RNA & Protein Synthesis Chapter 13

13.1: RNA

The Role of RNA The structure of DNA itself does not explain how a gene actually works. The answer came with the discovery of another nucleic acid, RNA RNA- Ribonucleic acid

Comparing DNA & RNA Both are made of nucleotides, linked together or not 3 important differences Different sugars DNA- deoxyribose RNA- ribose

DNA is double stranded, RNA is single stranded Different bases DNA has thymine RNA has uracil All other bases are the same

DNA cannot leave the nucleus, but the directions on building important molecules need to be carried to other parts of the cell. RNA acts like disposable copies of genetic information for these directions to be used.

Functions of RNA RNA has many functions, but they are all involved in one main role- protein synthesis.

3 types Messenger RNA (mRNA) Ribosomal RNA (rRNA) Transfer RNA (tRNA) Produced in the nucleus Complementary to DNA Carries the instructions found in DNA from the nucleus to other parts of the cell Ribosomal RNA (rRNA) Makes up the 2 subunits of a ribosome Ribosomes are the organelles that assemble proteins Transfer RNA (tRNA) Transports amino acids found in the cell to the ribosome Uses the coded message from mRNA

RNA Synthesis Understanding how RNA and proteins are made is essential to understanding how genes work.

transcription Most of the work of making RNA happens during the process of transcription Transcription- segments of DNA serve as templates to produce complementary RNA strands Transcription requires the use of an enzyme called RNA polymerase Binds to DNA, separating the bases by breaking the hydrogen bonds Uses one strand of the DNA to assemble a complementary chain of mRNA nucleotides

promoters RNA polymerase does not bind to just anywhere on the DNA molecule. There are regions along the DNA strand called promoters that act as signals to show where the enzyme should start building RNA. There are similar signals that also tell the enzyme to stop transcription.

RNA Editting RNA molecules require some editing before being read. Bits and pieces are cut out and pieced together before they are used in the next step of protein synthesis.

2 portions Introns- cut out and discarded Exons- remaining pieces spliced together to form the final strand of mRNA

Biologists don’t have a complete answer as to why cells use energy to make a large RNA molecule and then throw parts of that molecule away. Some pre-mRNA molecules may be cut and spliced in different ways in different tissues, making it possible for a single gene to produce several different forms of RNA. Introns and exons may also play a role in evolution, making it possible for very small changes in DNA sequences to have dramatic effects on how genes affect cellular function.

13.2: Ribosomes & protein Synthesis

The Genetic code The first step in decoding genetic messages is to transcribe DNA into RNA This transcribed information contains a code for making proteins Proteins are made by linking amino acids together in long chains called polypeptide chains There are as many as 20 different amino acids commonly found in polypeptide chains

The specific order the amino acids are joined together determine the properties of different proteins. The sequence can affect the shape in which determines its function.

We read the genetic code by looking at 3 bases at a time The language used in making proteins is called the genetic code which is made up of 4 different letter or bases (A, C, G, & U). We read the genetic code by looking at 3 bases at a time Codon- 3 base long message found on mRNA Codes for amino acids

How to read codons There are 4 different bases in RNA, so there are 64 different combinations of 3 base codons Some amino acids can be specified by multiple codons Leucine can be coded by the codons UUA, UUG, CUU, CUC, CUA, & CUG. Some may be more specific with only one codon combination Tryptophan can only be coded by UGG.

Start & stop codons These act as punctuation marks for the genetic code. Methionine is the start codon (AUG) that will signal the beginning of a new polypeptide chain. All polypeptide chains begin with methionine. Stop codons will signal an end to a polypeptide chain. UGA, UAA, & UAG

translation mRNA contains the instructions that give the order in which amino acids should be linked together. Ribosomes use the sequence of codons to assemble amino acids into polypeptide chains. Translation- decoding of codons into a polypeptide chain to eventually become a protein

Steps in translation Translation begins when a ribosome attaches to an mRNA molecule. Each codon will pass through the ribosome giving the directions for each amino acid.

tRNA brings the proper amino acids to the ribosome based on the codons read. tRNA molecules can only bring one amino specific amino acid at a time tRNA has 3 unpaired bases that are complementary to codons found on mRNA These 3 unpaired bases are called anticodons Pairs with codons showing exactly what sequence of amino acids in the polypeptide chain

Amino acids are brought to the ribosome one at a time and linked together. The chain continues to grow until the ribosome reads a stop codon which signals it to stop and release the polypeptide chain.

The molecular basis of heredity The central dogma of molecular biology is that the information is transferred from DNA to RNA to proteins. This illustrates or explains gene expression. Putting genetic information in action for living cells One of the most interesting discoveries about molecular biology is the near-universal nature of the genetic code All organisms use the same 4 letter/base genetic language and protein synthesis process.

13.3: Mutations

Types of mutations Cells can make mistakes in copying DNA, transcribing DNA, and even translating DNA. Mutations- heritable changes in genetic information Mutations can come in many different forms but fall in 2 basic categories Those that produce changes in single genes Those that produce changes in whole chromosomes

Gene mutations Mutations that involve changes in one or a few nucleotides are known as point mutations because they occur at a single point in the DNA sequence. They generally occur during replication. 2 types

Substitution- one base is changed to a different base Silent- amino acid does not change GGU changes to GGA but still codes for glycine

Missense- amino acid does change CAC changes to CAG which changes Histidine to Glutamine

Nonsense- amino acid changes to a stop codon UGU changes to UGA which changes the amino acid Cysteine into a stop codon to early

Frameshift mutations- shifts the “reading frame” of the genetic message Insertion- base is added to the sequence Deletion- base is removed from the sequence Changes almost every amino acid after the mutation and can alter a protein so much that it is unable to function properly

Chromosomal mutations Involve the change in the number or structure of chromosomes Can change the location of genes or the number of copies of genes 4 types

Deletion- whole segments of chromosomes are deleted, genes can be lost Duplication- segments of chromosomes are replicated and remain part of the chromosome Inversion- reverses segments of chromosomes so that they read backwards Translocation- segments of one chromosome are removed and attached to a different chromosome

Effects of mutations Genetic material can be altered by natural events or by artificial means. Mutations may or may not affect organisms

Many mutations are produced by errors in genetic processes such as DNA replication. The cellular machinery that replicates DNA inserts an incorrect base roughly once in every 10 million bases. Small changes in genes can gradually accumulate over time. Stressful environmental conditions may cause some bacteria to increase mutation rates.

mutagens Some mutations arise from mutagens- chemical or physical agents in the environment. Chemical- pesticides, some alkaloids, tobacco smoke, pollutants Physical- electromagnetic radiation such as X- rays and UV light

Harmful & helpful mutagens The effects of mutations on genes vary widely. Some have little or no effect Some produce beneficial variations Some negatively disrupt gene function.

Whether a mutation is negative or beneficial depends on how its DNA changes relative to the organism’s situation. Mutations are often thought of as harmful because they disrupt the normal function of genes. However, without mutations, organisms cannot evolve, because mutations are the source of genetic variability in a species.

Harmful effects The defective proteins produced by these mutations can disrupt normal biological activities, and result in genetic disorders and cancers.

Beneficial effects Mutations often produce proteins with new or altered functions that can be useful to organisms in different or changing environments. Organisms can become resistant to certain pesticides and antibiotics. Extra sets of chromosomes can be inherited which is called polyploidy. Polyploid plants are often larger and stronger than diploid plants.