Chapter 12: DNA & RNA

Section 12-1: DNA & RNA After it was established that traits were passed from one generation to the next, scientists wanted to know how this information passed from parental to offspring. The most important experiments done to answer this question were performed by: Frederick Griffith Oswald Avery Alfred Hershey & Martha Chase

Experiment Summary Frederick Griffith: Oswald Avery: Hershey-Chase: Used isolated bacteria that cause pneumonia. Disease-causing bacteria Harmless bacteria Showed that heat-killed bacteria could “transform”” harmless into disease-causing bacteria. Oswald Avery: Repeated Griffith’s experiment, using an extract from heat-killed bacteria. Treated extract with chemicals to eliminate macromolecules. Showed that DNA was the molecule causing the “transformation” in harmless bacteria. Hershey-Chase: Used bacteriophages and radioactive markers to corroborate previous results. Experiment would discriminate if what caused the “transformation” was nucleic acid or protein. Showed that DNA was the transferred material and not protein.

Griffith’s Experiment

Avery’s Experiment

Hershey-Chase Experiment

Structure of DNA DNA is composed of 4 Nucleic Acids, a deoxyribose and a phosphate group Depending on their chemical composition, they are classified in one of two categories: Purines – two rings Adenine Guanine Pyrimidines – one ring Thymine Cytosine

Structure of DNA Nucleic Acids DNA Backbone

Chargaff’s Rule Nucleic Acids pair between a purine and a pyrimidine. Adenines with Thymines Guanines with Cytosines The percent of Guanines and Cytosines are almost equal and the same is true for Adenine and Thymine. This is known as Chargaff’s Rule.

X-Ray Evidence Research done by Rosalind Franklin using X-Ray diffraction, showed the pattern that latter helped Watson and Crick determine the 3-D structure of DNA.

The Double Helix In 1953 Francis Crick and James Watson proposed the model that sustained the previously stated and it explained the DNA’s properties. Watson and Crick’s model of the DNA was a double helix, in which two strands were wound around each other.

The Double Helix

DNA Structure’s Summary Watson and Crick’s model explained all the previous data presented: The double helix explains the paring of nucleic acids (base paring). Chargaff’s Rule The helix structure explains the shape that Rosalind Franklin saw.

Section 12-2: Chromosomes and DNA Replication DNA molecules are quite long. The smallest human chromosome contains more than 30 million base pairs of DNA. To achieve the level of compaction needed, DNA is arranged into chromosomes.

DNA to Chromosome

DNA Replication When Watson & Crick presented their structural model of the DNA, their proposed structure also gave them insight into a possible method of coping the molecule. Each of the strands can serve as a mold for the new strand using base paring rules.

DNA Duplication Before each cell division, the DNA is copied by the process called replication. During DNA replication, the DNA molecule separates into two strands, then produces two new complementary strands following the rules of base pairing. Each strand of the double helix of DNA serves as a template, or model, for the new strand.

DNA Duplication

DNA Duplication

Prokaryotes vs. Eukaryotes The enzyme DNA Polymerase is involved in DNA Replication. This enzyme besides adding nucleotides it performs “poof-reading” to maximize correct pairing of bases Prokaryote Eukaryote

12-3: RNA and Protein Synthesis RNA’s structure is like DNA’s with three major differences: RNA is generally single strand (DNA is double) RNA has the nucleotide Uracil (DNA has Thymine) RNA has the sugar ribose (DNA has deoxyribose)

Types of RNA There are 3 major classes of RNA: mRNA: messenger RNA, takes information from DNA to the rest of the cell. rRNA: ribosomal RNA, combines with proteins to form the ribosomes. tRNA: transfer RNA, transfer amino acids to the ribosome to build proteins based on information in the mRNA.

Types of RNA Messenger RNA (mRNA)

Types of RNA Messenger RNA (mRNA) Ribosomal RNA (rRNA) RNA (orange); Protein (blue) Messenger RNA (mRNA) Ribosomal RNA (rRNA)

Types of RNA Messenger RNA (mRNA) Ribosomal RNA (rRNA) RNA (orange); Protein (red) Messenger RNA (mRNA) Ribosomal RNA (rRNA) Transfer RNA (tRNA)

Types of RNA

Transcription Process to make an mRNA using the DNA as template. Process mediated by enzyme, RNA Polymerase

Transcription

RNA Editing

Genetic Code mRNA is read in groups of 3 called triplets. Each triplet is called a codon. Each codon specify for a specific amino acid. Some amino acids can be specified by more than one codon.

Genetic Code Important codons: Start (AUG) Stop (UGA - UAA – UAG) Some amino acids have more than one codon. This means that the code is redundant. The reason why some codons code the same amino acid is because of the 3rd position of the codon.

Translation Process of “reading” the mRNA into protein. Process occurs within the ribosome (combination of rRNA and Protein). The ribosomes contains various sites within its structure for the entrance of the mRNA, the tRNA and the exit of the growing protein.

Translation

Genes and Proteins Genes contain the information to assemble proteins. Depending on the protein or changes done to these proteins, changes in phenotype can be noticed. Proteins catalyze the reactions that produce the pigment for the eyes or the antigens in the surface of red-blood cells.

12-4: Mutations Mutations are changes in the DNA sequence that affects genetic information. Mutations can occur either at the level of: Genes Point mutations (Substitutions) Frameshift mutations (Insertions) Chromosomes Deletion Duplication Insertion Translocation

Genetic Mutations Gene mutations are changes in the DNA sequence or changing the sequence of triplets read. Substitutions: changing a nucleotide of another. Missense & Nonsense Frameshift: changes in the reading frame. Addition or Deletions

Chromosomal Mutations Chromosomal mutations are changes in the structural make-up of the chromosomes. Inversion: changes in the order of genes with the chromosome. Deletions: elimination of a segment of a chromosomes. Translocation: exchange from one chromosome to another (not crossing over). Insertion: a piece of a chromosome is relocated to another chromosome.

12-5: Gene Regulation Only a handful o genes are expressed at a particular time. An expressed gene is a gene that is transcribed. Non-transcribed genes can be considered “silent genes”. Genes can turn “on” or “off”. Before After

Gene Regulation Not all sequences of DNA are genes, some sequences serve as promoters or binding sites for proteins, or signals for regulation (start and stop). These sequences that do not code for a transcript are called regulatory sites.

Gene Regulation in Prokaryotes Prokaryotes although simpler than Eukaryotes, utilize an operon to control the expression of their genes. An operon is a group of genes that operate (turn on or off) together. An example of a prokaryotic operon is the Lac Operon.

The Lac Operon The Lac gene does not need to be turned on all the time. Bacteria can use glucose as a source of energy. In the absence of glucose and presence of galactose, the Lac operon must turn on to produce the protein that will break down galactose. Gal  Glu + Glu

The Lac Operon Repressor – gene off Inducer – gene on

Gene Regulation in Eukaryotes Most eukaryotic genes are controlled individually and have regulatory sequences that are much more complex than those of the lac operon.

Regulation and Development With gene regulation, studies have shown that there specific genes that control the development of embryos. Hox genes are a series of genes that control organ and tissues that develop in various parts of the embryo. These genes determine the basic body plan for an animal.

Hox genes

Chordate Embryonic Comparison