Chapter 12DNA and RNA

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Understanding Genetics

Well, then…we need to understand the chemical makeup of the gene

I’d like to better understand genetics…

Understanding Genetics

Yeah, DNA can store genetic information, and transmit it to the next generation!

Sweet! Genes are made of DNA…

Understanding GeneticsFar out, maybe we’ll

find out how it decides traits and replicates itself, too!

Let’s study this DNA and figure out how it carries information

Understanding GeneticsDude – DNA can make

a copy of itself – that’s called replication!

Replicates? What on earth does that mean?

History and Structure

• In 1953, two scientists, James Watson & Francis Crick, built model of DNA out of tin & wire – Looked like a twisted ladder – Called the shape a Double

Helix (2 twisting strands)

Structure

• Sides of ladder are sugar & phosphate groups

• Rungs of ladder are nitrogen bases joined together by H bonds

DNA

Structure

Structure

• DNA = deoxyribonucleic acid – Long molecules of nucleotides

linked together like a chain

Structure

• Each nucleotide is made of 3 parts: – Phosphate Group– 5-Carbon Sugar

(deoxyribose)

– Nitrogen Base

NUCLEOTIDE!!!!

Structure

• 5-C sugar & phosphate group always the same for each nucleotide in DNA– Form sides of ladder

Structure• There are 4 different

nitrogen bases: – Adenine– Guanine– Thymine– Cytosine

Structure

• Adenine & Guanine classified as Purines – Because they are larger

molecules with double rings of Carbon & Nitrogen

Structure

• Cytosine & Thymine classified as Pyrimidines – Because they are made of a

single ring of Carbon & Nitrogen

Note the single and double rings. What else do you notice?

BASE PAIRING

• Since A always pairs with T (A-T) and C always pairs with G (C-G), DNA strands considered complementary to each other– They match– Think of the color wheel!

• Are the same in every organism

• The nucleotides are in a DIFFERENT order for each organism

• DNA is found in the nucleus of cells.

- contain both DNA and

protein tightly packed together to form a substance called chromatin.

• An area of coded DNA on a given chromosome that gives a certain trait.

• Traits such as height, eye and hair color is coded in your genes.

• To analyze chromosomes, cell biologists PHOTOGRAPH cells in MITOSIS.

• Next, they cut out the chromosomes and group them in pairs.

• This is called a KARYOTYPE

• Replication is the process of copying DNA

• During replication each strand serves as a pattern to make a new DNA molecule

• Enzymes (DNA polymerase) break hydrogen bonds between nitrogen bases

• Molecule unzips• Unzipped molecule

base pairs with free nucleotides

• Two copies of the organism’s genetic information that can be passed during mitosis or meiosis

Replication Summary

• In DNA replication, enzymes work to unzip & separate the double helix.

• Complementary bases are added to the open strands.

• Each new double helix is made of one old DNA strand & one new strand.

http://nobelprize.org/educational_games/medicine/dna_double_helix/

12-3: Protein Synthesis

• RNA is…• Single stranded• Contains ribose• Contains the base uracil

instead of thymine

• Three types:– Messenger (mRNA)

• Carry copies of instructions for assembling amino acids into proteins.

– Ribosomal (rRNA)• Along with proteins, rRNA makes up

ribosomes where proteins are assembled.

– Transfer (tRNA)• Transfers amino acids to the ribosome as it

is specified by coded messages in mRNA.

• Process of making an RNA copy of a DNA strand

• Results in the formation of a single-stranded RNA molecule

• mRNA carries the information for making proteins to the ribosomes

•Since directions for making proteins are in the nucleus the mRNA has to take the info to the cytoplasm where the ribosomes can make proteins!

• DNA molecule unzips• Free RNA nucleotides

pair with complementary DNA strands

• mRNA breaks away and the DNA strand rejoins

• Proteins are made by joining amino acids into long chains called polypeptides.

• Each polypeptide contains any combination of the 20 amino acids.– The properties of proteins are determined

by the order of each of these different amino acids.

• Each set of three nitrogen bases representing an amino acid is known as a codon—triplet code

• 64 combinations possible; 61 code for amino acids and 3 code for stop signals

• The process of converting information in a sequence of nitrogen bases in mRNA into a sequence of amino acids that make up a protein

• Decoding the message

• tRNA brings amino acids to the ribosomes so they can be assembled into proteins

So you may be thinking….

• Why do we need proteins?• Proteins:– Most are enzymes that code for….– Production of a pigment to control the

color of a flower– Production of red blood cell surface

antigen (determines blood type)– Regulate the rate and pattern of

growth throughout an organism (controls size and shape)

Mutations• Sometimes cells make mistakes when

copying their own DNA…these mistakes are called MUTATIONS!– Changes in the genetic material of the cell.

• Many mutations are not harmful, some even increase genetic variation in a species!

• Mutations that are harmful can cause genetic disorders and sometimes cancer.

Types of Mutations• Single base-pair substitution

• Deletion or insertion of one or more base pairs

• Major alteration in the structure of a chromosome.

• May or may not bring about a change in a phenotype. WHY?

Mutations-classified by location

• Cells– Somatic: occur in any cell in the body except gametes

(Localized cell death, altered cellular function, or tumors)– Sex-cells: occur in gametes (creates genetic diversity

and/or disease)

• Chromosome…in all of the cells!– Autosomal: occur within genes located on the autosomes

(chromosome pairs 1-22)– X-linked: occur on the X chromosome

Mutations: classified by type of change

POINT (changes base pair)

• If there is not a change in AA, then it is a silent mutation.

• Missense mutation: Change of one nucleotide, may change the AA that the codon called for.

• Nonsense mutation: Triplet will be changed into a stop codon, resulting in a termination of translation.

FRAMESHIFT

• Addition or deletion of a single letter, which causes all of the rest of the letters to be “MOVED” by one space.