Chapter 11: DNA- The Molecule of Heredity

History of DNA 1952: Hershey and Chase –Did experiments using radioactive viruses to infect bacteria –Discovered DNA was the genetic material of all living things 1953: Watson and Crick –Discovered the structure of DNA Made up of 2 chains of nucleotides held together by nitrogen bases Double helix

DNA in Cells Located in the nucleus of cells as chromosomes Packed tightly Consists of more than 30 million base pairs Complimentary DNA strands –Can use 1 strand to make a copy of the other strand using base pairing

Nucleotides Made up of DNA 3 parts to a nucleotide: –A simple sugar called Deoxyribose –A phosphate group –A nitrogen base

Nitrogen Bases 4 possible nitrogen bases: –Adenine (A) –Guanine (G) –Cytosine (C) –Thymine (T)

Adenine (A) and Guanine (G) Double-ringed nitrogen bases Called purines

Thymine (T) and Cytosine (C) Single-ringed nitrogen bases Called pyrimidines

Nucleotides join together to form long chains of complimentary base pairs –Adenine always pairs with Thymine (A-T or T-A) –Guanine always pairs with Cytosine (G-C or C-G)

Structure of DNA Nitrogen bases of the nucleotides hold 2 strands of DNA together with weak hydrogen bonds Twisted DNA  double helix

DNA Replication DNA is copied before cell division DNA will separate into 2 strands –Carried out by enzymes Unzips DNA by breaking hydrogen bonds to unwind the double helix –Each strand acts as a template or model to make new DNA strands Makes new complimentary strands through base-pairing

Example: –TACGTT – Old DNA strand ATGCAA – New DNA strand After DNA is replicated, DNA will have 1 old strand and 1 new strand

The Genetic Code Proteins have chains of amino acids A code is needed to convert messenger RNA (mRNA) into a protein 20 amino acids –Codon: a group of 3 Nitrogen bases that code for an amino acid 64 possible combinations of codons Some code for amino acids Some code for making proteins More than 1 codon can code for the same amino acid

Transcription Making a RNA copy of a part of DNA Makes messenger RNA (mRNA) 4 Steps: –Enzymes unzip the DNA –Free RNA nucleotides floating in the cytoplasm base pair with nucleotides on DNA strand (makes mRNA) –mRNA strand breaks away and DNA strands go back together –mRNA leaves nucleus and goes out to the cytoplasm Result of transcription: formation of 1 single-stranded RNA molecule

Two Types of DNA: RNA and DNA

DNARNA SugarDeoxyriboseRibose BasesG, C, A, TG, C, A, U (uracil) StructureDouble- stranded Single-stranded Location in a Cell Only in the nucleus In nucleus and cytoplasm Base PairingC-G and A-TC-G and A-U

Messenger RNA (mRNA) Brings instructions from DNA out of the nucleus and into the cytoplasm Moves toward the ribosomes

Ribosomal RNA (rRNA) Binds to messenger RNA Uses the instructions from DNA to put amino acids in the correct order

Transfer RNA (tRNA) Delivers the amino acids to the ribosomes to be made into a protein

DNA Controls Protein Synthesis What are proteins? –Long chains of amino acids –Key structures and regulators of cell functions Help with structural parts Enzymes  chemical reactions Help in transport through cell membrane

Making Proteins Protein production is similar to building car –DNA provides workers with instructions for making proteins –Workers build proteins (RNA) –Other workers bring parts (amino acids) to the assembly line

Translation Process of building proteins Takes place in the ribosomes Transfer RNA (tRNA) brings amino acids to the ribosomes –Attaches to only 1 type of amino acid –Amino acid will become bonded to 1 side of the tRNA –The other side of the tRNA has 3 nitrogen bases called an anticodon Pairs up with mRNA codon

Amino acids are joined by peptide bonds Anticodon bind to the codon of mRNA through base pairing –Example: Codon: CGA Anticodon: GCU A chain of amino acids form until a stop codon is reached –Translation will end –Amino acid strand is released from the ribosome to become proteins

Mutations Any change in the sequence of DNA Can be caused by errors in: –DNA replication –Transcription –Cell division –External agents

Mutations in Reproductive Cells: Birth Defects Within the egg or sperm cells Can produce new traits Can result in proteins that do not work (can kill organism) Could have positive effects –Faster –Stronger –Important in the evolution of a species

Mutations in Body Cells Not passed on to offspring May impair cell function Can affect genes that control cell division (cancer)

Point Mutation (substitution) Change in 1 N-base in DNA Example: CGATTACGC (normal DNA) CGATTTCGC (mutated DNA) Albinism –Inability to produce pigments –Lethal to plants

Frameshift Mutation 1 N-base is added or deleted Changes all codons from that point on Example: CGATTACGC CGAATTACGC (N-base added) Example: CGATTACGC CGTTACGC (N-base deleted) May cause no problems or can be severe More dangerous than point mutations

Chromosomal Mutations Involve many genes Usually very bad 4 types: –Deletions  taking away –Insertions  adding –Inversions  switching parts (ex: ab  ba) –Translocations  breaking off Many occur from improper separation during meiosis (nondisjunction)

Causes of Mutations Spontaneous or random mutations Mutagens (things that cause mutations) –Radiation, X-Rays, UV light, chemicals –Carcinogens