From Gene to Protein

Transcription

1 8.2 Structure of DNA From Gene to Protein deoxyribonucleic acid - (DNA) - the ultimate source of all information in a cell This information is used by the cell to produce the protein molecules which are responsible for virtually all the major functions of the organism.

2 8.2 Structure of DNA DNA is composed of four types of nucleotides. DNA is made up of a long, double chain of nucleotides. Each nucleotide has three parts. a phosphate group a deoxyribose sugar a nitrogen-containing base phosphate group nitrogen-containing base deoxyribose (sugar)

3 8.2 Structure of DNA The nitrogen containing bases are the only difference in the four nucleotides. - Purines (adenine and guanine) have a double ring - Pyrimidines (cytosine and thymine) have a single ring

4 8.2 Structure of DNA Watson and Crick determined the three-dimensional structure of DNA by building models. DNA is a double helix made up of a sugar-phosphate backbone on the outside with nitrogenous bases on the inside

5 8.2 Structure of DNA Watson and Crick s discovery built on the work of Rosalind Franklin and Erwin Chargaff. Franklin s x-ray images suggested that DNA was a double helix of even width. Chargaff s rules stated that A=T and C=G.

6 8.2 Structure of DNA Nucleotides always pair in the same way. The base-pairing rules show how nucleotides always pair up in DNA. A pairs with T C pairs with G Because a pyrimidine (single ring) pairs with a purine (double ring), the helix has a uniform width. G A C T

7 8.2 Structure of DNA The backbone is connected by covalent bonds between the phosphate and the sugar. The nitrogenous bases are connected by hydrogen bonds. hydrogen bond covalent bond

8 8.2 Structure of DNA ribonucleic acid (RNA) differs from DNA in three major ways. RNA has ribose as the 5-C sugar RNA has uracil instead of thymine RNA is a single-stranded structure

9 8.3 DNA Replication Replication copies the genetic information. A single strand of DNA serves as a template for a new strand. The rules of base pairing direct replication. DNA is replicated during the S (synthesis) stage of the cell cycle. Each body cell gets a complete set of identical DNA.

10 8.3 DNA Replication Proteins carry out the process of replication. DNA serves only as a template. Enzymes and other proteins do the actual work of replication. Enzymes unzip the double helix. Free-floating nucleotides form hydrogen bonds with the template strand. nucleotide The DNA molecule unzips in both directions.

11 8.3 DNA Replication DNA polymerase enzymes bond the nucleotides together to form the double helix. Polymerase enzymes form covalent bonds between nucleotides in the new strand. new strand nucleotide DNA polymerase

12 8.3 DNA Replication Two new molecules of DNA are formed, each with an original strand and a newly formed strand. DNA replication is semiconservative. original strand new strand Two molecules of DNA

13 8.3 DNA Replication Replication is fast and accurate. DNA replication starts at many points in eukaryotic chromosomes. There are many origins of replication in eukaryotic chromosomes. DNA polymerases can find and correct errors.

14 8.4 Transcription RNA carries DNA s instructions outside of the nucleus. Central Dogma of Molecular Biology - states that information flows in one direction - from DNA to RNA to proteins The central dogma includes three processes. Replication Transcription Translation replication transcription translation

15 8.4 Transcription DNA stores information using the four letter code of its nitrogenous bases (A, T, C, G) Depending on the arrangement of these letters a certain sequence of DNA, called a gene, can code for a specific type of protein. Proteins are polymers of amino acids

16 8.4 Transcription transcription - copies DNA to make a strand of mrna Transcription is catalyzed by RNA polymerase. RNA polymerase and other proteins form a transcription complex The transcription complex recognizes the start of a gene and unwinds a segment of it. start site transcription complex nucleotides

17 8.4 Transcription RNA nucleotides pair with one strand of the DNA RNA polymerase bonds the nucleotides together to form a strand of mrna The DNA helix winds again as the gene is transcribed DNA RNA polymerase moves along the DNA

18 8.4 Transcription The RNA strand detaches from the DNA once the gene is transcribed. It can now move out of the nucleus to a ribosome (on the RER or floating on the cytoplasm) to be translated. RNA

19 8.4 Transcription Transcription makes three types of RNA. messenger RNA (mrna) carries a copy of the message from the DNA out of the nucleus to a ribosome where it will be translated into a protein *contains codons sequence of 3 nucleotides that code for a specific amino acid ribosomal RNA (rrna) - forms part of ribosomes where proteins are made transfer RNA (trna) contains a sequence of 3 nucleotides (called an anticodon) that brings the amino acid called for by the mrna to a ribosome

20 8.5 Translation Amino acids are coded by mrna base sequences. translation - converts mrna messages into a long chain of amino acids (polypeptide) codon - (contained on the mrna) - sequence of three nucleotides that codes for an amino acid codon for methionine (Met) codon for leucine (Leu)

21 8.5 Translation one start codon - (AUG = methionine) tells ribosome to start translation three stop codons tells the ribosome to stop translation The same amino acid may be coded for by more than one codon The genetic code matches each RNA codon with its amino acid or function.

22 8.5 Translation A change in the order in which codons are read changes the resulting protein. Universal Genetic Code - regardless of the organism, codons code for the same amino acid

23 8.5 Translation Amino acids are linked to become a protein. An anticodon is a set of three nucleotides that is complementary to an mrna codon. An anticodon is carried by a trna.

24 8.5 Translation Ribosomes consist of two subunits. The large subunit has three binding sites for trna. The small subunit binds to mrna.

25 8.5 Translation For translation to begin, trna binds to a start codon and signals the ribosome to assemble. A complementary trna molecule binds to the exposed codon of the mrna, bringing its amino acid close to the first amino acid.

26 8.5 Translation The ribosome helps form a peptide bond between the neighboring amino acids. The trna then releases the amino acid once the peptide bond is formed. The ribosome pulls the mrna strand the length of one codon.

27 8.5 Translation The now empty trna molecule exits the ribosome. The next complementary trna molecule binds to the next exposed mrna codon, bringing another amino acid. Once the peptide bond is formed, the trna releases the amino acid and exits the ribosome. This continues until a stop codon is reached.

28 8.5 Translation Once the stop codon is reached, the ribosome releases the polypeptide and disassembles. The polypeptide eventually folds into a protein and performs whatever function it does in the cell (enzymes, hemoglobin, insulin etc.)