Biology

DNARNAProteins

REVIEW! What is DNA?Deoxyribonucleic Acid (DNA)

Monomers made up of nucleotides:Nucleotides consist of:

A five carbon sugar, deoxyriboseo Four in it’s ring, one extending above the ringo Missing one oxygen when compared to ribose

Phosphate groupo Is the source of the “acid” in nucleic acid

Nitrogenous base (Adenine, Guanine, Cytosine, Thymine)o A ring consisting of nitrogen and carbon atoms with

various functional groups attached o Double ring= purines (A and G)o Single ring= pyrimidines (T and C)

Double helix consists of:Sugar-phosphate backbone held by covalent bondsNitrogen bases are hydrogen bonded together; A pairs with T

and C pairs with G

REVIEW! Nucleotides

Protein synthesis: overviewDNA inherited by an organism specifies traits

by dictating the synthesis of proteins.However, a gene does not build a protein

directly; it dispatches instruction in the form of RNA, which in turn programs protein synthesis.

Message from DNA in the nucleus of the cell is sent on RNA to protein synthesis in the cytoplasm.

Two main stages: TranscriptionTranslation

Protein Synthesis: OverviewTwo main stages:

TranscriptionThe transfer of genetic information from DNA into an

RNA moleculeOccurs in the eukaryotic cell nucleusRNA is transcribed from a template DNA strand

TranslationTransfer of the information in RNA into a protein.

TranscriptionDetails:

1. Initiation- Promoter is the nucleotide sequence on DNA that

marks where transcription of a gene begins and ends; “start” signal

Promoter serves as a specific binding site for RNA polymerase and determines which of the two strands of the DNA double helix is used as the template.

TranscriptionElongation-

RNA elongates As RNA synthesis continues, the RNA strand peels

away from its DNA template, allowing the two separated DNA strands to come back together in the region already transcribed.

Transcription3. Termination-

RNA polymerase reaches a sequence of bases in the DNA template called a terminator.

Signals the end of the gene; at that point, the polymerase molecule detaches from the RNA molecule and the gene.

mRNA (messenger RNA) or “transcript” exits the nucleus via the nuclear pores and enter the cytoplasm

Transcription animationhttp://www-class.unl.edu/biochem/gp2/

m_biology/animation/gene/gene_a2.html

RNA processingBefore mRNA leaves the nucleus, it is modified

or processed.1. addition of extra nucleotides to the ends of

the transcriptInclude addition of a small cap (a single G

nucleotide) at one end and a long tail (a chain of 50 to 250 A’s) at the other end

Cap and tail facilitate the export of the mRNA from the nucleus, protecting the transcript from attack by cellular enzymes, and help ribosomes bind to the mRNA

Cap and tail are NOT translated into protein.

http://vcell.ndsu.edu/animations/mrnaprocessing/movie.htm

RNA processing2. RNA splicing

Cutting-and-pasting process catalyzed by a complex of proteins and small RNA molecules, but sometime the RNA transcript itself catalyzes the process.

Introns“intervening sequences”; internal noncoding regionsGet removed from transcript before it leaves nucleus

ExonsCoding regions; parts of a gene that are expressed as amino

acidsJoined to produce an mRNA molecule with a continuous coding

sequenceCap and tail are considered parts of the first and last exons,

although are not translated into proteins.

http://student.ccbcmd.edu/biotutorials/protsyn/exon.html

RNA processing

More animationshttp://www.pbs.org/wgbh/aso/tryit/dna/prot

ein.htmlhttp://www.wisc-online.com/objects/

index_tj.asp?objID=AP1302

Translation- overviewA typical gene consists or hundreds or thousands

of nucleotides in a specific sequence, which get transcribed onto mRNA.

Translation is the conversion of nucleic acid language into polypeptide language

There are 20 different amino acids. A cell has a supply of amino acids in cytoplasm,

either obtained by food or made from other chemicals.

Flow of information from gene to protein is based on a triplet code: genetic instructions for the a.a. sequence of a polypeptide chain are written in DNA and mRNA as a series of three-base pairs, or codons.

Translation- tRNATo convert the codons of nucleic acids on

mRNA to the amino acids of proteins, a cell employs a molecular interpreter, called transfer RNA (tRNA)

tRNA molecules are responsible for matching amino acids to the appropriate codons to form the new polypeptide.

tRNA’s unique structure enables it to be able to: 1. pick up the appropriate amino acids 2. recognize the appropriate codons in the mRNA

Translation- tRNAat one end of the folded molecule contains

a special triplet of bases called an anticodon.Complementary to a codon triplet on mRNAAnticodon recognizes a particular codon

triplet on mRNAAt the other end of the tRNA molecule is a

site where an amino acid can attach.

Translation- tRNA

Translation- rRNA

Ribosomal RNA (rRNA)Organelle in the cytoplasm that coordinates the

functioning of mRNA and tRNA and actually makes polypeptides.

Consists of two subunits: large and small

TranslationInitiation-

mRNA arrives at the ribosomeTranslation begins at AUG, the start codon.Each transfer RNA has an anticodon whose

bases are complimentary to the bases of a codon on the mRNA strand.

TranslationElongation-

The ribosome moves along the mRNA binding new tRNA molecules and amino acids

Amino acids form peptide bonds

TranslationTermination

The polypeptide chain continues to grow until a “stop” codon on the mRNA molecule is reached.

The ribsome then releases the mRNA molecule and the newly formed polypeptide chain

Translation Animationhttp://www-class.unl.edu/biochem/gp2/

m_biology/animation/gene/gene_a3.html

PolysomeSeveral ribosomes can translate an mRNA at the same time, forming what is called a polysome.

Peptide Bond Formation

Free ribosomes vs. bound ribosomesFree ribosomes

Found in cytoplasmSynthesize proteins for use primarily within

the cellBound ribosomes

Found on rough ERSynthesize proteins primarily for secretion or

for lysosomes

Free ribosomes vs. bound ribosomes

After protein synthesis…Each polypeptide coils and folds, assuming

a 3-D shape, its tertiary structure.Several polypeptides may come together,

forming a protein with quaternary structure.Overall significance:

Process whereby genes control the structures and activities of cells

The way genotypes determine phenotypes; proteins made from the original DNA nucleotides determine the appearance and capabilities of the cell and organism!

MutationsMutation is any change in the nucleotide

sequence of DNA.Can involve large regions of a chromosome

or just a single nucleotide pair, as in sickle cell diseaseIn one of the two kinds of polypeptides in the

hemoglobin protein, the sickle-cell individual has a single different amino acid.

This small difference is caused by a change of a single nucleotide in the coding strand of DNA. Only ONE base pair!

Mutations on DNATwo general categories:

Base substitutionAlso known as a point mutationReplacement of one nucleotide with another.Depending on how the base substitution is translated, it can

result in no change in the protein (due to redundancy of genetic code), an insignficant change, or a change that significantly affects the individual.Occasionally, it leads to an improved protein that enhances

the success of the mutant organism and its descendants.More frequently, its harmful.

o May cause changes in protein that prevent it from functionally normally.

o If stop codon is a result of mutation and protein is shortened, it may not function at all.

Mutations on DNABase insertions or deletions

Also known as frameshift mutationOften has a disastrous effectAdding or subtracting nucleotides may result

in an alteration of the reading frame of the messageall the nucleotides that are “downstream” of the

insertion or deletion will be regrouped into different codons.

Result will most likely by a nonfunctional polypeptide

Mutations on DNAWhat causes mutations?

Mutagenesis, or the production of mutations, can occur in a number of ways.Spontaneous mutations: errors that occur during

DNA replication or recombination are called.Mutagen, a physical or chemical agent that causes

mutationsPhysical mutagen: high-energy radiation, such as

X-rays and UV lightChemical mutagen: consists of chemicals that

cause incorrect DNA bases pairs, such as asbestos.

Mutations on DNACan also be helpful both in nature and in

the laboratory.It is because of mutations that there is such

a rich diversity of genes in the living world, that make evolution by natural selection possible.

Also essential tools for geneticists.Whether naturally occurring or created in the

laboratory, mutations create the different alleles needed for genetic research.

Mutations- Chromosome Number

NondisjunctionMembers of a chromosome fail to separate.Can lead to an abnormal chromosome number

in any sexually reproducing diploid organism.For example, if there is nondisjunction affecting

human chromosome 21 during meiosis I, half the resulting gametes will carry an extra chromosome 21. Then, if one of these gametes unites with a normal

gamete, trisomy 21 (Down Syndrome) will result.

Mutations- Chromosome Number

Mutations- Chromosome Structure

Abnormalities in chromosome structure:Breakage of a chromosome can lead to a

variety of rearrangements affecting the genes of that chromosome:1. deletion: if a fragment of a chromosome is lost.

Usually cause serious physical and mental problems.Deletion of chromosome 5 causes cri du chat

syndrome: child is mentally retarded, has a small head with unusual facial features, and has a cry that sounds like the mewing of a distressed cats. Usually die in infancy or early childhood.

Mutations- Chromosome Structure

2.duplication: if a fragment from one chromosome joins to a sister chromatid or homologous chromosome.

3.inversion: if a fragment reattaches to the original chromosome but in the reverse direction.Less likely than deletions or duplications to produce

harmful effects, because all genes are still present in normal number

4. translocation: moves a segment from one chromosome to another nonhomologous chromosomeCrossing over between nonhomologous

chromosomes!

Mutations- Chromosome Structure

KaryotypeThe term karyotype refers to the chromosome

complement of a cell or a whole organism. A karyotype is an ordered display of magnified

images of an individual’s chromosomes arranged in pairs, starting with the longest.

In particular, it shows the number, size, and shape of the chromosomes as seen during metaphase of mitosis.

Chromosome numbers vary considerably among organisms and may differ between closely related species.

Karytype

Karyotypes are prepared from the nuclei of cultured white blood cells that are ‘frozen’ at the metaphase stage of mitosis. Shows the chromosomes condensed and doubled

A photograph of the chromosomes is then cut up and the chromosomes are rearranged on a grid so that the homologous pairs are placed together.

Homologous pairs are identified by their general shape, length, and the pattern of banding produced by a special staining technique.

Karyotype

Male karyotypeHas 44 autosomes, a single X chromosome, and

a Y chromosome (written as 44 + XY)Female karyotype

Shows two X chromosomes (written as 44 + XX)

Karyotype- Normal

Karyotype- Abnormal