cellular reproduction mrs. daniels december 2003

Cellular Reproduction

Mrs. Daniels

December 2003

Purpose of Cell DivisionPurpose of Cell Division

• The perpetuation of life depends on reproduction of cells or cell division

• The cell theory states that cells arise only from other living cells

• If cells cannot be manufactured, then cell division is the only means by which new cells can be made

• For unicellular organisms, the division of one cell to form two in fact reproduces an entire organism

• Ex. An amoeba

• For multicellular organisms, cell division allows growth and development from a fertilized egg (start with the joining of two cells and becomes many cells over time)

• Also, enables organisms to replace damaged or dead cells

Binary FissionBinary Fission

• Bacteria reproduce (divide) by a process known as binary fission

• Binary means 2 parts• Fission means to split• The process is literally the splitting of

an organism into two parts• Before that can happen, everything

must be copied in the cell.

Passing along genetic information

Passing along genetic information

• In order for any cell to reproduce, it must have a means by which it can pass along genetic information

• In order to maintain its own DNA, it must make a copy of its genetic material to give to the new cell.

• Genome: total collection of DNA unique to each species

• In binary fission, the chromosome is replicated and each copy stays attached to the plasma membrane on adjacent sites

• The membrane grows between the two sites• The bacterium grows to nearly twice its size

and the plasma membrane pinches inward• Finally, a cell wall forms across the bacterium

between the two chromosomes which ultimately divides the initial cell into two daughter cells

Eukaryotic cells are more complex

Eukaryotic cells are more complex

• There are tens or even hundreds of thousands of genes in eukaryotic organisms, so the genes have to be organized into multiple chromosomes

• The new organization helps to cut down on replication errors.

• (typically only 1 error per 100,000 cell divisions)• Think of it as putting files into folders or categories,

rather than having one big list of files (it is easier to find what you’re looking for)

DNA TerminologyDNA Terminology

• Chromosomes: threadlike structures in eukaryotic nuclei composed of DNA and proteins

• We have 23 pairs = 46 total• Chromatin: an uncoiled DNA-protein

complex• Chromatin coils up to form the

chromosomes

Figure 9-7Page 180Figure 9-7Page 180

Microtubules

Centromereregion

Sisterchromatids

Kinetochore

1.0µm

MitosisMitosis

• Eukaryotic organisms cannot simply divide as bacteria (prokaryotes) did by binary fission.

• We must use a process called mitosis which comes from the word mitos meaning thread

• Mitosis: process by which two identical daughter cells result from one original parent cell

• Specifically, it is the division of the nucleus

MitosisMitosis

• Before mitosis, a cell copies its genome by duplicating every chromosome.

• Each chromosome that has been copied remains attached to the original chromosome. The two pieces are called sister chromatids and are held together by a centromere

• The sister chromatids are pulled apart during mitosis forming two complete, identical sets of chromosomes.

MitosisMitosis

• Stages of mitosis: prophase, metaphase, anaphase, telophase

• Prior to mitosis is interphase• Following mitosis is cytokinesis:

cytoplasmic division that forms two separate daughter cells, which each contain their own nucleus.

Prophase of whitefish cell mitosis. LM X360.

Credit: © Carolina Biological/Visuals Unlimited 214212

Metaphase of whitefish cell mitosis. The metaphase stage follows prophase. During this time the chromosomes align at the metaphase plate. LM X360.


Anaphase of whitefish cell mitosis. During this stage the paired chromosomes separate and being to move to opposite ends of the cell. LM X360.


Telophase of whitefish cell mitosis. During telophase the chromosomes are barricaded off into two distinct nuclei in the emerging daughter cells. LM X360.


Ring ofcontractilemicrofilaments(actin andmyosinfilaments)

Cleavagefurrow

10µm(a)

Daughter cells created as a result of mitosis. LM X360.


Onion root tip mitosis - Interphase stage.


Onion root tip mitosis - Prophase stage.


Onion root tip mitosis - Metaphase stage.


Onion root tip mitosis - Early anaphase stage.


Onion root tip mitosis - Late anaphase stage.


Onion root tip mitosis - Telophase stage.


Eventually onelarge vesicle

exists

New plasmamembranes

(from vesiclemembranes)

Vesicles gatheron cell'smidplane

Cell plateforming

Plasmamembrane

Cellwall

Small vesiclesfuse, forming

larger vesicles

New cell walls(from vesicle

contents)

(b)

Onion root tip mitosis - Daughter cells.


Cell CycleCell Cycle

• The process is continuously repeating itself in a cycle…the CELL cycle

• Some cells complete one life cycle in an hour, others take more than 24 hours

• Some cells rarely or never divide once they are formed.

• Ex. Nerve and muscle cells

Cell cycleCell cycle

• M phase: this is the shortest phase in the cell cycle

• This is the phase in which division is actually occurring

• It includes mitosis and cytokinesis• Interphase: the non-dividing phase in

which the cell spends ~90% of its time.• The cell grows, copies its chromosomes,

and prepares for division

• Interphase consists of three periods:• 1. G1 phase- 1st growth phase• G stands for gap

• 2. S phase - synthesis phase when DNA is synthesized as chromosomes are duplicated

• S stands for synthesis

• 3. G2 phase - second growth phase

INTERPHASE

S(Synthesis phase)

G1

(First gap phase)

G2

(Second gapphase)

M PHASE (Mitosis and cytokinesis)

Controls for Cell DivisionControls for Cell Division

• Normal growth,development, and maintenance depend on timing and rate of mitosis

• Various cells divide at different rates and patterns:• Human skin = divide frequently• Liver cells = only when needed (wound repair)• Nerve, muscle, and other specialized cells = don’t

divide in mature humans


• Some factors influence cell division:• 1. Contents of cell medium• -if essential nutrients are lacking, the

cell will not divide• -if specific regulatory growth factors

are missing, the cell will not divide• Ex. Platelet-derived growth factor

(PDGF)


• 2. Cell density:• If there is a crowding of cells, the density-

dependent inhibition will kick in and stop division

• 3. G1 phase of the cell cycle:

• There is a restriction point which occurs late in the G1 phase of the cell cycle

• If the cell is destined or “supposed to” divide, it continues on…otherwise it is stopped (G0 phase)


• 4. Cell size: because of the size to volume ratio, the cell must have adequate volume of cytoplasm enabling it to attain a certain size before DNA synthesis can occur

• -this helps the daughter cells to maintain a stable cell size and not get progressively smaller with each division

• -if there is not enough cytoplasm, the cell will not pass the restriction point

Mitotic ClockMitotic Clock

• Once the cell passes the restriction point, it is destined to divide

• The cell will begin the S phase and then it must continue through the G2 and M phases

• Research is still being conducted on how the switches that control the exact sequence of events in cell division occur

• We do know that each step must be completed before progressing to the next step


• Regulatory proteins help keep everything in synch.

• Some of these proteins are protein kinases which

• Catalyze the transfer of phosphate groups• Kinase activity is in turn controlled by proteins

called cyclins• The cyclin attaches to the cyclin-dependent

kinase and activates it when necessary


• Cyclin levels (concentrations) vary throughout the cell cycle

• They are very high during mitosis and very low during the rest of the cell cycle

• Excess cyclin is destroyed following mitosis since it is not needed.

• The levels begin to grow again shortly before mitosis.

CancerCancer

• Cancer cells do not respond normally to controls on cell division.

• They divide excessively, invade other tissues, and if unchecked can kill the entire organism

CancerCancer

• Cancer cells in culture do NOT stop growing in response to cell density

• They grow until the nutrients are exhausted• Cancer cells that stop dividing do so at random

and not at the restriction point• Normal cells only divide ~20-50 times and then

stop• Cancer cells continue dividing indefinitely until

the nutrients are gone

• Once a normal cell has moved beyond the normal controls of the cell cycle, it is said to be transformed

• Transformation: the conversion of a euk cell in tissue culture to a condition of unregulated growth

• Tumors : unregulated growing mass of cells within otherwise normal tissue

• Tumors can be benign or malignant• Benign: remain uninvasive of normal tissues• Malignant: have the ability to spread to

other body parts• Malignant cells are also abnormal in other

ways: unusual # of chromosomes, strange metabolism, loss of attachments or junctions to neighboring cells

• The spreading of cancer beyond the original site is referred to as metastasis

DNA is the genetic material of cells

DNA is the genetic material of cells• It was once believed that proteins

were what carried genetic information• Evidence arose that proved this

hypothesis wrong and deduced that DNA was actually the carrier of genetic information

• Evidence included:• Griffith’s experiments• The bacteriophage experiments• Circumstantial evidence:

• DNA is doubled prior to mitosis• During mitosis, the doubled DNA is equally

divided into two daughter cells• An organism’s diploid cells have twice the

DNA as its haploid gametes

• Paper chromatography of nitrogenous bases indicated base ratios

DNA ReplicationDNA Replication• Two DNA strands separate• Each strand is a template for

assembling a complementary strand• Nucleotides line up singly along the

template strand in accordance with the base-pairing rules.

• Enzymes link the nucleotides together at their sugar-phosphate groups.

• Is a semi-conservative process.

Mechanism of DNA Replication

Mechanism of DNA Replication

• The helical molecule must untwist while it copies its two antiparallel strands simultaneously (requires dozens of enzymes and other proteins)

• It occurs very rapidly. It takes only a few hours to copy the 6 billion bases of a single human cell.

• Only one in a billion nucleotides is incorrectly paired.

Replication the ProcessReplication the Process

• There is a specific sequence of nucleotides which indicates the site called the “origin of replication”

• In order to initiate replication, specific proteins bind to the origin

• The double helix opens at the origin & replication forks spread in both directions

• There is only one origin in bacterial or viral DNA, but hundreds or thousands in euks

• 1. Strand separation-• Helicase enzymes are responsible for

unwinding the DNA.• Single-stranded binding proteins keep

the strands apart and stabilize the unwound DNA until new strands can be made

• 2. Synthesis of the New DNA Strands:• -enzymes called DNA polymerases

catalyze synthesis of a new DNA strand• New nucleotides align themselves along

the templates of the old DNA strands in accordance with base-pairing rules

• DNA polymerase actually links the nucleotides to the growing strand.

• They can only grow in the 5’ --> 3’ direction since new nucleotides are added only to the 3’ end of the growing strand

• Hydrolysis of ATP and GTP provides energy necessary to synthesize the new DNA strands

• Continuous synthesis of both DNA strands at a replication fork is not possible because:

• -the sugar phosphate backbones are anti-parallel

• -the polarity of DNA (3’ end has a hydroxyl group and the 5’ end has a phosphate)

• -DNA polymerase can only elongate strands in the 5’ to 3’ direction

• How do we solve the problem of not having continuous synthesis?

• Leading strand and lagging strands• Leading strand: the DNA strand which

is synthesized as a single polymer in the 5’ to 3’ direction.

• Lagging strand: the DNA strand that is discontinuously synthesized against the overall direction of replication

Lagging Strand Replication

Lagging Strand Replication

• the lagging strand is produced as a series of short segments called Okazaki fragments which are synthesized in the 5’ to 3’ direction

• The many fragments are ligated by DNA ligase, a linking enzyme that catalyzes the formation of a covalent bond between the 3’ end of each new O. fragment to the 5’ end of the growing chain

• 3. Priming DNA Synthesis:• -before new DNA strands can form, there

must be small pre-existing primers to start the addition of new nucleotides

• Primer: short RNA segment that is complementary to a DNA segment (necessary to start replication); is ~ 10 bases long in humans

• -a primer is necessary because DNA polymerase can only add nucleotides to a polynucleotide that is already correctly base-paired with a complementary strand

• Only one primer is necessary for the leading strand, but many primers are required to replicate the lagging strand

• -an RNA primer must initiate the synthesis of each Okazaki fragment

• -the many Okazaki fragments are ligated (linked) in two steps to produce a continuous DNA strand:

• 1. DNA polymerase removes the RNA primer and replaces it with DNA

• 2. DNA ligase catalyzes the linkage between the 3’ end of each new Okazaki fragment to the 5’ end of the growing chain.

Enzymes Proofread DNAEnzymes Proofread DNA

• -initial pairing errors occur at a rate of 1 in 10,000, but errors in a complete DNA molecule are only about 1 in 1,000,000,000.

• -the great improvement is due to enzymes which proofread and correct mistakes

• Mismatch repair and Excision repair

• Mismatch repair: corrects mistakes while DNA is being synthesized

• polymerase (and other proteins in eukaryotes) detects an incorrectly paired nucleotide, removes the incorrect one, and replaces it with the correct one before continuing with synthesis

• Excision repair: corrects accidental changes in existing DNA

• More than 50 types of DNA repair enzymes• One repair enzyme (of the 50) will remove

incorrect nucleotides and the gap is filled in by DNA polymerase and DNA ligase

Transcription & Translation

Transcription & Translation

• RNA links DNA’s genetic instructions for making proteins to the process of protein synthesis

• It copies or “transcribes” the message from DNA in the nucleus, transfers it to the cytoplasm, and interprets or “translates” it into a protein

What protein will be made?

What protein will be made?

• The linear sequence of nucleotides in the DNA ultimately determines the linear sequence of amino acids that will make up a particular protein

• -remember: proteins are made up of a varying combination of the 20 amino acids that are linked together by peptide bonds

DefinitionsDefinitions• Transcription: the synthesis of RNA using DNA as

a template• -a portion of the DNA is unwound, a segment of it

is copied into mRNA, the mRNA leaves the nucleus and the DNA recoils

• Translation: Synthesis of a polypeptide, which occurs under the direction of the mRNA

• -translation occurs on the ribosomes (made up of rRNA) which match the codons on the mRNA with the anticodons on the tRNA to put the amino acids in the correct sequence

The codeThe code

• The flow of information from a gene (DNA) to a protein is based on a triplet code

• The three-nucleotide “words” found on the mRNA are called codons

• Each codon matches up with a complementary sequence on the tRNA, which holds the accompanying amino acid

Where does it occur?Where does it occur?

• We already know that transcription occurs in the nucleus, but where in the long DNA chain will transcription occur?

• Specific DNA nucleotide sequences mark where transcription of a gene will begin (initiation) and where it will end (termination)

• Between these two points is the “transcription unit”

Steps to TranscriptionSteps to Transcription

• Transcription occurs in three main stages:

• 1. Polymerase binding and initiation• 2. Elongation of the mRNA strand• 3. Termination

1. Polymerase binding & initiation

1. Polymerase binding & initiation• RNA polymerases bind to DNA at regions

called promoters• (in euks, this promoter region is ~100

nucleotides long)• Transcription factors, such as a TATA box

upstream of the promoter, are DNA-binding proteins which bind to the DNA near the promoter so that the RNA polymerases can recognize where to begin

2. Elongation of the mRNA strand

2. Elongation of the mRNA strand

• Once initiation of transcription occurs, RNA polymerase moves along the DNA doing two things:

• -untwists and opens a short (~10 bases) segment of DNA to be a template for the RNA nucleotides

• -links incoming RNA nucleotides to the 3’ end of the elongating strand of mRNA

• !!! Notice it happens in the 5’ to 3’ direction!!!

3. Termination of Transcription

3. Termination of Transcription

• Transcription proceeds until RNA polymerase reaches a “termination site” on the DNA

• Terminator sequence: DNA sequence that signals RNA polymerase

After TranscriptionAfter Transcription

• mRNA is ready for immediate translation in the cytoplasm, except in eukaryotes

• Before a eukaryotic mRNA is exported from the nucleus, it is processed in 2 ways:

• -both ends are covalently altered• -introns are removed and spliceosomes

splice the remaining mRNA together

Alteration of the mRNA ends

Alteration of the mRNA ends

• During mRNA processing, both the 5’ and the 3’ ends are altered

• 1. the 5’ end is capped with a leader sequence (a non-coding, untranslated sequence of mRNA) which helps with ribosomal recognition

• 2. A trailer sequence (again untranlated) and a sequence of ~200 adenine nucleotides are added to the 3’ end of the mRNA

• -helps with export of mRNA from nucleus to cytoplasm (keeps it from degrading)

Meiosis & Sexual Reproduction…the connection

• Heredity: Continuity of biological traits from one generation to the next, which results from the transmission of genes from parents to offspring

• Variation: Inherited differences among individuals of the same species

• Genetics: The scientific study of heredity and variation.

Chromosomes and inheritance• Genes: units of hereditary information that are

made of DNA and are located on chromosomes• Locus: specific location on a chromosome that

contains a gene• They have a specific sequence of nucleotides,

which code for the production of specific proteins• The action of these proteins results in the

organism’s inherited traits

• Inheritance is possible because

• -DNA is replicated with with precision and is passed along from parent to offspring

• -sperm and ovum (egg) carrying each parent’s genes are combined in the nucleus of the fertilized egg

• -depends upon the behavior of the chromosomes

Asexual vs. Sexual Reproduction

• Single parent• One parent passes on all

its genes to offspring• Offspring are genetically

identical to the parent

• Results in a “clone” with rare genetic differences from mutations

• Two parents• Each parent passes on

half its genes to offspring• Offspring have unique

combination of genes from both parents

• Results in greater genetic variation

• The human life cycle follows the same basic pattern found in all sexually reproducing organisms

• Meiosis and fertilization result in alternation between the haploid and diploid condition

• Haploid: cells contain one set of chromosomes (in gametes; n)

• Diploid: cells contain two sets of chromosomes (2n)

Alternation of Generations

Figure 9-16aPage 190

Multicellular diploidorganism (2n)

Gametes (n)

Fertilization

Zygote (2n)

Meiosis

Mitosis

(a) Animals

Life cycle vocabulary• Life cycle: sequence of stages in an organism’s

reproductive history, from conception to production of its own offspring

• Somatic cell: any cell other than a gametes

• -human somatic cells contain 46 chromosomes distinguishable by differences in size, position of the centromere, and staining or banding pattern

• -chromosomes can be matched into homologous pairs and arranged into a karyotype

• Karyotype: a display or photomicrograph of an individual’s somatic-cell metaphase chromosomes that are arranged in a standard sequence

• -often made with lymphocytes in humans

• -can be used to screen for chromosomal abnormalities

• Homologous chromosomes: a pair of chromosomes that have the same size, centromere position and staining pattern

• -homologues carry the same genetic loci, with the exception of sex chromosomes

• Autosomes: chromosomes that are not sex chromosomes

• Sex chromosomes:

• XX combination for females

• XY combination for males

• Humans have 22 pairs of autosomes and 1 pair of sex chromosomes

• One homologue is inherited from each parent

• Zygote: a diploid cell that results from the union of two haploid gametes (2n)

• -contains the maternal and paternal haploid sets of chromosomes (from the gametes)

• -as a human develops from a zygote to a sexually mature adult, the zygote’s genetic information is passed with precision to all somatic cells by mitosis

• Gametes are the only cells in the body NOT produced by mitosis…they are produced by the process of meiosis

Meiosis• Like mitosis, meiosis is preceded by replication

of chromosomes

• Meiosis differs from mitosis in that this single replication is followed by two consective cell divisions: meiosis I and II

• These cell divisions produce 4 daughter cells, which each contain half the number of chromosomes as the original cells

Figure 9-12(1)Page 186

MEIOSIS ISisterchromatids

Homologouschromosomes

INTERPHASEInterphase preceding meiosis;DNA replicates.

PROPHASE IHomologous chromosomes synapse,forming tetrads; nuclear envelopebreaks down.

METAPHASE ITetrads line up on cell's midplane.Tetrads held together at chiasmata(sites of prior crossing-over).

ANAPHASE IHomologous chromosomes separateand move to opposite poles. Note thatsister chromatids remain attached attheir centromeres.

TELOPHASE IOne of each pair ofhomologous chromosomesis at each pole. Cytokinesisoccurs.

Figure 9-12(2)Page 186

MEIOSIS II

PROPHASE IIChromosomes condense againfollowing a brief period of interkinesis.DNA does not replicate again.

METAPHASE IIChromosomes line up alongcell's midplane.

ANAPHASE IISister chromatids separate, andchromosomes move to oppositepoles.

TELOPHASE IINuclei formed at opposite poles ofeach cell. Cytokinesis occurs.

HAPLOID CELLSFour gametes (animal)or four spores (plant)are produced.

• MEIOSIS I

• Synapsis occurs to form tetrads. Chiasmata appear as evidence that crossing over has occurred

• Tetrads align on the metaphase plate

• Pairs of homologues separate…not sister chromosomes

• MITOSIS

• Neither synapsis nor crossing over occurs

• Individual chromosomes align on metaphase plate

• Centromeres divide and sister chromatids separate

Prophase

Metaphase

Anaphase

Figure 9-14Page 188

Chiasmata

Kinetochores

Sisterchromatids

Sisterchromatids

(a)

Chiasmata

Kinetochores

Sisterchromatids

(b)

cellular reproduction mrs. daniels december 2003

Documents

initial cell

cell divisionsthink

new cells

cell wall forms

reproduction of cells

identical daughter cells

daughter cellseukaryotic

separate daughter cells