Nucleic Acids DNA and RNA DNA Structure and

Nucleic Acids DNA and RNA

DNA Structure and Replication

Structure

What is the essential vocabulary? ü DNA stands for Deoxyribonucleic Acid ü Nucleic acids are a group of large molecules that store and transmit genetic information. (RNA is another example. ) ü DNA structure is called a Double helix – two strands, parallel with each other, resembling a twisted ladder

What are the components of DNA? ü DNA is made up of repeating parts called nucleotides, like beads that repeat on a necklace. ü The nucleotide has 3 parts to it: ü a phosphate, phosphate ü a sugar (deoxyribose), ü a nitrogenous base

What are the components of DNA? q The strong outer portion, or backbone, of the DNA is formed by alternating sugar and phosphate groups of each nucleotide (use covalent bonds). q On the interior of the molecule, the nitrogenous bases are connected like puzzle pieces via relatively weak hydrogen bonds. It is the ORDER of these bases that forms the genetic code, just like the order of letters in the alphabet forms our languages.

What is The Rule of Complementary Base Pairing? Adenine (A) Thymine (T) Cytosine (C) Guanine (G) 2 Hydrogen bonds 3 Hydrogen bonds

Replication

What is DNA replication? q During DNA replication, the DNA molecule (parent strands) produces two new complementary (daughter) strands. q Each strand of DNA serves as a template for the new strand so the new DNA molecule has one new strand one original strand. This is known as the semiconservative model

Let's look at the details: 1. An enzyme called helicase unwinds the double-stranded DNA 2. An enzyme complex called DNA polymerase "walks" down the DNA strands and adds new nucleotides to each strand. The nucleotides pair with the complementary nucleotides on the existing strand (A with T, G with C). 3. A subunit of the DNA polymerase proofreads the new DNA 4. The new copies automatically wind up again (Note: The two new strands are still attached (sister chromatids!!)

DNA Replication Overview A T C G T A A T G C (a) Parent molecule q Each strand of the DNA double helix has all the information needed to reconstruct the other half by the mechanism of base pairing. q Because each strand can be used to make the other strand, they are said to be complementary.

DNA Replication Overview A T C G T A A T G C (a) Parent molecule (b) Separation of strands

DNA Replication Overview A T A T C G C G T A T A T G C G C (a) Parent molecule (b) Separation of strands (c) “Daughter” DNA molecules, each consisting of one parental strand one new strand

DNA & Common Ancestors All organisms from prokaryotes to eukaryotes use the same basic components of DNA, making the genetic code universal • Organisms that share a recent common ancestor share many similarities in their genetic code since DNA is passed on from generation to generation. •

RNA AND PROTEIN SYNTHESIS

Transcription

Transcription ü Transcription is the process during which RNA molecules are produced by copying part of the nucleotide sequence of DNA. ü First part of Protein Synthesis

How are DNA and RNA Similar/Different? RNA and DNA are nucleic acids q RNA, like DNA, consists of a long chain of nucleotides q There are three main differences between RNA and DNA: q The sugar in RNA is ribose, the sugar in DNA is deoxyribose. q RNA is single stranded, stranded DNA is double stranded q RNA contains uracil (U) DNA contains thymine (T). q

What are three types of RNA? Abbrev. m. RNA t. RNA r. RNA Name Function Carries copies of the Messenger instructions for assembling RNA proteins from DNA to ribosomes. Transfer RNA Structure Singlestranded Reads m. RNA sequence and transfers each amino acid to A clover leaf the ribosome. Binds corresponding amino Ribosomal acids to a growing peptide RNA chain Complex, with proteins

How does transcription work? RNA polymerase binds to DNA and separates the DNA strands. 2. RNA polymerase then adds complementary RNA nucleotides to one strand of DNA (template strand) to form a strand of m. RNA. 3. When RNA polymerase reaches a termination signal in the DNA, the DNA and new m. RNA are released. 1.

How does transcription work? DNA template strand 3 A C C A A A C T T 5 G G T C G A G T G G C T T C A 5 3 DNA molecule Gene 1 TRANSCRIPTION Gene 2 m. RNA U G G 5 U U U G G C U C A 3 Codon TRANSLATION Protein Trp Amino acid Phe Gly Ser Gene 3

Translation

How do proteins function? Proteins are made by joining amino acids together into long chains, called polypeptides chains q Each polypeptide contains a combination of any or all of the 20 different amino acids. q The properties of a protein are determined by the specific amino acids in a polypeptide chain and the order in which different amino acids are joined together. q q The sequence of amino acids influences the shape of the protein, which in turn determines its function.

What is the genetic code? The “language” of m. RNA instructions is called the genetic code q RNA contains four different bases: A, U, G, C. q The genetic code is read three letters at a time, each three-letter word is known as a codon q q There are 64 possible three-base codons.

What is the essential vocabulary? ü Translation – Using m. RNA code as a template to assemble proteins. ü Genetic code – Instructions needed to convert DNA into proteins. ü Codon – Three consecutive nucleotides in m. RNA that specify a single amino acid. ü Anticodon – Three adjacent nucleotides located on t. RNA that binds to the complementary m. RNA codon.

To use the codon table below, start at the middle of the circle with the first letter of the codon. Move out to the second ring to find the second letter of the codon. Then find the third and final letter among the smallest set of letters in the third ring from the center. Next to the third letter is the amino acid that the codon specifies. In the codon table above, start on the left with the first letter of the codon. On the top find the second letter of the codon and find the box where the 2 bases intercept (There should be 4 amino acids listed). Look to the right and find the 3 rd letter of the codon that intercepts with the box. The amino acid level with that 3 rd base is the amino acid that the codon specifies.

Important!!! Only m. RNA codons can be used with a codon table. • DNA – No!!! • t. RNA – No!!! • r. RNA – No!!!

How does a cell interpret DNA? q Consider the following RNA sequence: 5’ UCGCACGGU 3’ q This sequence will be read three bases at a time. Write out the codons. q Use the codon table and write out the amino acids that the codons represent.

How does a cell interpret DNA? Consider the following RNA sequence and identify the amino acids it will code for. 5’ AUGCGCUUCUAA Consider the following DNA sequence and identify the amino acids it will code for. 3’ CTTGGAATG 5’ 3’

How does translation work? q Messenger RNA is transcribed in the nucleus and then enters the cytoplasm for translation.

How does translation work? 1. INITIATION A ribosome attaches to an m. RNA molecule in the cytoplasm and a t. RNA molecule carrying methionine binds to ribosome. The codon on the t. RNA matches the first (“start”) codon which is always AUG.

How does translation work? 2. ELONGATION As the ribosome reads each codon of m. RNA, the t. RNA with the specified amino acid attaches at the ribosome (t. RNA anticodons bind to m. RNA codons). One at a time, the ribosome then attaches each amino acid to the growing chain.

How does translation work? 3. TERMINATION The polypeptide chain continues to grow until the ribosome reaches a “stop” codon on the m. RNA molecule.

How does translation work? 4. DISASSEMBLY Ribosome complex falls apart releasing both the newly formed polypeptide and the m. RNA molecule, completing the process of translation

Mutations

Vocabulary Words ü -gen: suffix meaning something that produces. ü Mutagen: An agent that can induce or increase the frequency of mutation in an organism. ü Chemicals ü Ultraviolet light ü Radioactive elements ü Carcinogen: Substance that increases the risk of mutations that lead to cancer

What are mutations? q Mutations are changes in the genetic material. q Now and then cells make mistakes in copying their own DNA: q Insert an incorrect base. q Skip a base as the new strand is put together. q Mutations that produce changes in a single gene are known as gene mutations q Occurs during DNA replication q Mutations that produce changes in whole chromosomes are known as chromosomal mutations q Occurs during Meiosis

What are mutations? q Mutations in eggs or sperm (gametes) affect future generations by producing offspring with new characteristics. q May result in embryo not surviving. q May provide organism with an advantage. (EVOLUTION) EVOLUTION q Mutations in body (somatic) cells affect only the individual and may result in cancer.

What are some types of gene mutations? Gene mutations involving changes in one or a few nucleotides are known as point mutations q Occur at a single point in the DNA sequence. q Substitutions, Substitutions in which one base is changed to another. q Insertions in which an extra base is inserted into the DNA sequence q Deletions, Deletions in which a base is removed from the DNA sequence. q

Insertions and Deletions are also known as Frameshift Mutations Frameshift mutations are mutations that shift the “reading frame” of the genetic message. q The groups of 3 bases (codons) are changed from the point of the mutation and may change every amino acid that follows. q Can alter a protein so much that is unable to perform its normal function. q

What are the effects of point mutations? q Find the product of translation for the following DNA sequences: TACAAACCGAACATT TACAAATCGAACATT TACAAACCTAACATT

Silent Mutations • Sometimes a substitution mutation creates a codon that specifies the same amino acid as the original. • The resulting protein is the same as the original. There is no affect on the cell or organism. • Much of your DNA is unused. Mutations in these unused portions of DNA do not affect the cell or organism.

What are the effects of frameshift mutations? q Find the product of translation for the following DNA sequences: TACAAACCGAACATT TACAACCGAACATT A

What are chromosomal mutations? Chromosomal mutations involve changes in the number or structure of chromosomes. q. Occur during crossing over. Deletions: when all or part of a chromosome fails to cross over and genes are now missing. q Duplications: when all or part of a chromosome fails to cross over and there are now extra copies of genes. q Inversions: during crossing over, part of the chromosome attaches in reverse order. q Translocation occurs when parts of one chromosome crosses over to another chromosome pair that is NOT its homolog. q

What is the significance of mutations? Most mutations are neutral, neutral meaning that they have little or no effect on the expression of genes or the function of the proteins for which they code. q Mutations that cause dramatic changes in protein structure or gene activity are often harmful, harmful producing defective proteins that disrupt normal biological activities. q Some mutations turn out to be beneficial. q Mutation are also a source of genetic variability in a species. q