Chapter 4- Sesay emphasized

DNA

sugar- deoxyribose

nitrogenous base

phosphate- has thymine instead of uracil

nucleotides

DNA is a polymer of nucleotides (a sugar, nitrogenous base, and phosphate)

Nitrogenous bases:

Purines- adenine and guanine (pure as gold)

pyramidines- cytosine, thymine, uracil, (cut the pie)

The Five

Nitrogenous

Bases found in

DNA and RNA

Purines: Adenine (A) and Guanine (G) (double ring)

Pyrimidines: Cytosine (C), Thymine (T), Uracil (U) (you cut the pie

Dna has ATCG

What is the essential function of DNA?

The essential function of DNA is to carry instructions (genes)

for the synthesis of proteins. genes only contain information for production of proteins

What is a Gene?

segment of DNA coding for the synthesis of a specific

protein/aka polypeptide

• Humans have about 20,000 genes

• Only about 2% of total DNA code for protein

• Other 98% is noncoding DNA- they regulate gene activity

• Plays role in chromosome structure

• Regulates gene activity

Chromatin—

fine filamentous DNA material complexed

with proteins called histones

not dividing- loose chromatin

Occurs as 46 chromosomes in most cells

• 6 feet long thread packed in cell nucleus of 5 μm diameter

• In nondividing cells, chromatin is so slender it cannot be seen with

light microscope

• Granular appearance under electron microscope

chromatin structure

Histones—

proteins around which DNA winds

• Histones cluster in groups of eight molecules

transition from chromatin to chromosome

when chromatin transitions to chromsome it loses histones and coils tighter.

how does chromatin ‘behave”?

In a nondividing cell, the chromatin is not static

• Structure and location changes moment to moment

according to genetic activity of cell

• Genes get turned off and on:

Ribonucleic acids (RNAs)

General structure:

• Contain the sugar ribose

• nitrogenous Bases A, U, G, C; uracil (U) replaces thymine found in DNA

• Single nucleotide chain (not a double helix) except in short regions

• Smaller than DNA, can have less than 100 or just over 10,000

bases per molecule

• Functions mainly in cytoplasm

Three RNAs are important for protein synthesis

Messenger RNA (mRNA)

• Ribosomal RNA (rRNA)

• Transfer RNA (tRNA)

Genome—

all the DNA in one 23-chromosome set

• 3.1 billion nucleotide pairs in human genome

• Individual variation comes from single-nucleotide polymorphisms

4.2b The Genetic Code

4.2b The Genetic Code

Base triplet—

sequence of three DNA nucleotides that stands

for one amino acid

codon- must know start and stop codons

3-base sequence in mRNA

in nucleus- speaks codon language

cytoplasm- speaks amino acid/protein language

• 64 possible codons available to represent 20 amino acids; 61

code for amino acids, 3 are stop codons

• Start codon—AUG codes for methionine, and begins the amino

acid sequence of the protein

• Stop codons—UAG, UGA, and UAA: signal “end of message,” like a period at the end of a sentence

Transcription—

copying genetic instructions from DNA to

mRNA in the nucleus.

RNA polymerase rewinds the DNA helix behind it; allows gene to be

transcribed by several polymerase molecules.

MRNA is small enough to leave nucleus.

• Terminator: stop sequence at the end of a gene

Pre-mRNA

is “immature” RNA produced by transcription;

undergoes processing

has introns

• Enzymes within the nucleus remove introns from the preRNA and

splice exons together to make mRNA (only has exons)

Alternative splicing—

pre-mRNA is spliced- introns are removed

variations in the way exons are

spliced allow for a variety of proteins to be produced from

one gene

• One gene can code for more than one protein

• Exons can be spliced together into a variety of different mRNAs

Translation—

nucleotide language converted into amino acid

language

A ribosome “reads” the mRNA codons (group of three nucleotides) and forms a protein in the cytoplasm

Three main participants in translation:

• Messenger RNA (mRNA)

• Carries code from nucleus to cytoplasm

• Has protein cap that is recognition site for ribosome

• Transfer RNA (tRNA)

• Finds free amino acid in cytosol, Delivers a single amino acid to the ribosome

• Contains an anticodon—series of 3 nucleotides that are

complementary to the mRNA codon

Initiator tRNA always carries AUG methione.

• Ribosomes

• Organelles that read the message and build a peptide chain

• Contain a large subunit and a small subunit

• Free in cytosol, on rough ER, and on nuclear envelope

• E site, P site, and A site involved in translation process

• Messenger RNA (mRNA)

• Carries code from nucleus to cytoplasm

• Has protein cap that is recognition site for ribosome

image of protein processing and secretion- very important image

4.2f Synthesizing Compounds Other Than Proteins

Cells synthesize glycogen, fat, steroids, phospholipids, pigments,

and other compounds

• No genes for these products, but their synthesis is under

indirect genetic control

• They are produced by enzymatic reactions, and enzymes are

proteins encoded by genes

image of

Testosterone

Synthesis by DNA

DNA replication- def on exam

dna is double stranded.

chromatin- very thin and wrapped around histones

first we must unwind from the histones.

now its in chromosome state

now chromsome unzips

creates a complementary pair (by adding complementary base pair)

4.3a DNA Replication1

Four steps of DNA replication: Definitely on exam

1. Unwinding the helix from the histones

2. Unzipping small portion of helix by DNA helicase enzyme;

separates the strands, forming a replication fork

3. Enzyme called DNA polymerase moves along each strand,

reads the exposed bases, and synthesizes complementary

strands. towards replication fork on one strand, and away on the other strand- On the discontinuous strand, segments(okazaki fragments) are connected by DNA ligase enzyme.

• Semiconservative replication—each new DNA molecule

contains old (parental) DNA and newly synthesized DNA

4. New histones synthesized in cytoplasm, transported into the nucleus- used to organize new DNA strands into nucleosomes.

image of semi-conservative DNA replication- this picture will be on exam

G0

(G zero) phase- on test

•

describes cells that have left the cycle and cease dividing

for a long time (or permanently)

Regulation of Cell Division 3

Checkpoints

within the cell cycle

• Controlled by protein called Cyclin–Cdk complexes- adds protein to other proteins thus inhibiting or activitating it. cyclin binds to CDK and triggers cascade of reactions that prepare cell for next steps.

• Start or G1 checkpoint

• Either allows the cell to proceed toward the S phase or, if it doesn’t,

the cell goes into the noncycling G0 phase

• G2 checkpoint

• Late in the G2 phase determines whether the cell is able to proceed

to mitosis

• A third M checkpoint at the transition from metaphase to

anaphase determines whether the cell can proceed to

anaphase, leading to separation of its sister chromatids

Homologous chromosomes

Two members of each pair of the 46 chromosomes•

1 chromosome from each pair inherited from each parent

• 22 pairs of autosomes

• Look alike and carry the same genes

• 1 pair of sex chromosomes

• Female has homologous pair of X chromosomes

• Male has one X and one much smaller Y chromosome

Alleles—

different forms of a particular gene

• Alleles are found at same locus on homologous chromosomes

4.4h Epigenetics

Epigenetic effects—

gene expression is altered without

changes in base sequence;

• Can be passed to offspring, called epigenetic inheritance

• Examples:

• DNA methylation: mechanism of epigenetic change in which methyl

groups (—CH3) are added to DNA

• Variations in packaging of chromatin

• Gene silencing by noncoding RNA (ncRNA)

• Many epigenetic changes are normal, but some may

trigger diseases such as cancer

Oncogenes

cause cell division to accelerate out of control

• Excessive production of growth factors or growth-factor

receptors that stimulate mitosis