To truly understand genetics, biologists first had to discover the chemical nature of genes Identifying the structure that carries genetic information makes it possible to understand how genes control the inherited characteristics of living things
Griffith- discovered transformation, a change in genes that is caused when cells take up foreign genetic material Avery-concluded that DNA is responsible for transformation Hershey and Chase experiment concluded that DNA is the hereditary material, not proteins. Watson and Crick determined the structure of DNA
Scientists figured that genes would have to do 3 things 1. genes had to carry information from one generation to the next 2. genes have to put that information to work by determining the heritable characteristics of organisms 3. genes had to be easily copied (all of the cell s genetic information is replicated every time a cell divides)
DNA is a long molecule made up of units called nucleotides 5 carbon sugar (deoxyribose) Phosphate group Nitrogen base Adenine Thymine Guanine Cytosine
Nitrogen base Phosphate Deoxyribose sugar
Purines- two rings in their structure Adenine & Guanine Pyrimidines one ring in their structure Thymine & Cytosine
1953: James Watson & Francis Crick discovered structure of DNA molecule (with the help from Franklin s X-rays) Double Helix- two strands of nucleotides wound around a central axis Looks like a twisted ladder or spiral staircase
Sides of the DNA helix are made of sugar (deoxyribose) and phosphates Bases are in the middle & are held together by Hydrogen bonds strong enough to hold bases together, but weak enough to be broken during replication
Hydrogen bonds can form only between certain base pairs said to be complementary Adenine (A) & Thymine (T) Guanine (G) & Cytosine (C) Explained Chargaff s rule on why [A] = [T] and [G] = [C]
DNA is often compared to a ladder or a spiral staircase. Look at Figure 4 (pg. 296) and answer the following questions. 1. How is the structure of DNA similar to that of a ladder or spiral staircase? 2. How is it different from that of a ladder or spiral staircase?
In prokaryotes: DNA floats freely in the cytoplasm (because they have no nucleus) DNA is contained on one circular chromosome that holds the cell s genetic information In eukaryotes: 1000 times more DNA than prokaryotes; much more complicated DNA located in a cell s nucleus on multiple chromosomes Remember DNA condenses to form chromosomes
Before a cell divides, DNA is duplicated in a process called replication - ensures each resulting cell will have a complete set of DNA molecules Prokaryotes: DNA replication begins @ a single point and proceeds in two directions until entire chromosome is replicated Eukaryotes: DNA replication occurs @ hundreds of places and proceeds in both directions until each chromosome is copied Replication forks: sites where separation & replication occur
The enzyme DNA helicase unzip a molecule of DNA by breaking the Hydrogen bonds between the base pairs This causes the 2 strands of DNA to unwind Each strand serves as a template for attachment of complimentary bases (A-T & G-C) For example: TACGTT would match up with ATGCAA
DNA replication involves several enzymes & regulatory molecules Principle enzyme of DNA replication is DNA polymerase- it polymerizes (puts together) individual nucleotides to produce DNA Also proofreads each new DNA strand to maximize the odds that each new molecule is a perfect copy of the original DNA
Genes are coded DNA instructions that control the production of proteins within the cell The first step in decoding these genetic messages is to copy part of the nucleotide sequence from DNA into RNA (ribonucleic acid) which carry out the process of making proteins
3 main differences between DNA & RNA: 1. Sugar of RNA is ribose DNA s sugar is deoxyribose 2. RNA is a single strand of nucleotides DNA is a double strand of nucleotides 3. RNA s nitrogen bases are: Uracil, Adenine, Guanine & Cytosine DNA has Thymine, Adenine, Guanine & Cytosine
RNA is a disposable copy of a segment of DNA and is a working copy of a single gene The ability to copy a single DNA sequence into RNA makes it possible for a single gene to produce hundreds or thousands of RNA molecules
Assembly of amino acids into proteins is controlled by RNA 3 main types of RNA: 1. messenger RNA (mrna): carries copies of instructions for assembling amino acids into proteins by serving as messengers from DNA to rest of the cell
2. ribosomal RNA (rrna): makes up ribosome (site of protein assembly) & helps form peptide bonds that hold amino acids together in a protein 3. transfer RNA (trna): transfers each amino acid to ribosome for protein assembly
Transcription: RNA molecules are produced by copying part of the nucleotide sequence of DNA into a complimentary strand of mrna (DNA mrna) RNA polymerase binds to DNA & separates the DNA strands so that one strand of the DNA nucleotides can serve as a template from which nucleotides are assembled into a strand of mrna
RNA polymerase will bind only to regions of DNA called promoters Promoters have specific base sequences and act as signals in DNA to indicate to the enzyme where to bind to make RNA Similar signals in DNA cause transcription to stop when the new RNA molecule is completed
Proteins are made by joining amino acids into long chains called polypeptides; each of which contains any or all of the 20 different amino acids Properties of proteins are determined by the order in which the amino acids are joined together Bases of DNA and RNA must be translated into a particular order of amino acids to form polypeptides
The language of mrna instructions is called the genetic code Written in 4 bases: (A)Adenine, (U) Uracil, (C) Cytosine, and (G) Guanine Read 3 letters at a time: codon- 3 consecutive nucleotides that specify a single amino acid
4 different bases; allows for 64 different codons in the genetic code Some amino acids can be specified by more than one codon AUG is the start codon; signals the initiation of protein synthesis as well as coding for the amino acid methionine 3 stop codons; UAA, UAG, UGA signal the end of a polypeptide
To decipher the genetic code: UCGCACGGU Break down into codons: UCG CAC GGU Serine -Histidine -Glycine
The decoding of an mrna message into a protein is known as translation (mrna amino acid sequence) The cell uses information from messenger RNA to produce proteins
1. mrna is transcribed in nucleus & released into cytoplasm
Each trna molecule has an amino acid attached to one end & a region of 3 unpaired bases on the other The 3 bases on the trna molecule are called an anticodon and are complimentary to one of the mrna codons trna anticodon
2. mrna attaches to a ribosome. As codons move through the ribosome, the proper amino acid is brought to the ribosome (via trna) and attached to the growing polypeptide chain.
3. Ribosome will form a peptide bond between the 1 st & 2 nd amino acids. The ribosome then moves to each consecutive codon.
4. The polypeptide chain continues to grow until the ribosome reaches a stop codon on the mrna. It then releases the newly formed polypeptide & the mrna molecule & completes the process of translation
Every once in a while, cells make mistakes in copying their own DNA An incorrect base can be inserted or sometimes a base is skipped as the new DNA is being assembled Mutation : changes in DNA sequence that affect genetic information
Mutation : changes in DNA sequence that affect genetic information Gene mutations: result from changes in a single gene Chromosomal mutations: involve changes in whole chromosome
Point mutation: affect one nucleotide Occur at a single point in the DNA sequence Often one nucleotide is substituted for another Generally change just one amino acid Usually not lethal
Frameshift mutation- results from the insertion or deletion of a nucleotide Since genetic code is read in groups of three, adding or deleting a nucleotide shifts all resulting amino acids This mutation can alter a protein so much that it is unable to perform its normal functions Most likely lethal
Chromosomal mutations involves changes in the number or structure of chromosomes May change the locations of genes on chromosomes & even the number of copies of some genes
Deletion: Involves the loss of all or part of a chromosome Duplication: A segment of a chromosome is repeated
Inversion: Part of a chromosome becomes oriented in the reverse of its usual direction Translocation: Part of one chromosome breaks off and attaches to another chromosome
The nucleus of a human cell contains more than 1 meter of DNA: must be folded to fit into the tiny space of a cell s nucleus chromatin- DNA & proteins (histones) tightly packed together DNA & histone molecules form beadlike structure called a nucleosome
Nucleosomes pack with one another to form a thick fiber which is shortened by a system of loops & coils This makes chromosomes visible & may aid in their separation during mitosis Important because a mistake in DNA folding could harm a cell s ability to reproduce Nucleosomes may also play a role in how genes are read to make proteins
Nucleosomes pack with one another to form a thick fiber which is shortened by a system of loops & coils This makes chromosomes visible & may aid in their separation during mitosis Important because a mistake in DNA folding could harm a cell s ability to reproduce Nucleosomes may also play a role in how genes are read to make proteins