out by an  enzyme complex known as  RNA polymerase.

2. Faithfully copies one of two DNA strands 3. RNA polymerase binds to a special region (DNA

sequence) called the promoter. 4. RNA polymerase unwinds about 1 turn of DNA (about

10 base pairs) to produce a single stranded template. (hydrogen bonds of the DNA are broken by the unwinding process)

5. The first two nucleotides are hydrogen-bonded to DNA and are joined 5' to 3'

6. Continue to add new nucleotides to the 3' end.

Transcription cont.

7. As synthesis proceeds, the RNA polymerase moves in the 3' to 5' direction on the copied strand.

8. When a specific terminator sequence is reached, transcription stops.

9. Enzymes and the single stranded RNA product are then released.

10.The orientation of the RNA is determined by the promoter, that is, by which strand contain the sequence that is recognized by RNA polymerase. This can be oriented in either direction.

NOTE: DNA is always read 3' to 5' so that the new RNA is made in the 5' to 3' direction. 

TRANSCRIPTION

Many RNA polymerase molecules may process the same transcription unit at

the same time.

See animation of transcription

http://vcell.ndsu.nodak.edu/animations/transcription/movie.htm

TRANSLATION

- Involves the transfer of biological information from the 4 nucleotide code of nucleic acids (A,C,G,T) into the 20 amino acid code of proteins

- Each 3 nucleotide coding unit that specifies an amino acid is called a codon. A codon is a group of three consecutive nucleotides required to specify a single amino acid

TRANSLATION (3 easy steps)

1. INITIATION

- an mRNA, a ribosome, and the first tRNA molecule come together.

- The tRNA containing MET will recognize the start signal and bind the “P” site of the ribosome complex.

TRANSLATION cont.2. ELONGATION

- The incoming tRNA binds the A site (the only tRNA able to attach is complementary to the codon of the A site on the mRNA.)

- A peptide bond forms bewteen amino acids in A and P sites. (growing peptide now at A site)

- The ribosome complex moves so the growing peptide is now in P site, tRNA from Met is in the Exit and released, a new tRNA containing another amino acid binds the A site.

TRANSLATION cont.

3. TERMINATION- A "stop" codon (UAA, UGA, or UAG) signals the end of the process. - An enzyme called the releasing factor binds the P site freeing the PROTEIN - The mRNA molecule is released from the ribosome complex which then falls apart- All mRNA messages are eventually degraded when the protein no longer needs to be made.

Translation

http://www-class.unl.edu/biochem/gp2/m_biology/animation/gene/gene_a3.htmlAnimation:

Key Points to remember

• Transcription: DNA to RNA– DNA is read 3’ to 5’ while RNA is made 5’ to 3’

• Translation: making a polypeptide chain from mRNA– mRNA is read 5’ to 3’.

• Complementary base pairing. A with T, C with G. In RNA the T is replaced by Uracil (U).

Practice Problem:

• Look at the following DNA template strand. Figure out the complementary mRNA sequence, the tRNA anticodons, and the appropriate amino acid sequence

3’- A G C T T A C C G T G G - 5’