Chapter 12 Notes, DNA, RNA, and Protein Synthesis

The Discovery of DNA By the early 1900's, scientists knew that genes and chromosomes were responsible for traits being inherited from parents to offspring. However, the key component of the chromosomes that actually contained the genetic information remained a mystery. Chemical analysis of chromosomes told them that the genetic material had to be either proteins or nucleic acids (DNA), but they didn't know which one was responsible for carrying the genetic information.

Griffith's Experiment In 1928, a British bacteriologist by the name of Fredrick Griffith performed an experiment to try to discover what the genetic material was. Griffith injected two different strains of a bacteria (Streptococcus pneumoniae) into mice. One strain was covered in a sugar coat and one was not. The strain that had the sugar coat he called the smooth or S strain. The strain that lacked the sugar coat he called the rough or R strain.

The Results of Griffith's Experiment The smooth strain was the virulent (disease causing) strain. The rough strain was not. This was the result of his experiments Mice + smooth (virulent) strain = dead mice Mice + rough (nonvirulent) strain = live mice Mice + smooth (virulent) strain after the smooth strain had been killed with heat = live mice Mice + rough (nonvirulent) strain + heat killed smooth (virulent) strain = dead mice

Explanation of Griffith's Experiment Griffith concluded that a disease causing factor was transforming the rough (nonvirulent) strain into the smooth (virulent) strain of bacteria

Hershey and Chase Experiment In 1952, a bacteriologist by the name of Alfred Hershey, and a geneticist by the name of Martha Chase provided conclusive evidence that DNA was in fact the transforming factor. Their experiment involved a special type of virus called a bacteriophage. A bacteriophage is a virus that attacks bacteria. The bacteriophage was ideal for this experiment because it was made of the two key components (protein and DNA) which were thought to be possible molecules responsible for inheritance.

Hershey and Chase Experiment Hershey and Chase used a technique called radioactive labeling to trace both the protein and the DNA of the bacteriophage after infecting a bacteria. Once the virus infected the bacteria with its genetic material, they monitored which radioactive material was inherited by the bacteria. This would tell them if the genetic material was in the proteins or the DNA.

Hershey and Chase Experiment

The Structure and Composition of DNA Scientists were now confident that they had discovered what the genetic material was, but questions about the structure of DNA and how DNA communicated information remained. What they discovered is that DNA is made up of nucleotides. A nucleotide is a sugar molecule, a phosphate molecule, and a nitrogenous base.

DNA Structure and Composition In the DNA there are four different nitrogenous bases Adenine Guanine Cytosine Thymine Uracil (In RNA, replaces Thymine)

Chargaff's Rule In the 1950s, Erwin Chargaff discovered that in every organism the amount of guanine and cytosine, and the amount of adenine and thymine was nearly equal. This is called Chargaff's rule.

The Double Helix In 1951, Rosalind Franklin used X-rays to photograph DNA. Photo 51 showed that the DNA molecule was in the shape of a twisted ladder known as a double helix.

The Double Helix

Watson and Crick James Watson and Francis Crick used data from Chargaff and Franklin's photo to build the first accurate model of DNA.

The Structure of DNA DNA is like a twisted ladder made up of alternating strands of deoxyribose and phosphate. The rails of the ladder are joined by the bases. (adenine, guanine, cytosine, and thymine)

Complementary Base Pairing Each nitrogen base pairs up with another base in what is known as complementary base pairing. Purine bases pair with pyrimidine bases. Adenine and Guanine are called purines. Cytosine and Thymine are called pyrimidines. Adenine always pairs with Thymine. Guanine always pairs with Cytosine.

Purines and Pyrimidines

Complementary Base Pairing

Orientation of the DNA Another important feature of the DNA structure is the orientation of the strands. The two strands DNA are antiparrellel, meaning they run parallel but in opposite directions. This orientation is important to understand because it determines how DNA replicates. One end is referred to as the 5' (five-prime) and the other is referred to as the 3' (three-prime).

DNA Orientation We will discuss the importance of this orientation later

How Does DNA Fit Inside A Cell? Just one strand of DNA in one chromosome can be up to 245 million base pairs long! And remember humans have 46 chromosomes It has been estimated that if all the DNA from just one cell of a human's body was unwound, it would stretch about 6 ft long! That means the DNA in one cell is about 100,000 times longer than the cell itself! And amazingly, it all fits into the nucleus, which only takes up about 10% of the cell's volume!

How Does DNA Fit Inside A Cell? So how does all that information fit into a cell? DNA coils tightly around small balls of proteins called histones. Histones and phosphates from the DNA combine together to form nucleosomes. Nucleosomes combine together to form chromatin fibers, and the chromatin fibers combine together to form the chromosomes.

Chromosomes and Histones

Semiconservative Replication When Watson and Crick created their model of the DNA double helix, they also proposed a possible way that DNA might get replicated. The way they proposed DNA gets replicated is called semiconservative replication. In semiconservative replication, one of the strands always gets copied and the other strand is a copy from the original parent or template strand. The process is similar to how sourdough bread is made. In order to make it you need a starter batch (original template).

Semiconservative Replication Semiconservative Replication occurs in three stages: unwinding, base pairing, and joining During unwinding, an enzyme called DNA helicase unwinds or unzips the DNA double helix. After the strands are unwound, another enzyme called DNA polymerase, adds nucleotides to the new strand in complementary base pairs.

Semiconservative Replication Because the strands are antiparallel, one of the strands can be continously replicated and is therefore called the leading strand. The other strand, called the lagging strand, has to be replicated in reverse order in sections of nucleotides called Okazaki fragments.

Semiconservative Replication The Okazaki fragments are then glued together by another enzyme called DNA ligase

Semiconservative Replication

Semiconservative Replication

The Central Dogma It is now known that DNA contains a code that is transcribed and translated by another substance called RNA (ribonucleic acid). RNA guides the synthesis of proteins. This is what is known as the Central Dogma. DNA is transcribed by Messenger RNA. Messenger RNA carries the information. Ribosomal and Transfer RNA translate the code to make proteins.

The Central Dogma

What is RNA? RNA is similar to DNA. It is a nucleic acid. RNA contains the sugar ribose instead of deoxyribose. Instead of using Thymine as one of its base pairs, Thymine is replaced by Uracil. Another major difference between RNA and DNA, is that RNA is single stranded. There are three main types of RNA that play a role in protein synthesis They are Messenger RNA (mRNA for short), Ribosomal RNA (rRNA) and Transfer RNA (tRNA).

Messenger RNA (mRNA) The job or role of mRNA is transcription. To transcribe means to copy or rewrite something. Transcription, as it applies to DNA means to copy or rewrite the DNA code. This is the role of messenger RNA (mRNA). The messenger RNA enters the nucleus, the DNA strand is unzipped and copied. Then the messenger RNA leaves the nucleus with the code.

Messenger RNA and Transcription After the DNA is unzipped in the nucleus, an enzyme comes along called RNA polymerase. RNA polymerase assists mRNA in recording what information is found on a portion of the DNA strand, called the template strand. Messenger RNA records the code in complementary base pairs, similar to the way DNA bases are paired during replication except when the base pair Adenine is encountered, Adenine pairs with Uracil instead of Thymine.

Messenger RNA and Transcription After the mRNA is transcribed, mRNA can then leave the nucleus through nuclear pores and enter into the cytoplasm to find tRNA and rRNA.

Translation and Transcription

Ribosomes, Transfer RNA, and Translation After a mRNA finds a rRNA, the code is read and translated by interpreters called transfer RNA. Transfer RNA (tRNA) interprets the code by reading the bases in groups of three called Codons. Transfer RNA molecules each have their own Anticodon that only matches with a specific codon.

Ribosomes, Transfer RNA, and Translation

The Code The DNA code is made up of a three-base code system. Each codon matches with a specific anticodon and a specific amino acid. By joining multiple amino acids together, proteins can be assembled.