Chapter 12 -1

GLENCOE ch 12 Living Systems Chapter 9 Bio 30

Genes are made of DNA The following scientists proved that DNA is the genetic material • Fredrick Griffith (1928) • Oswald Avery ( 1944) • Hershey and Chase (1952)

Fredrick Griffith (1928) • The discovery of the genetic role of DNA began with research by Frederick Griffith. • He studied Streptococcus pneumoniae, a bacterium that causes pneumonia in mammals. – One strain, the R strain, was harmless. – The other strain, the S strain, was pathogenic.

• When Griffith mixed heat-killed S strain with live R strain bacteria and injected this into a mouse it died. • He recovered the pathogenic strain from the dead mouse’s blood. • Some harmless bacteria had been “transformed” into the deadly strain • For the next 14 years scientists tried to identify the transforming substance. • video

Transforming Substance? • Could be:

–DNA –Proteins Because scientists already knew chromosomes consist of these substances. So the debate started.

Oswald Avery ( 1944) • Treated Griffith’s mixture with – Protein digesting enzymes  remove all proteins – DNA digesting enzymes remove all DNA

Avery contd. • Is Protein the transforming factor? – treated Griffith’s mixture of heat treated deadly strain and live harmless strains with protein-destroying enzymes grew the strains – The bacterial colonies were still transformed

• Concluded that protein could not be the transforming factor

Avery contd. • Is DNA the transforming factor? – treated the mixture with DNA-destroying enzymes grew the strains – The bacterial colonies failed to transform

• Concluded that DNA is the genetic material of the cell Scientists were still skeptical  proteins made of 20 AAs, DNA only 4 bases

Hershey and Chase (1952) • Used viruses to prove that DNA is the genetic material. – Viruses consist of a DNA (sometimes RNA) enclosed by a protective coat of protein. – To replicate, a virus infects a host cell and takes over the cell’s metabolic machinery. – Viruses that specifically attack bacteria are called bacteriophages or just phages.

• Conclusion Phage DNA entered the bacterial cell, proteins did not • DNA carries the genetic information. • Although scientists were now confident that DNA was the genetic material they still did not understand the structure of DNA and how it communicated information

Watson and Crick • Prior to Watsons and Crick’s discovery other scientists had already made some discoveries… • In the1920 Levine had determined the basic structure of nucleotides • Chargaff had analyzed the bases and determined the ratios of each. Chargaff’s rule will be looked at.

Watson and Crick • In 1953, Watson and Crick • discovered the structure of • DNA • DNA is composed of a double helix

The Nucleotides- the basic structure that makes up DNA • • • • •

The Structure of DNA S = sugar P = phosphate B = base (A, G, C, T) VIDEO

The 4 Bases (rungs) • • • • •

DNA Contains Four Bases: A = Adenine G = Guanine T = Thymine C = Cytosine

Chargaff’s Law • DNA Base Pairing Rules: • A always pairs with T • G always pairs with C

DNA is the Genetic Material Therefore it must (2) Replicate faithfully. (3) Have the coding capacity to generate proteins and other products for all cellular functions. • “A genetic material must carry out two jobs: duplicate itself and control the development of the rest of the cell in a specific way.” • -Francis Crick

Replication of DNA • Daughter DNA molecules contain one parental • strand and one newly-replicated strand

Semiconservative model:

Replication(Semi-Conservative Model) • Each strand acts as a template for the synthesis of a new DNA molecule by the sequential addition of complementary base pairs, thereby generating a new DNA strand that is the complementary sequence to the parental DNA. Each daughter DNA molecule ends up with one of the original strands and one newly synthesized strand.

Base Pairing • The two strands of the double helix are first separated by enzymes. With the assistance of other enzymes, spare parts available inside the cell are bound to the individual strands following the rules of complementary base pairing: adenine (A) to thymine (T) and guanine (G) to cytosine (C). Two strands of DNA are obtained from one, having produced two daughter molecules which are identical to one another and to the parent molecule.

Amino acids • Amino Acids are the chemical units or "building blocks" of the body that make up proteins. Protein substances make up the muscles, tendons, organs, glands, nails, and hair. Growth, repair and maintenance of all cells are dependent upon them. Next to water, protein makes up the greatest portion of our body weight.

Genes • A gene can be defined as a region of DNA that controls a hereditary characteristic. It usually corresponds to a sequence used in the production of a specific protein • A gene carries biological information in a form that must be copied and transmitted from each cell to all its progeny. • Genes can be as short as 1000 base pairs or as long as several hundred thousand base pairs.

Codons • The genetic code consists of 64 triplets of nucleotides. These triplets are called codons.With three exceptions, each codon encodes for one of the 20 amino acids used in the synthesis of proteins. That produces some redundancy in the code: most of the amino acids being encoded by more than one codon.

Codons • The genetic code consists of 64 triplets of nucleotides. These triplets are called codons.With three exceptions, each codon encodes for one of the 20 amino acids used in the synthesis of proteins. That produces some redundancy in the code: most of the amino acids being encoded by more than one codon.

To Recap • Codons are 3 bases which code for amino acids which form long chains called polypeptides of which 2 or ore may join together to make proteins . Genes are related in that they contain the information for making proteins. • The DNA code is on page 241 of your chapter handout – be able to decipher the code and also (not in your chapter ) know the code for RNA

RNA (FYI not on test) • RNA: Ribonucleic acid. Single-stranded where DNA is double-stranded, messenger RNA carries single pages of instructions out of the nucleus to places they're needed throughout the cell. Transfer RNA helps translate the mRNA message into chains of amino acids in the ribosomes.

Chromosome Structure (not on test) • Prokaryotes – DNA is in cytoplasm • Eukaryotes –DNA is organized into chromosomes – the DNA tightly coils around protiens called histones Nucleosome: Subunit of chromatin composed of a short length of DNA wrapped around a core of histone proteins.

Nucleosomes cont. • The human genome contains about 3 billion nucleotide pairs organized as 23 chromosomes pairs. If uncoiled, the DNA contained by each of those chromosomes would measure between 1.7 and 8.5 cm (0.67 to 3.35 inches) long. This is too long to fit into a cell. Moreover, if chromosomes were composed of extended DNA, it is difficult to imagine how the DNA could be replicated and segregated into two daughter cells without breaking down.

End of Section 1 • DNA is packaged into a compact structure with the help of specialized proteins called histones. The complex DNA plus histones in eukaryotic cells is called chromatin.

Assessment • 1. • Griffith showed that bacteria could be transformed by the transfer of genetic material • Avery showed the DNA was the transforming factor

Assessment • 2. • Franklin’s data showed a helix shape • Chargaff’s data suggested how the bases pair

Assessment • 3.

Assessment • 4. DNA coils around histones to form nucleosomes which group together to form chromatin fibres which supercoil to make the chromosome