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PowerPoint® Lecture Slides prepared by Janice Meeking, Mount Royal College

C H A P T E R

Copyright © 2010 Pearson Education, Inc.

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Cells: The Living Units: Part D


Cell Cycle

• Defines changes from formation of the cell until it reproduces

• Includes:

• Interphase

• Cell division (mitotic phase)


Interphase

• Period from cell formation to cell division

• Nuclear material called chromatin

• Four subphases:

• G1 (gap 1)—vigorous growth and metabolism

• G0—gap phase in cells that permanently cease dividing

• S (synthetic)—DNA replication

• G2 (gap 2)—preparation for division

Copyright © 2010 Pearson Education, Inc. Figure 3.31

G1

Growth

SGrowth and DNA

synthesis G2

Growth and finalpreparations for

divisionM

G2 checkpoint

G1 checkpoint(restriction point)


Centrosomes(each has 2centrioles)

Nucleolus

Interphase

Plasmamembrane

Nuclearenvelope

Chromatin

Interphase


DNA Replication

• DNA helices begin unwinding from the nucleosomes

• Helicase untwists the double helix and exposes complementary chains

• The Y-shaped site of replication is the replication fork

• Each nucleotide strand serves as a template for building a new complementary strand


DNA Replication

• DNA polymerase only works in one direction

• Continuous leading strand is synthesized

• Discontinuous lagging strand is synthesized in segments

• DNA ligase splices together short segments of discontinuous strand


DNA Replication

• End result: two DNA molecules formed from the original

• This process is called semiconservative replication


AdenineThymineCytosineGuanine Old (template) strand

Two new strands (leading and lagging)synthesized in opposite directions

DNA polymerase

DNA polymerase

Laggingstrand

Leading strand

Free nucleotides

Old strand acts as atemplate for synthesisof new strandChromosome

Helicase unwindsthe double helix andexposes the bases

Old DNA

Replicationfork


DNA Replication

PLAYPLAY Animation: DNA Replication


Cell Division

• Mitotic (M) phase of the cell cycle

• Essential for body growth and tissue repair

• Does not occur in most mature cells of nervous tissue, skeletal muscle, and cardiac muscle


Cell Division

• Includes two distinct events:

1. Mitosis—four stages of nuclear division:

• Prophase

• Metaphase

• Anaphase

• Telophase

2. Cytokinesis—division of cytoplasm by cleavage furrow


G1

Growth

SGrowth and DNA

synthesis G2

Growth and finalpreparations for

divisionM

G2 checkpoint

G1 checkpoint(restriction point)


Cell Division

PLAYPLAY Animation: Mitosis


Prophase

• Chromosomes become visible, each with two chromatids joined at a centromere

• Centrosomes separate and migrate toward opposite poles

• Mitotic spindles and asters form


Prophase

• Nuclear envelope fragments

• Kinetochore microtubules attach to kinetochore of centromeres and draw them toward the equator of the cell

• Polar microtubules assist in forcing the poles apart


Early mitoticspindle

Early Prophase

Centromere

Aster

Chromosomeconsisting of twosister chromatids

Early Prophase


Spindle pole

Kinetochore Kinetochoremicrotubule

Polar microtubule

Late Prophase

Fragmentsof nuclearenvelope

Late Prophase


Metaphase

• Centromeres of chromosomes are aligned at the equator

• This plane midway between the poles is called the metaphase plate


Spindle

MetaphaseplateMetaphase

Metaphase


Anaphase

• Shortest phase

• Centromeres of chromosomes split simultaneously—each chromatid now becomes a chromosome

• Chromosomes (V shaped) are pulled toward poles by motor proteins of kinetochores

• Polar microtubules continue forcing the poles apart


Anaphase

Daughterchromosomes

Anaphase


Telophase

• Begins when chromosome movement stops

• The two sets of chromosomes uncoil to form chromatin

• New nuclear membrane forms around each chromatin mass

• Nucleoli reappear

• Spindle disappears


Cytokinesis

• Begins during late anaphase

• Ring of actin microfilaments contracts to form a cleavage furrow

• Two daughter cells are pinched apart, each containing a nucleus identical to the original


Contractilering atcleavagefurrow

Nuclearenvelopeforming

Nucleolusforming

Telophase

Telophase and Cytokinesis


Control of Cell Division

• “Go” signals:

• Critical volume of cell when area of membrane is inadequate for exchange

• Chemicals (e.g., growth factors, hormones, cyclins, and cyclin-dependent kinases (Cdks))


Control of Cell Division

• “Stop” signals:

• Contact inhibition

• Growth-inhibiting factors produced by repressor genes


Protein Synthesis

• DNA is the master blueprint for protein synthesis

• Gene: Segment of DNA with blueprint for one polypeptide

• Triplets of nucleotide bases form genetic library

• Each triplet specifies coding for an amino acid

PLAYPLAY Animation: DNA and RNA


Nuclearpores

mRNA

Pre-mRNARNA Processing

Transcription

Translation

DNA

Nuclearenvelope

Ribosome

Polypeptide


Roles of the Three Main Types of RNA

• Messenger RNA (mRNA)

• Carries instructions for building a polypeptide, from gene in DNA to ribosomes in cytoplasm



• Ribosomal RNA (rRNA)

• A structural component of ribosomes that, along with tRNA, helps translate message from mRNA



• Transfer RNAs (tRNAs)

• Bind to amino acids and pair with bases of codons of mRNA at ribosome to begin process of protein synthesis


Transcription

• Transfers DNA gene base sequence to a complementary base sequence of an mRNA

• Transcription factor

• Loosens histones from DNA in area to be transcribed

• Binds to promoter, a DNA sequence specifying start site of gene to be transcribed

• Mediates the binding of RNA polymerase to promoter


Transcription

• RNA polymerase

• Enzyme that oversees synthesis of mRNA

• Unwinds DNA template

• Adds complementary RNA nucleotides on DNA template and joins them together

• Stops when it reaches termination signal

• mRNA pulls off the DNA template, is further processed by enzymes, and enters cytosol


RNA polymerase

RNA polymerase

RNApolymerase

DNA

Coding strand

Template strandPromoterregion

Terminationsignal

mRNA

mRNA

Template strand

mRNA transcript

Completed mRNA transcript

Rewindingof DNA

Coding strand of DNA

DNA-RNA hybrid region

The DNA-RNA hybrid: At any given moment, 16–18 base pairs ofDNA are unwound and the most recently made RNA is still bound toDNA. This small region is called the DNA-RNA hybrid.

Templatestrand

Unwindingof DNA

RNA nucleotides

Direction oftranscription

Initiation: With the help of transcription factors, RNApolymerase binds to the promoter, pries apart the two DNA strands,and initiates mRNA synthesis at the start point on the template strand.

Termination: mRNA synthesis ends when the termination signalis reached. RNA polymerase and the completed mRNA transcript arereleased.

Elongation: As the RNA polymerase moves along the templatestrand, elongating the mRNA transcript one base at a time, it unwindsthe DNA double helix before it and rewinds the double helix behind it.

1

2

3


mRNA Template strand

mRNA transcript


2


RNApolymerase



3


RNApolymerase

mRNA

Rewindingof DNA



The DNA-RNA hybrid: At any given moment, 16–18 base pairsof DNA are unwound and the most recently made RNA is stillbound to DNA. This small region is called the DNA-RNA hybrid.

Templatestrand

Unwindingof DNA

RNA nucleotidesDirection oftranscription


RNA polymerase

RNA polymerase

RNApolymerase

DNA

Coding strand


Terminationsignal

mRNA

mRNA

Template strand

mRNA transcript


Rewindingof DNA



The DNA-RNA hybrid: At any given moment, 16–18 base pairs ofDNA are unwound and the most recently made RNA is still bound toDNA. This small region is called the DNA-RNA hybrid.

Templatestrand

Unwindingof DNA

RNA nucleotides

Direction oftranscription




1

2

3


Translation

• Converts base sequence of nucleic acids into the amino acid sequence of proteins

• Involves mRNAs, tRNAs, and rRNAs


Genetic Code

• Each three-base sequence on DNA is represented by a codon

• Codon—complementary three-base sequence on mRNA


SECOND BASE

UUG

UUA

UUC

UUUPhe

Leu

CUG

CUA

CUC

CUU

Leu

AUA

AUC

AUU

Ile

GUG

GUA

GUC

GUU

Val

UCG

UCA

UCC

UCU

Ser

CCG

CCA

CCC

CCU

Pro

ACG

ACA

ACC

ACU

Thr

GCG

GCA

GCC

GCU

Ala

UAC

UAUTyr

CAG

CAA

CAC

CAUHis

Gln

AAG

AAA

AAC

AAUAsn

Lys

GAG

GAA

GAC

GAUAsp

Glu

UGC

UGUCys

Trp

CGG

CGA

CGC

CGU

Arg

AGG

AGA

AGC

AGUSer

Arg

GGG

GGA

GGC

GGU

Gly

UAA Stop UGA Stop

AUGMet orStart

UAG Stop UGG

U C A G

G

A

C

U

G

A

C

U

G

A

C

U

G

A

C

U

U

C

A

G


Translation

• mRNA attaches to a small ribosomal subunit that moves along the mRNA to the start codon

• Large ribosomal unit attaches, forming a functional ribosome

• Anticodon of a tRNA binds to its complementary codon and adds its amino acid to the forming protein chain

• New amino acids are added by other tRNAs as ribosome moves along rRNA, until stop codon is reached


1

2

3

4

Leu

Leu

Energized by ATP, the correct aminoacid is attached to each species oftRNA by aminoacyl-tRNA synthetaseenzyme.

Amino acid

tRNA

Aminoacyl-tRNAsynthetase

G A A

tRNA “head”bearinganticodon

Psite A

siteE

site

Ile

Pro

A AU U UC C C

CG G

G

Largeribosomalsubunit

Smallribosomalsubunit

Direction ofribosome advancePortion of mRNA

already translated

Codon15

Codon16

Codon17

Nucleus

mRNA

Released mRNA

Nuclearmembrane

Nuclear pore

RNA polymerase

Templatestrand ofDNA

After mRNA synthesis in thenucleus, mRNA leaves the nucleusand attaches to a ribosome.

Translation begins as incomingaminoacyl-tRNA recognizes thecomplementary codon calling forit at the A site on the ribosome. Ithydrogen-bonds to the codon viaits anticodon.

As the ribosome moves alongthe mRNA, and each codon isread in sequence, a new aminoacid is added to the growingprotein chain and the tRNA inthe A site is translocated to theP site.

Once its amino acid is releasedfrom the P site, tRNA is ratchetedto the E site and then released toreenter the cytoplasmic pool,ready to be recharged with a newamino acid. The polypeptide isreleased when the stop codon isread.

GA A

U

UA


Developmental Aspects of Cells

• All cells of the body contain the same DNA but are not identical

• Chemical signals in the embryo channel cells into specific developmental pathways by turning some genes off

• Development of specific and distinctive features in cells is called cell differentiation

• Elimination of excess, injured, or aged cells occurs through programmed rapid cell death (apoptosis) followed by phagocytosis


Theories of Cell Aging

•Wear and tear theory: Little chemical insults and free radicals have cumulative effects

• Immune system disorders: Autoimmune responses and progressive weakening of the immune response

• Genetic theory: Cessation of mitosis and cell aging are programmed into genes. Telomeres (strings of nucleotides on the ends of chromosomes) may determine the number of times a cell can divide.

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