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The CellularLevel of

Organization3.6-3.9August 24, 2012

ReviewWhich organelles are surrounded by a membrane

and which are not?

How do large particles enter and exit the nucleus?

How is DNA packed in the nucleus?

3.6 Protein SynthesisObjectives

Describe the sequence of events in proteinsynthesis

Transcription & TranslationCells make proteins by transcribing and

translating the genetic information contained inDNA

The genetic code is a set of rule that relatesthe base triplet sequences of DNA to thecorresponding codons of RNA and the aminoacids they specific

DNA

RNA

Protein

In transcription, the genetic information in thesequence of base triplets in DNA serves as atemplate for copying the information into acomplementary sequence of codons for mRNA

Transcription begins on DNA in a region calledthe promoter

Regions of DNA that code for proteinsynthesis are called exons

Regions that do not are called introns

Newly synthesized pre-mRNA is modified beforeleaving the nucleus

In translation, the nucleotide sequence of mRNAspecifies the amino acid sequence of a protein

The mRNA binds to a ribosome, specific aminoacids attach to tRNA, and anticodons of tRNAbind to codons of mRNA, bring specific aminoacids into position on a growing polypeptide

Translation begins at the start codon and endsat the stop codon

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3.7 Cell DivisionObjectives

Discuss the stages, events, and significance ofsomatic and reproductive cell division

Describe the signals that induce somatic celldivision

Somatic Cell DivisionCell division is the process by which cells reproduce

themselvesConsists of nuclear division (mitosis or meiosis)

and cytoplasmic division (cytokinesis)Cell division that replaces cells or adds new ones is

called somatic cell division and involves mitosis andcytokinesis

Cell division that results in the production of gametes(sperm and ova) is called reproductive cell divisionand involves meiosis and cytokinesis

The cell cycle is an orderly sequence of events inwhich a somatic cell duplicates its contents and dividesin twoConsists of interphase and a mitotic phase

Human somatic cells contain 23 pairs of homologouschromosomesDiploid = 2n

Before the mitotic phase, the DNA molecules, orchromosomes, replicate themselves so that identicalsets of chromosomes can be passed on to the nextgeneration of cells

Interphase is when the cell is not dividing

The cell replicates its DNA

Consists of three phases, G1, S, and G2

Replication of DNA occurs in the S phase

During the G2 phase, enzymes and otherproteins are synthesized and centrosomereplication is completed

The mitotic phaseco nsists of a nuclear division(mitosis) and a cytoplasmic division (cytokinesis)to form two identical cells

MitosisMitosis is splitting the chromosomes and distributing two

identical sets of chromosomes into separate and equal nuclei In prophase the chromatin fibers change into chromosomes In metaphase the microtubules align the centromeres of the

chromatid pairs at the metaphase plate In anaphase the chromatid pairs split at the centromere and

move to opposite poles of the cell; the chromatids are nowcalled chromosomes

In telophase the two identical nuclei are formed around theidentical sets of chromosomes now in their chromatin form

CytokinesisDivision of a cell’s cytoplasm to form two

identical cellsUsually begins in late anaphaseThe plasma membrane constricts at its middle,

forming a cleavage furrowThe cell eventually splits into two daughter cells.Interphase begins when cytokinesis is complete

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EarlyLate(d) ANAPHASE

Pericentriolar material

NucleolusNuclear envelopeChromatinPlasma membraneCytosol

(a) INTERPHASE

CentriolesCentrosome:

(f) IDENTICAL CELLS IN INTERPHASE

Cleavage furrow

(e) TELOPHASE

(c) METAPHASE

Cleavage furrow

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5

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LateEarly (b) PROPHASE

Fragments ofnuclear envelope

Mitotic spindle(microtubules)

Kinetochore

Metaphase plate

Chromosome

all at 700xLM

Centromere

Chromosome(two chromatidsjoined atcentromere

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A cell can either remain alive and functioning withoutdividing, grow and divide, or die

The control of cell division depends on specific cyclin-dependent protein kinases and cyclins

Apoptosis is normal, programmed cell deathIt first occurs during embryological development

and continues throughout the lifetime of anorganism

Certain genes regulate both cell division andapoptosisAbnormalities in these genes are associated with

a wide variety of diseases and disorders

Reproductive Cell DivisionDuring sexual reproduction, each new organism is

the result of the union of two gametes (fertilization),one from each parent

Meiosis is reproductive cell division that occurs in thegonads (ovaries and testes) that produces gameteswith half the number of chromosomesHaploid cells are gametes that contain a single set

of 23 chromosomesFertilization restores the diploid number of

chromosomes (46)

Meiosis occurs in two successive stages: meiosis Iand meiosis II

Each of these two stages has 4 phases: prophase,metaphase, anaphase, and telophase

Meiosis I begins with a diploid cell and ends withtwo cells having the haploid number ofchromosomes

In Meiosis II, each of the two haploid cells divides,and the net result is four haploid gametes that aregenetically different from the original diploidstarting cell

3.8 Cellular DiversityObjectives

Describe how cells differ in size and shape

The average adult has nearly100 trillion cells

There are about 200 differenttypes of cells

Cells come in a variety ofshapes and sizes

Shape is related tofunction

Cellular diversity permitsorganization of cells intomore complex tissues andorgans

3.9 Aging & CellsObjectives

Describe the cellular changes that occur withaging

Aging is a normal process accompanied byprogressive alteration of the body’shomeostatic adaptive responses

Aging produces observable changes instructure and function and increasesvulnerability to environmental stress anddisease

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Telomeres, specific DNA sequences found only atthe tips of each chromosome, protect them fromerosion and from sticking to one anotherThe telomeres shorten after each cycle of cell

divisionOver time the telomeres may be completely

gone, as well as some of the functionalchromosomal material, contributing to aging andthe death of cell

It has been found that individuals who experiencehigh levels of stress have significantly shortertelomere length

Many theories of aging have been proposed,including genetically programmed cessation ofcell division, buildup of free radicals, and anintensified autoimmune response

Clinical Connection -A Focus on Cancer

Cancer is a group of diseases characterized byuncontrolled or abnormal cell divisionWhen cells divide without control, the excess

tissue that develops is called a tumor (neoplasm)Malignant tumors are cancerous and are able to

metastasizeBenign tumors do not metastasize

Types of Cancer• The name of the cancer is derived from the type of

tissue in which it develops

• Carcinoma - arise from epithelial cells

• Melanoma - cancerous growth of melanocytes

• Sarcoma - arising from muscle cells andconnective tissue

• Leukemia - cancer of blood-forming organscharacterized by rapid growth of abnormalleukocytes

• Lymphoma - disease of lymphatic tissue

Growth & Spread of Cancer Cells of malignant tumors duplicate rapidly and

continuously As they invade tissues, they often trigger angiogensis, the

growth of new networks of blood vessels As the cancer grows, it begins to compete with normal

tissues for space and nutrientsThe normal tissue decreases in size and dies

Some malignant cells may invade body cavities or enter theblood or lymph, then circulate and establish secondarytumors

The pain associated with cancer develops when tumorspress on nerves or blocks passageways so that secretionsbuild up pressure

Causes of CancerEnvironmental agents such as carcinogens induce

mutations

Oncogenes can be inappropriately activated

Some cancers have viral origins and are calledoncogenic viruses, that can cause cancer bystimulating abnormal proliferation of cells

Inflammation

Carcinogenesis: A Multistep ProcessAs much as 10 distinct mutations may have to

accumulate in a cell before it becomes cancerousThe tumor begins as an area of increased cell

proliferation that results from one mutationThe growth progresses to abnormal, but

noncancerous growths called adenomasAfter two or three additional mutations, the

mutation of the tumor suppressor gene p53occurs and the carcinoma develops

Treatment of CancerSurgeryChemotherapy involves the use of drugs to cause

the death of cancer cellsRadiation breaks chromosomes, thus blocking cell

divisionVirotherapy is a new technique that uses viruses

to kill cancersThe diversity of cancer cells makes its difficult to

treatResearchers are investigating the role of

metastasis regulatory genes

Case Study