Lecture 8DNA, Genes, Protein Synthesis

Covers (DNA) 9.1, 11.1-11.4 (Protein Synthesis) 12.1-12.3

(Mutations)11.5 & 12.4Regulation (12.5)

Big Picture

Cells must reproduce and carry all info needed to make another cell!

• We have learned a lot about the cell:– Cell Theory (all living things are made of cells)– Components of the cell (mitochondria, organelles)– How cells get and use energy (cell respiration)

• If all living things come from cells, these cells must reproduce themselves (WE all started from ONE cell!)

• Cells must also carry ALL of the hereditary info needed to make another cell just like it.

The Cell Cycle*

• Cells follow a repeating pattern– Divide (mitosis-asexual OR meiosis-sexual)^^– Grow– (Possibly) differentiate– Divide again

• This pattern is called the Cell Cycle.• The Cell Cycle for cells that become egg and sperm (Meiosis)

is different than the cell cycle for all other bodily cells (Mitosis).

• ^^More in Lecture 9

Cell Differentiation

• We all started as one cell, yet we have billions of cells in our body.

• Stem cells have the ability to differentiate into MANY different types of mature cells (liver cells, lung cells, brain cells)

• Once a cell has differentiated, it is only capable of making more of it’s mature type of cell. (Lung cells only make more lung cells.)

• Finally, some cells are not capable of dividing at all once the organism reaches a certain stage of development. Heart and brain cells are examples.

Transition

• Now let’s talk about what DNA is, then we will discuss DNA Replication (part of the cell cycle, but impt to know at this point), and then we will discuss genes and how the info they carry becomes a living organism.

Hereditary Info: DNA*

• The hereditary info (the blueprint which tells daughter cells what they are – the mitochondria, organelles, cell membrane, all of the proteins & enzymes required for a cell to live, grow and acquire energy) is carried in the NUCLEUS of the parent cell and packaged as a molecule called DNA.

• DNA is arranged in chromosomes.• Each chromosome is a single molecule of

DNA.

General Pic of cell with nucleus & DNA

DNA*

• DNA is a nucleic acid (recall Lecture 3)• It is a double-stranded molecule made up of

billions of subunits called nucleotides.• Each nucleotide consists of:

– A phosphate group– A sugar molecule (deoxyribose)– A nitrogen-containing base

• 4 types of bases (C, T, G, A)• Cytosine, Thymine, Guanine, Adenine

sugar

phosphate

sugar

phosphate

sugar

phosphate

base = adenine

base = thymine base = cytosine

sugar

phosphate

base = �guanine

DNA Nucleotides

Fig. 11-3

DNA*• The phosphate group of one nucleotide bonds to

the sugar of the next nucleotide in the same strand. (sugar-phosphate backbone)

• The nitrogenous base on each DNA strand forms hydrogen bonds with a complementary base on the opposite strand (remember DNA is double stranded.) – A to T– G to C

• DNA therefore has a ladder type of structure, which twists to form a double helix.

The Watson-Crick Model of DNA Structure

freephosphate

phosphate

hydrogenbonds

freesugar

freephosphate

nucleotide nucleotide

base(cytosine)

sugar

free sugar

(a) Hydrogen bonds hold complementarybasepairs together in DNA

(b) Two DNA strands form adouble helix

(c) Four turns of aDNA double helix

Fig. 11-5

The Structure of DNA

Fig. 9-1

T A

C G

G C

C

T

A

A

C

A

A

T

T

G

C G

A T

nucleotide

phosphate

base

sugar

(a) A single strand of DNA (b) The double helix

History

• Chargaff’s Rule: the DNA of any species has equal amounts of A and T and equal amounts of G and C.

• 1953: After 20 years of trying to discover the nature of hereditary info (what molecule carried the info), Watson & Crick were finally able to prove the molecule of heredity was DNA and they uncovered the double helix shape and the complementary base pairing.

DNA & Chromosomes*• One entire “copy” of your DNA is in every cell of

your body, and stretched out would be 6 feet long! (exception: egg and sperm)

• DNA is packaged into chromosomes (each human chromosome has 50-250 million nucleotides.)

• Each species on earth has a different number of chromosomes (Humans 46, Dogs 78, etc)

• Every species has an EVEN number of chromosomes, because we INHERIT ½ of our chromosomes from mom and ½ of our chromosomes from dad.* except mules/donkey?

histone proteins

protein scaffold

DNA double helix

DNA wound around histone proteins

Folded chromosome,fully condensed in adividing cell

Coiled DNA/histone beads

Loops attached to a protein scaffold; this stage of partial condensation typically occurs in a nondividing cell

1

2

3

4

5

Chromosome Structure

Fig. 9-4

sexchromosomes

The Karyotype of a Human Male

Fig. 9-6

REPLICATION OF DNA

• DNA is replicated each time a cell prepares to divide (mitosis & meiosis)

• Recall DNA is a double helix• In order to make a copy of the entire DNA

“message”, the parent cell’s DNA will UNWIND and use each SINGLE strand as a template to make TWO double strands (one to stay in the parent cell and one to go into the daughter cell)

Basic Features of DNA Replication

free nucleotides

The parental DNAis unwound

New DNA strandsare synthesized withbases complementaryto the parental astrands

Each new double helix is composedof one parental strand (blue) and onenew strand (red)

Parental DNAdouble helix1

2

4

3

Fig. 11-6

Material needed for DNA replication

• Parental DNA strands• Free nucleotides (ATGC’s) hanging out in the

nucleus• Enzymes that facilitate the process

REPLICATION OF DNA*

• DNA helicase opens strand• DNA Polymerase moves along each single

strand and facilitates the creation of a second strand of nucleotides that are the COMPLEMENT to each nucleotide on parent strand

• Parent strand: ATCGCCCGGGAAATTT• Daughter strand: TAGCGGGCCCTTTAAA

REPLICATION OF DNA

REPLICATION OF DNA

• When complete, each new DNA molecule consists of a parent strand and a newly made daughter strand….they wind around each other to become a double helix.

Two identical DNAdouble helices, eachwith one parentalstrand (blue) andone new strand (red)

One DNAdouble helix

DNA replication

Semiconservative Replication of DNA

Fig. 11-7

How does DNA make an organism?

• How can DNA “know” to make a bird’s feathers, or a snake’s scales, or human skin?

• How can DNA “know” how to make blue eyes, brown eyes, etc?

• Answer: GENES! The pattern and order of A, T, G, and C on each chromosome determines what an organism will look like.

• Ex: Alphabet has only 26 letters, but can make MILLIONS of words!

Genes

• A gene is a sequence of DNA that occupies a specific location (or locus) on a chromosome

• Chromosomes vary on the number of genes contained on each one (Chromosome 1 has more than 3,000 genes!)

• Most genes contain the information needed to direct the synthesis of ONE protein.

• Proteins form all cell structures, enzymes and signalling molecules in the cell.

DRAW PIC OF GENE

How do Genes “become” proteins?*

• Recall that genes are the “instructions” to make proteins.

• DNA must go through PROTEIN SYNTHESIS in order to make the protein that corresponds to each gene.

From DNA to Proteins: Protein Synthesis*

• The area of DNA that corresponds to a gene is transcribed into messenger RNA (mRNA.)

• mRNA: also a nucleic acid, like DNA. Made of nucleotides A, U, G, C (no T in mRNA).

• mRNA is then translated into a protein• Protein is then “sent” to specific places within

the cell (or outside the cell) to carry out its particular job (structural, enzyme, signalling protein)

• Therefore: DNA>>>>mRNA>>>>Protein

Protein Synthesis*

• Composed of THREE events:– Transcription: creation of mRNA from DNA

template– Translation: creation of a protein from mRNA

template (recall that proteins are strings of Amino Acids)

– Processing of Protein: amino acid “string” is processed and becomes finished protein

Genetic Information Flows from DNA to RNA to Protein

Fig. 12-2

(a) Transcription

Translation of the mRNA produces a protein molecule with an amino acid sequence determined by the nucleotide sequence in the mRNA

(b) Translation

Transcription of thegene produces anmRNA with anucleotide sequencecomplementary to oneof the DNA strands

DNA

messenger RNA

protein

ribosome

(cytoplasm)(nucleus)

gene

Transcription

• Creation of mRNA from DNA• Occurs IN THE NUCLEUS• An enzyme (RNA Polymerase) makes this

process happen by “opening up” the DNA molecule and facilitating the copying of a piece of DNA (gene)

• Material needed for transcription:– RNA nucleotides: A, U, G, C– RNA Polymerase

Transcription*

• RNA Polymerase must know WHERE on the DNA to start and stop transcription.

• Promoter: specific sequence of nucleotides that “signals” the beginning of a gene (TATAA: called the TATA box)

• Termination Signal: specific sequence of nucleotides that “signals” the end of a gene.

DNA TO mRNA

Translation

• mRNA to protein• Happens in the CYTOPLASM (mRNA moves

out of the nucleus for translation)• Material needed for translation:

– The mRNA strand– Transfer RNA (tRNA)– Amino Acids (AA’s)– Ribosome (organelle in the cell – its only job is

translation)

Translation

Translation: Ribosomes & tRNA

• Ribosome is made of 2 subunits that clasp the mRNA molecule

• Ribosome has binding site for mRNA and tRNA molecules• Transfer RNA (tRNA) is a molecule that “carries” amino acids

into the ribosome to be linked together as a new protein• A group of three nucleotide bases (anticodons) protrudes

from each tRNA• Complementary base pairing between tRNA and mRNA

directs correct amino acid into growing protein chain.

Translation

Completion of Protein*

• As the “string” of AA’s comes off the ribosome complex, it will begin to twist and turn into a mature protein.

• Twists and turns depend on the AA and it’s polarity (functional groups on each AA.) AA’s that are negatively charged will be drawn to those that are positively charged, etc.

Gene Mutation

• Now that we know how genes are the blueprint for all proteins that make up a cell, let’s consider what would happen if a gene is mutated.

• Would we still have the correct protein? • Would that protein function normally?

Result of Gene Mutation*

• the sequence of ATGC is incorrect in the sequence of DNA that makes up the gene

• This would result in an altered mRNA molecule (because it is made from incorrect DNA sequence)

• This COULD result in altered AA being placed in the growing protein (remember mRNA is template for the AA that the tRNA brings into the ribosome)

HOW do mutations happen?

• Incorrect base pair put into DNA as it is being replicated, called point mutations

• There are also insertion, deletion, inversion and translocation mutations

• Environmental conditions can damage DNA by increasing the frequency of base pair errors:– Chemicals– Radiation– This damage can happen during dna replication and

even when DNA is not replicating!

Mutations

Fig. 11-8a

Mutations

Fig. 11-8b

Mutations

Fig. 11-8c

Mutations

Fig. 11-8d

Result of mutation: a different protein?*

• 1.) Protein is non functional (if it’s a protein necessary for life, organism could die) EXAMPLE: HEMOGLOBIN (Table 12-4)

• 2.) No effect on protein• 3.) Protein works “better” than the original

(sometimes called wild-type) protein, and this is the basis of evolution!!!!

Ways errors are prevented/fixed*

• Mutations in the ATGC code of DNA are minimized:– DNA is replicated with high accuracy in large part due to the specificity

of hydrogen bonding between complimentary base pairs (if an incorrect nucelotide tried to slide in, it would not bond to the nucleotide on the other strand.)

– Proofreading of DNA: DNA repair enzymes proofread the new DNA strand. Incorrect bases are put in once every 1,000-100,000 base pairs

– Repair of incorrect nucleotides in DNA: repair enzymes fix many of these errors.

NEW TERM: ALLELE

• alternate form of a gene, the result of a mutation in the DNA.

Finally, regulation of protein synthesis*

• Although there are 20,000 genes on your DNA, EACH CELL is only producing a small number of proteins at any given time.

• Some genes are expressed (made into proteins) by ALL cells EX: making DNA Polymerase to replicate DNA for mitosis or RNA polymerase for transcription.

• Other genes are expressed only in certain cells at certain times under certain conditions: ex: casein, milk protein, only made in female cells of the breast and only when woman is breast feeeding

Regulation of Protein Synthesis*

• 1.) cells can control the frequency at which a gene is transcribed

• 2.) Proteins may require modification before they become functional

• 3.) cell can control the rate that proteins are degraded.

An Overview of Information Flow in a Eukaryotic Cell

Fig. 12-10

Translation

Modification

Degradation

activeprotein

aminoacids

substrate

product5

4

3

Transcription

mRNA

pre-mRNA tRNA

tRNA

1

mRNAprocessing

2

rRNAproteins

ribosomes

amino acids

(cytoplasm)

(nucleus)

Cells can control the frequency oftranscription

Different mRNAsmay be producedfrom a single gene

Cells can control thestability and rate oftranslation of particular mRNAs

If the active proteinis an enzyme, it willcatalyze a chemicalreaction in the cell

Cells can regulate a protein’s activityby degrading it

Cells can regulate a protein’s activityby modifying it

DNA

inactiveprotein

mRNA