Genes and proteins updated

Genes and Proteins

Genetic Role and Structure of DNA

• Hershey and Chase experiments– Showed that viruses inject their DNA into bacteria

and direct bacteria to replicate it for them• DNA not protein is genetic material

• DNA– Double helix– Made of EQUAL amounts of nucleotides: Adenine,

Guanine, Cytosine, Thymine– Each part of helix is complementary to other, run in

opposite directions– 3 prime and 5 prime ends

Genetic Role and Structure of DNA• Hershey and Chase experiments and Rosalind Franklin– Showed that viruses inject their DNA into bacteria and

direct bacteria to replicate it for them• DNA not protein is genetic material

Alfred Hershey Alfred Hershey

The Structure of DNA (review)

DNA has (a) a double helix structure and (b) phosphodiester bonds. The (c) major and minor grooves are binding sites for DNA binding proteins during processes such as transcription (the copying of RNA from DNA) and replication.

Base Pairing

Double Helix Structure

DNA vs. RNA• Same basic structure• Instead of thymine use uracil to pair with adenine• Single helix• Helps code for and make proteins• Types of RNA– Messenger RNA (mRNA) – carries info. For a specific

protein. Segment is codon– Ribosomal RNA (rRNA) – combines with proteins to

form ribosome– Transfer RNA (tRNA) – connectors to bind an mRNA

codon to a specific RNA

DNA vs. RNADNA encodes the genetic instructions used in development and function; transfers to daughter cells

RNA plays an active role within cells by catalyzing biological reactions, controlling gene expression, or sensing and communicating responses to cellular signals

The Genetic Code• Genome – All the genetic material in cells– All different sizes depending on complexity of

organisms• Chromosome – Package of DNA and associated

proteins– You have 23 pairs or 46 total chromosomes

• Gene – sequence of DNA on a chromosome that codes for a specific protein or RNA molecule

Protein Synthesis

• Transcription – Copying a gene’s DNA to a complementary RNA molecule, occurs in nucleus

• Translation – Copying translating an mRNA strand into the language of amino acids

Transcription and Translation

Transcription occurs within the nucleus; translation occurs outside (still within the cell)

Transcription• Transcription – Copying a gene’s DNA to a

complementary RNA molecule, occurs in nucleus– Just copying the “words”

• Occurs in 3 Steps– Initiation – Enzymes unzip DNA double helix, RNA

polymerase binds to promoter• Promoter – DNA sequence at the beginning of a gene

– Elongation – RNA polymerase, adds nucleotides from 3’ to 5’ end making RNA molecule

– Termination – RNA polymerase gets to termination sequence at end of gene, separates and releases new RNA molecule

Transcription

Preparation for Translation

• Bacteria and archaea begin translation as RNA molecule is being transcribed

• Eukaryotic cells – mRNA can’t cross nuclear membrane– Add nucleotide cap to end of 5’ end of mRNA– Add 100-200 adenines – “poly a-tail”– Helps ribosomes attach to 5’ end of mRNA– Helps prevent degradation of mRNA– Introns removed from RNA, exons spliced together

The mRNA then leaves the nucleus of the cell; ribosomes translate from mRNA.

Translation

• Translate DNA/RNA language into a amino acid language to make protein

• Uses– mRNA – carries codon information– tRNA – binds to mRNA and amino acid– Ribosome – anchors mRNA

• Functional Unit = Codon – 3 base pair “word” that coincides with an amino acid– Genetic Code

Large subunit• 5,080 RNA bases • ~49 proteins

Small subunit• 1,900 RNA bases • ~33 proteins

Small subunit

Large subunit

Ribosome

tRNA

Tertiary structure of tRNA. CCA tail in yellow, Acceptor stem in purple, Variable loop in orange, D arm in red, Anticodon arm in blue with Anticodon in black, T arm in green.

tRNA and the ribosome combined

Triplet Codon

Translation

• Occurs in 3 Steps– Initiation – at 5’ end “start” codon (AUG) codes for

methionine, calls large subunit, start polypeptide– Elongation – tRNA brings 2nd amino acid,

covalently bonds with 1st amino acid, release 1st tRNA, get another tRNA and so on to make poly peptide chain

– Termination – “Stop” codon (UGA, UAG, or UAA). NO amino acid corresponds to “stop”, release factors, release last tRNA, ribosomal units separate, polypeptide chain released

Steps in Translation

Steps in Translation

Polypeptide to Protein

• Polypeptide chain is NOT a protein• Chain folds in cytoplasm to get 3-D structure• Errors can occur– Wrong amino acid sequence “messes” up folding• Cystic fibrosis

– Error in folding with correct sequence• Alzheimer Disease – incorrect folding of amyloid, forms

mass in brain

– Error in joining polypeptide chains• Misfire in types or how joined. Hemoglobin

How Do Prokaryotic Cells Express Proteins?

• Operons


• Operons – Used by bacteria– Group of genes plus promoter and

operator/repressor• Operator – DNA sequence between promoter and

protein encoding region• Repressor – Protein that binds to operator to inhibit

transcription


• Lac Operons: lactose turns off the repressor


• TRP Operon : presence of TRP = active repressor

How Do Eukaryotic Cells Express Proteins?

• All cells in an organism contain identical DNA sequences.– i.e. cloning experiments

on plants


• All cells in an organism contain identical DNA sequences.– i.e. cloning experiments on animals


• Transcription Factors– Eukaryotic Cells– Bind to DNA at different sequences to control

transcription– Can bind to promoter or enhancer– Respond to external stimuli to signal gene to “turn on”

• Signaling molecule binds to outside of cell, triggering reactions inside

– Defects can cause disease or be used as drugs• Cancer• RU486


How Do Eukaryotic Cells Express Proteins?• DNA Availability– If DNA not “unwound” from double helix, cannot do

transcription• Molecules bind to DNA and either not allow to unwind or wind it

even tighter

How Do Cells Express Proteins?

• RNA Processing– Removal of introns to change what proteins are

coded for


• Methylation of certain portions of DNA can deactivate those genes

• X-inactivation

The coloration of tortoiseshell and calico cats is a visible manifestation of X-inactivation. The black and orange alleles of a fur coloration gene reside on the X chromosome. For any given patch of fur, the inactivation of an X chromosome that carries one gene results in the fur color of the other, active gene.

Mutations

• Mutation – Change in cell’s DNA sequence• Not always harmful, can lead to genetic variability• Point Mutation – changes 1 or a few base pairs in a

gene– Substitution – replacement of 1 DNA base pair with

another• Silent – mutation codes for same protein• Missense – mutation codes for different amino acid, changing

proteins shape (ex. Sickle cell anemia)• Nonsense – mutation codes for “stop” codon instead of amino

acid – makes shorter peptide chain

Mutations: Substitution

Missense

Mutations

• Base Insertions and Deletions– 1 or more nucleotides are added or subtracted from gene

• Frameshift Mutation – adds or deleted nucleotides in any number other than multiple of 3– Disrupts codon reading, alters amino acid sequence

• Expanding Repeats– Number of a 2 or 4 nucleotide sequence increases over

several generations• Symptoms get more and more severe• Huntington’s Disease – makes extra glutamines, makes fibrous

clumps in brain

Mutations: Deletions

Mutations: Deletions

How are these Mutations Produced?

• Unmatched base pairs can possibly lead to changes in the DNA structure

How are these Mutations Produced?

• Unmatched base pairs can possibly lead to changes in the DNA structure

Wild-Type Sequence

DNA CTG ACT CCT GAG GAG AAG TCT

Protein Leu Thr Pro Glu Glu Lys Ser

Amino Acid Position

3 4 5 6 7 8 9

Sickle Cell Sequence

DNA CTG ACT CCT GTG GAG AAG TCT

Mutations

• Causes– Spontaneous – DNA replication error– Mutagens – external agent that induces mutations

• UV Radiation, x-rays, chemical weapons, nuclear energy, tobacco

– During Meiosis– Transposons – jumping pieces of DNA

• Types– Germline – occurs in cells that give rise to sperms and eggs

• Things that run in families

– Somatic – occurs in non-sex cells• DOES NOT get passed on

Science

Genes and proteins updated