Chapter 13

DNA, RNA, & Proteins

DNA

Deoxyribonucleic acid (DNA) is the material that contains the information that determines inherited characteristics. In other words, it contains our genes.

The History of DNA

3 major experiments lead to the conclusion that DNA is the genetic material in cells. These experiments were performed by Griffith, Avery, Hershey, and Chase.

Griffith – Experimented on mice and observed some harmless strains of bacteria could change into harmful strains. He called this transformation. His study led to the conclusion that genetic material could be transferred between cells.Transformation- a change in genotype that is caused when cells take up foreign genetic material.

Avery – 1940- wanted to determine if the transforming agent in Griffith’s experiments was protein, DNA, or RNA. Discovered that DNA is the nucleic acid that is responsible for transformation.He did this by using enzymes to destroy each agent (one at a time) and observing if R cells were transformed into S cells.

Hershey-Chase experiment

Studied bacteriophages (viruses that infect bacterial cells & cause the cells to produce viruses)Found that the genetic material of viruses was DNA, not proteins

DNA Structure

DNA is a relatively simple molecule that is composed of a 5 carbon sugar (deoxyribose), a phosphate group, and 4 different subunits known as nitrogenous bases:-Adenine (A) -Guanine (G) -Thymine (T) -Cytosine (C)

Gene- a segment of DNA that is located in a chromosome and that codes for a specific hereditary trait.

Chargaff’s Rules for base pairing

•Adenine always binds to Thymine A---T•Guanine always binds to Cytosine G---C•When two nitrogenous bases are bound together, they are known as a base pair.•These paired bases are called complimentary because they fit together like puzzle pieces.

Purine- a nitrogenous base that has a double ring structure (A & G)Pyrimidine-a nitrogenous base that has a single ring structure (T & C)

Watson & Crick

Built the first 3-D model of DNA using the information from Chargaff, Rosalind Franklin (x-rayed a DNA molecule), & their own knowledge of chemical bonding. Watson & Crick won a Nobel prize for their discovery.

DNA Replication

1. Proteins called helicases separate the 2 original DNA strands

2. Complimentary nucleotides (A, T, G, C) are added to each strand by DNA polymerase

3. Two DNA molecules are formed that are identical to the original DNA molecule

DNA replication- the process of making a copy of DNA

DNA helicase- an enzyme that unwinds the DNA double helix during DNA replication

DNA polymerase- an enzyme that catalyzes (speeds up) the formation of the DNA molecule

DNA replication in prokaryotes (binary fission)1. Original Site of Replication

Fork2. Each of the Two Strands of

DNA separate from each other and synthesis of a complementary strand on each parental strand begins

3. Loop of DNA Extending Out of The Plane of the Parental Bacterial Chromosome

4. Rotation Around Axis

BR - Copy the diagram and the question. Then, answer the question.

Which sequence of bases do the question marks represent?

SWP – trans-?

EQ – Why is RNA needed to make proteins?

C C T A T G ? ? ?

G G A T A C C T G

DNA Replication Practice

•TTG GAG CGT GCT

•GCA CAT TTA CGA

•AAC GGC CTG CAG

Ribonucleic acid

RNA

RNA- a nucleic acid that is essential in taking the genetic information from DNA and building proteins

3 types:messenger RNA (mRNA)transfer RNA (tRNA)ribosomal RNA (rRNA)

mRNA

When DNA is transcribed into RNA, mRNA is the type of RNA made. mRNA is complementary to the DNA sequence of a gene. The mRNA carries the instructions for making a protein from a gene and delivers them to the site of translation.

tRNA

During translation, tRNA “reads” the mRNA sequence (codons) and builds amino acids from those codes

rRNA

ribosomal RNA- RNA component of ribosomes. Provides a mechanism for decoding mRNA into amino acids & interacts with tRNA during translation.

RNA Stucture:-a single strand version of DNA-contains 4 nitrogenous bases (A,U,C,G)-Uracil instead of Thymine-A binds to U-G binds to C

Differences in RNA & DNARNA- Single strandedsugar is riboseuracil instead of thyminelocated in the nucleus & cytoplasm

DNA- double strandedsugar is deoxyribosethymine instead of uracillocated in the nucleus

Transcription- the information in a specific region of DNA (a gene) is copied into mRNA

3 steps:

1. Initiation- RNA polymerase binds to a DNA sequence in the gene known as the promoter (the start point)

2. Elongation- The two DNA strands unwind and separate, exposing the bases. Complementary RNA nucleotides are added by RNA polymerase, growing a strand of mRNA.

3. Termination- The RNA polymerase eventually reaches a ‘stop’ code at the end of a set of genes. The mRNA strand is released from the template and the DNA strands close up and reform the double helix.

The mRNA made in transcription leaves the nucleus and takes its

sequence to the ribosome for protein synthesis

(translation).

Translation

Step 1: Initiation: mRNA is inserted into the small subunit of a ribosome (at the 5l end). When the RNA encounters the start sequence (AUG) , an initiator tRNA binds to the ribosome.

Step 2: Elongation- tRNA reads the codons (3 nucleotide sequence that codes for amino acids), binding anticodons to the mRNA strand & linking amino acids together.

Step 3: Termination- The end of translation occurs when the ribosome reaches a STOP codon (UAA, UAG, UGA). There are not any anticodons for the STOP codons. The long chain of amino acids (a polypeptide) is then released from the ribosome.

The Genetic Code: 3-letter wordsCodon-3 nucleotide sequence (ex: AUC)

Codon is found on the mRNAAnticodon- 3 nucleotide sequence

complementary to mRNA codon (ex: UAG)Found on the tRNA

Genetic Code- made of nucleic acids, based on codons that represent a specific amino acid.

Waste not…..Want notSeveral proteins can be made from the

same amino acid.

PracticeDNA → ATG CTG GAC TGA GTA TGAmRNA → tRNA →AA →(protein)

DNA → CAG GCT ATC GAG CGC TTGmRNA →tRNA →AA →(protein)