DNA and RNA Notes Part 2 Protein Synthesis

Genes and Proteins Proteins form and become key cell structures and regulators of cell functions Sequence of amino acid makes specific proteins The sequence of nucleotides in each gene contains information for assembling amino acids. It is estimated that each human cell contains about 80,000 genes

RNA RNA – ribonucleic acid 4 Differences Between DNA and RNA RNA DNA Single Stranded Double Stranded Sugar is Ribose Sugar is Deoxyribose Adenine, Guanine, Adenine, Guanine, Cytosine, Uracil Cytosine, Thymine RNA can leave the nucleus DNA can not leave nucleus  

Instead of thymine, RNA uses uracil. Notice that RNA is single-stranded instead of double-stranded. -ose represents sugars and DNA and RNA both have different sugars.

DNA – A C G T G A A G C T G T A C A G T C A G G C T A RNA – U G C A ……now do the rest your self remember Thymine is replaced with uracil in RNA!

What is the role of RNA in the cell: DNA provides workers with the instructions for making the proteins RNA takes the DNA instructions on how a protein is made then amino acid by amino acid they assemble the protein. 3 Types of RNA that help to build proteins Messenger RNA (mRNA) – brings information from the DNA in the nucleus to the ribosome Ribosomal RNA (rRNA) – what ribosomes are made of and they clamp onto the mRNA and use its information to assemble amino acids Transfer RNA (tRNA) – transports amino acids to the ribosome to be assembled into a protein.

Transcription Transcription – enzymes make an RNA copy of a portion of a DNA strand The process of transcription is similar to replication except Transcription results in single strand of RNA Does not transcribe the entire strand of DNA

Steps of Transcription From DNA to RNA The process of transcription begins as enzymes, RNA polymerase, unzips the molecule of DNA As the DNA molecule unzips, RNA polymerase assembles RNA nucleotides using one strand of the DNA as a template. Only the 3’  5’ template strand of DNA is transcribed. The RNA complimentary strand grows in the 5’  3’ direction. Transcription continues until RNA polymerase reaches a special sequence of nucleotides. mRNA molecule breaks away and mRNA leaves the nucleus and enters the cytoplasm.

The Genetic Code Proteins are built from amino acids 20 different amino acids Codon – each set of 3 nitrogen bases represents an amino acid, which is also called a triplet code The order of nitrogen bases in DNA can determine the type and order of amino acids in a protein 64 different combinations 61 code for amino acids 3 signal to stop protein synthesis More than 1 codon can code for the same amino acid

Codons represent the same amino acids in all organisms Gives evidence of evolution Start codon is AUG (methionine) Stop codons are UAA, UAG, UGA

Translation From mRNA to Protein Translation – the process of converting the information in a sequence of nitrogen bases in mRNA into a sequence of amino acids Takes place at the ribosomes in the cytoplasm

The Role of Transfer RNA To bring the amino acids to the ribosome Correct translation of the mRNA message depends upon the joining of each mRNA codon with the correct tRNA molecule tRNA recognizes the mRNA codon, because tRNA has a sequence of three nucleotides that are complement of the nucleotides in the codon Anti-Codon – tRNA nucleotides Ex: mRNA – A-C-A tRNA – U-G-U

DNA – T–A–C–A–G –G –T–C–G –T–T–A–C–G–G–A–C–T mRNA – tRNA – Amino Acids

Translating the mRNA Code Steps of Translation The first codon of the mRNA strand attaches to a ribosome tRNA molecules, each carrying a specific amino acid approach the ribosome tRNA anticodon pairs with mRNA codon The first codon on mRNA is AUG which codes for amino acid methionine. AUG is the start codon for protein synthesis A new tRNA molecule carrying an amino acid will pair with the mRNA codon As the process continues a chain of amino acids is formed until the ribosome reaches a stop codon on the mRNA strand UAA, UAG, UGA

12-4 Mutations Mutation: A Change in DNA Mutation – any change in the DNA sequence that also changes the protein it codes for Mutations in Reproductive Cells If mutation occurs in egg or sperm the altered gene would become part of the genetic makeup of the offspring Sometimes the mutation is so severe that the embryo does not survive In rare cases a gene mutation may have positive effects

Mutations in Body Cells If the cells DNA is changed this mutation would not be passed on to offspring But the mutations can cause harm to the individual

2 Types of Mutations in DNA Point Mutation Is a change in a single base pair in DNA A change in a single letter changes the meaning of this sentence 2. Frameshift Mutation A mutation in which a single base is added or deleted from DNA This mutation would cause nearly every amino acid in the protein after the deletion to be changed.

Chromosomal Mutations Chromosomal Mutations – changes in chromosomes during replication. Parts can be broken or lost. They occur in all living organisms, but they are especially common in plants Although rare, changes in an organisms chromosome structure do occur. Chromosomal mutations are rarely passed on to the next generation because: The zygote usually dies The mature organism is usually sterile

4 Types of Chromosomal Mutations Deletion – a fragment of a chromosome breaks off, it can be lost when a cell divides

2. Duplication – the chromosome fragment attaches to its homologous chromosome, which will then carry two copies of a certain set of genes

3. Inversion – fragment reattaches to the original chromosome in the reverse orientation

4. Translocation – a fragment may join a nonhomologous chromosome

Causes of Mutations Spontaneous Mutations – a mistake in base pairing during DNA replication. It occurs at random or at any given moment Mutagen – any agent that can cause a change in DNA Ex. Chemicals, radiation, high temperatures

Repairing DNA When mistakes do occur repair mechanisms fix mutations Proofreading Enzymes – reads the DNA strand and checks it for mistakes Repair Enzymes – fixes any mistakes in the DNA strand

Mistakes in Meiosis Sometimes accidents occur during meiosis and chromosomes fail to separate correctly Nondisjunction – failure of homologous chromosomes to separate During meiosis I one chromosome from each pair is supposed to move to opposite poles but occasionally both chromosomes of a pair move to the same pole

Trisomy – 1 extra chromosome (47) Ex: extra chromosome on pair number 21 – down syndrome Monosomy – missing 1 chromosome (45) Ex: missing chromosome on pair number 23 – turner syndrome Tetraploid – 2 extra chromosomes (48) Polyploids – organisms with more than the usual number of chromosome sets Is rare in animals and almost always results in death.