Chapter 12 DNA & RNA

Deoxyribonucleic acid 12 – 1 DNA Deoxyribonucleic acid

In the middle of the 1900’s biologists were wondering how genes work In the middle of the 1900’s biologists were wondering how genes work. What they are made of, and how they determine the characteristics of organism If the structures that carry genetic information could be identified, it might be possible to understand how genes control the inherited characteristics of living things

How does DNA do the following things? Carry info from one generation to the next Put info to work by determining the inheritable characteristics Copies itself, every time a cell divides

Nucleotides Units that make up DNA molecule Made of three parts 5 carbon sugar (deoxyribose) Phosphate group Nitrogen bases

4 kinds of nitrogen bases Adenine (A) Guanine (G) Cytosine (C) Thymine (T)

Chargaff’s Rule A=T and G=C

X-Ray Evidence Rosalind Franklin British Scientist Used a technique called X-Ray diffraction Provided important clues about the structure of DNA

X-Ray Evidence There were 2 strands Strands were twisted around each other (helix) The nitrogen bases are in the middle

Watson & Crick

The Double Helix Francis Crick & James Watson Trying to understand the structure of DNA by building models Unsuccessful until early 1953, Watson was shown a copy of Franklin’s X-ray pattern “The instant I saw the picture my mouth fell open and my pulse began to race.” James Watson Within weeks Watson and Crick had figured out the structure of DNA Published their results in a historic one page paper in April of 1953

Watson and Crick later discovered what held the two strands together Hydrogen bonds could form between certain nitrogen bases and provide enough force to hold the two strands together Hydrogen bonds could only form between certain base pairs adenine and thymine and guanine and cytosine This principal is called Base pairing This explains Chargaff’s Rule

12 – 2 Chromosomes and DNA Replication

To extract DNA for analysis, you need to know where to find it and how its organized DNA is located in the nucleus DNA is organized into chromosomes

Prokaryotic Cells Prokaryotic cells have a single circular DNA molecule that contains nearly all of its genetic information Located in the cytoplasm

Eukaryotic Cells Much more complex 1000 times the amount of DNA as prokaryotes DNA is located in the nucleus in the form of chromosomes

Chromosome Structure Q: If eukaryotic DNA can contain a meter or more of DNA, how does it get packed in so tight into chromosomes? A: Eukaryotic chromosomes contain both DNA and protein that form a substance called chromatin

Histones Proteins that coil up DNA DNA + histone molecules form a bead-like structure called a nucleosome Nucleosomes pack together to form thick fibers that loop and coil together to form chromosomes

DNA Replication When Watson and Crick discovered the double helix structure of DNA they recognized immediately how DNA could copy itself The strands are complementary If you could separate the two strands, the rules of base pairing would allow you to reconstruct the base sequence of the other strand

Replication When the DNA splits into 2 strands, then produces 2 new strands following the rules of base pairing

How Replication Occurs Replication is carried out by enzymes Before DNA replicates, the double helix must unwind and unzip There are many regulatory molecules used in replication

DNA polymerase Joins individual nucleotides to produce a DNA molecule, which is a polymer Also proof reads each new DNA strand

The Steps of Replication DNA unwinds DNA unzips The bases attach from a supply in the cytoplasm Sugar and phosphate groups form the side of each new strand

Do Now Place the following steps of Replication in order. DNA Unzips DNA unwinds Sugar and phosphate groups form the side of each new strand The bases attach from a supply in the cytoplasm

Do Now Place the following steps of Replication in order. DNA replication results in 2 DNA molecules Each with two new strands One with two new strands and the other with two original strands Each with one new strand and one original strand Each with two original strands 1. DNA unwinds 2. DNA Unzips 3. The bases attach from a supply in the cytoplasm 4. Sugar and phosphate groups form the side of each new strand

12 – 3 RNA and Protein Synthesis

Genes Coded DNA instructions that control the production of proteins

DNA never leaves the nucleus, therefore the code must be copied into RNA, or ribonucleic acid There are 3 main differences between RNA and DNA It has the sugar ribose, instead of deoxyribose RNA is single stranded RNA contains uracil in place of thymine

RNA is like a disposable copy of a segment of DNA RNA is like a working copy of a single gene

Types of RNA Messanger RNA (mRNA) Serve as messangers from DNA to the rest of the cell 2. Ribosomal RNA (rRNA) Type of RNA that makes up parts of ribosomes 3. Transfer RNA (tRNA) Transfers each amino acid to the ribosome as it is specified by the mRNA

Types of RNA

Transcription RNA molecules are produced by copying part of the DNA sequence into RNA Transcription requires an enzyme known as RNA polymerase During transcription, RNA polymerase binds to DNA and separates the DNA strands. RNA polymerase then uses one strand of DNA as a template from which nucleotides are assembled into a strand of RNA.

Q: How does RNA polymerase “know” where to start and stop making a RNA copy of DNA? A: promoters Signals in DNA that indicate to the enzyme where to bind to make RNA Similar signals in DNA cause transcription to stop

RNA Editing Remember, a lot of DNA doesn’t code for proteins Introns – not involved in coding for proteins Exons – code for proteins

The introns get cut out of the RNA molecules before the final mRNA is made

The Genetic Code Proteins are made by joining amino acids into long chains called polypeptides Each polypeptide contains a combination of any or all of the 20 different amino acids The properties of proteins are determined by the order in which different amino acids are joined together The language of mRNA instructions is called the genetic code

The code is read three letters at a time Each 3 letter “word” is called a codon Each codon corresponds to an amino acid that can be added to the polypeptide

UCGCACGGU UCG-CAC-GGU This sequence would be read three bases at a time as: UCG-CAC-GGU The codons represent the different amino acids: Serine-Histidine-Glycine

Translation The sequence of nucleotide bases in an mRNA molecule serves as instructions for the order in which amino acids are joined to make a protein Proteins are put together on ribosomes

Translation Decoding mRNA into a protein

Steps of Translation mRNA is transcribed from DNA in the nucleus and released into the cytoplasm mRNA attaches to a ribosome 3. as each codon of the mRNA molecule moves through the ribosome, the proper amino acid is transferred to the growing amino acid chain by tRNA tRNA carries only one kind of amino acid and three unpaired bases called the anticodon

4. The amino acid chain continues to grow until the ribosome reaches a stop codon on the mRNA molecule

The Roles of RNA and DNA You can compare the different roles played by DNA and RNA molecules in directing protein synthesis to the two types of plans used by builders. A master plan has all the information needed to construct a building. But builders never bring the valuable master plan to the building site, where it might be damaged or lost. Instead, they prepare inexpensive, disposable copies of the master plan called blueprints. The master plan is safely stored in an office, and the blueprints are taken to the job site. Similarly, the cell uses the vital DNA “master plan” to prepare RNA “blueprints.” The DNA molecule remains in the safety of the nucleus, while RNA molecules go to the protein-building sites in the cytoplasm—the ribosomes

Genes and Proteins Q: If most genes contain nothing more than instructions for assembling proteins, what do proteins have to do with traits? A: Everything, proteins are microscopic tools designed to build or operate a component of a living cell

12 – 4 Mutations

Mutations Changes in the genetic material

Point mutations Changes in one or a few nucleotides Ex.) substitutions, insertions, deletions

Frameshift mutations Mutation that shifts the “reading” frame of the genetic message by inserting or deleting a nucleotide

Chromosomal Mutations

Significance of Mutations Most mutations don’t do anything Mutations that cause drastic changes in proteins produce defective proteins that disrupt normal biological activities Mutations are also a source of genetic variability which can be beneficial

Polyploidy When plants produce triploid (3N) or tetraploid (4N) organisms These plants are often larger and stronger

Do Now Look at the bottom strand of DNA on the window blinds Suppose the second T was changed to a C How would this specifically alter the resulting amino acid chain? What kind of mutation is this?

Do Now #2 What if we got rid of the first G in the bottom strand of DNA How would this specifically alter the amino acid chain produced? What kind of mutation is this?

Do Now #3 How are substitution/point mutations and frameshift mutations similar? How are they different?