RNA & DNA

Compare RNA & DNAEach nucleotide in both DNA and RNA is made up of a

• 5-carbon sugar, • phosphate group,• nitrogenous base.

Contrast RNA & DNA(1) ribose sugar in RNA instead of deoxyribose.

(2) single-stranded RNA not double-stranded.

(3) uracil in RNA in place of thymine.

Comparing RNA and DNA – These chemical

differences make it easy for the enzymes in the cell to tell DNA and RNA apart.

– You can see the differences in structure in the figures to the right.

RNA and DNA Compare DNA and RNA to the information needed to construct a building. • A master plan has all the

information needed to do the construction, just as DNA has all the information needed to carry genetic information and to build proteins.

• A blueprint is a copy of the master plan that contains just the essential information, just as RNA copies the information needed to perform specific tasks.

Functions of RNA

You can think of an RNA molecule, – as a disposable copy of a

segment of DNA, – as a working copy (or blueprint)

of a single gene,– as involved in protein synthesis,– as controlling the assembly of

amino acids into proteins.

Different types of RNA specialize in different aspects of these jobs.

Functions of RNA

The three main types of RNA are – messenger RNA, – ribosomal RNA, and – transfer RNA.

Messenger RNA

Most genes contain instructions for assembling amino acids into proteins.

Messenger RNA – abbreviated as mRNA,– copies instructions for

assembling proteins from DNA,

– carries information from DNA to other parts of the cell.

Ribosomal RNAProteins are assembled on ribosomes, small organelles composed of two subunits.

Ribosomal RNA • abbreviated as rRNA,• ribosome subunits are made up

of several rRNA molecules and as many as 80 different proteins,

• receives information from mRNA.

Transfer RNA

• abbreviated as tRNA,• transfers each amino acid to

the ribosome as proteins are built,

• uses coded messages in mRNA to know which proteins need to be built.

Transcription

• Synthesis of RNA molecule from DNA molecule,

• When most of the work of making RNA takes place,

• Segments of DNA serve as templates to produce complementary RNA molecules,

• Complementary RNA molecules leave the nucleus with instructions on proteins or other substances that need to be produced by the cell.

Transcription

RNA polymerase• Enzyme used in

transcription• Similar to DNA

polymerase• Binds to DNA, • Uses one strand of DNA

as a template,• Assembles nucleotides

from DNA into a complementary strand of RNA.

The Genetic Code• The first step in decoding genetic

messages is to transcribe a nucleotide base sequence from DNA to RNA,

• The transcribed information contains a code for making proteins,

• The sequence for an amino acid is read 3 letters at a time,

• Each 3 letter “word” in mRNA is known as a codon,

• Each codon specifies the production of a single amino acid.

The Genetic Code

– Proteins are made by joining amino acids together into long chains, called polypeptides.

– As many as 20 different amino acids are commonly found in polypeptides.

The Genetic Code

Properties of different proteins depend on:– the specific amino acids in

a polypeptide, – the order in which they are

joined.

The sequence of amino acids influences:– the shape of the protein, – the function of the protein.

The Genetic Code

RNA contains four different bases: – adenine, – cytosine, – guanine, – uracil.

These bases form a “language,” or genetic code, with just four “letters”: A, C, G, and U.

How to Read Codons Because there are four different bases in RNA,

– there are 64 possible three-base codons in the genetic code,

– (4 × 4 × 4 = 64) – The circular table shows

the amino acid for each of the 64 codons.

– To read a codon, start at the middle of the circle and move outward.

How to Read Codons

Most amino acids can be specified by more than one codon.

For example, – six different codons

(UUA, UUG, CUU, CUC, CUA, and CUG) specify leucine.

– only one codon (UGG) specifies the amino acid tryptophan.

Start and Stop Codons

The genetic code has punctuation marks.

The methionine codon AUG serves as the initiation, or “start,” codon for protein synthesis.

Following the start codon, mRNA is read, three bases at a time, until it reaches one of three different “stop” codons, which end translation. (UAA, UAG, UGA)

Translation

What role does the ribosome play in assembling proteins?

Ribosomes use the sequence of codons in mRNA to assemble amino acids into polypeptide chains.

Translation

– The decoding of an mRNA message into a protein,

– A set of instructions that gives the order in which amino acids should be joined to produce a polypeptide,

– Requires the folding of one or more polypeptide chains,

– The sequence of codons in mRNA used to assemble amino acids into polypeptide chains in the ribosome.

Steps in Translation

– Messenger RNA is transcribed in the nucleus and then enters the cytoplasm for translation.

Steps in Translation

• Each tRNA molecule carries just one kind of amino acid.

• In addition, each tRNA molecule has three unpaired bases, collectively called the anticodon—which is complementary to one mRNA codon.

• The tRNA molecule for methionine has the anticodon UAC, which pairs with the methionine codon, AUG.

Steps in Translation

(1) A ribosome attaches to an mRNA molecule in the cytoplasm.

(2) The ribosome then reads each codon of mRNA,

(3) tRNA is directed to bring the specified amino acid into the ribosome,

(4) The ribosome attaches each amino acid to the growing chain one at a time.

Steps in Translation

(5) The ribosome has a second binding site for a tRNA molecule for the next codon.

For example: • If that next codon is UUC,

a tRNA molecule with an AAG anticodon brings the amino acid phenylalanine into the ribosome.

Steps in Translation

(6) The ribosome helps form a peptide bond between

the first and second amino acids—methionine and phenylalanine.

• At the same time, the bond holding the first tRNA molecule to its amino acid is broken.

Steps in Translation

(7) That released tRNA then moves into a third binding site, from which it exits the ribosome.

(8) The ribosome then moves to the third codon, where tRNA brings it the amino acid specified by the third codon.

Steps in Translation

(9) The polypeptide chain continues to grow until the ribosome reaches a “stop” codon on the mRNA molecule.

(10) When the ribosome reaches a stop codon, it releases both the newly formed

polypeptide and the mRNA molecule, completing the process of translation.

Mutations• Mutations are inheritable changes

in genetic information• The word mutation comes from the

Latin word mutare, meaning “to change,”

• Types of mutations: – mistakes made in copying DNA,– inserting the wrong base as

DNA is copied,– skipping a base as a strand is

put together,

Types of Mutations

All mutations fall into two basic categories: • Gene mutations:

– produce changes in a single gene.

• Chromosomal mutations: – produce changes in whole

chromosomes.

Gene Mutations

Point mutations • Involve changes in one or a few

nucleotides,• Occur at a single point in the

DNA sequence, • Generally occur during

replication.• If a gene in one cell is altered,

the alteration can be passed on to every cell that develops from the original one.

• Include substitutions, insertions and deletions

Substitutions In a substitution mutation, • one base is changed to a different base, • usually no more than a single amino acid is

affected,• sometimes there is no effect at all.

Insertions and Deletions • Insertions and deletions are point mutations in which

one base is inserted or removed from the DNA sequence.

• If a nucleotide is added or deleted, the bases are still read in groups of three, but now those groupings shift in every codon that follows the mutation.

Insertions and Deletions Insertions and deletions are also called frameshift mutations because they:• shift the “reading frame” of the genetic message.• change every amino acid that follows the point of the

mutation • can alter a protein so much that it is unable to perform its

normal functions.

Chromosomal Mutations • Involve changes in the number or structure of chromosomes,• Can change the location of genes on chromosomes,• Can change the number of copies of some genes.

There are four types of chromosomal mutations: • deletion, • duplication, • inversion, • translocation.

Chromosomal Mutations

Deletion involves the loss of all or part of a chromosome.

Chromosomal Mutations

Duplication produces an extra copy of all or part of a chromosome.

Chromosomal Mutations

Inversion reverses the direction of parts of a chromosome.

Chromosomal Mutations

Translocation occurs when part of one chromosome breaks off and attaches to another.

Effects of Mutations

How do mutations affect genes?

The effects of mutations on genes vary widely. – Some have little or no effect– Some produce beneficial

variations. – Some negatively disrupt gene

function.– Mutations often produce proteins

with new or altered functions that can be useful to organisms in different or changing environments

Mutagens

Mutagens are chemical or physical agents in the environment that can cause mutations to occur.

Chemical mutagens include:– pesticides, – plant alkaloids, – tobacco smoke, – environmental

pollutants.

Mutagens

Physical mutagens include some forms of electromagnetic radiation, such as X-rays and ultraviolet light.

If these chemical and physical mutagens interact with DNA, they can produce mutations at high rates.