Ch. 3: Cells the Living Units (pt 4)

Cell Cycle

• Interphase: cells grow and carry out functions

• Cell Division (mitotic phase): divide into 2 cells

Interphase

• period from cell formation to cell division

• G1 (gap 1) - vigorous growth and metabolism

  • G0 : cells that permanently cease dividing

• S (synthetic) - DNA replication

• G2 (gap 2) - preparation for division

DNA replication

• cells make copy of DNA prior to division

• DNA helices separated into replication bubbles with replication forks at each end

  • strand acts as template for complementary strand

• DNA polymerase begins adding nucleotides at RNA primer

• two identical DNA molecules formed at end

• semiconservative process

Cell Division

• meiosis: produce gametes

• mitotic cell division: produce identical daughter cells

  • essential for tissue growth and repair

    • occurs continuously in some cells (skin, intestinal lining)

  • none in most mature cells of nervous, skeletal and cardiac muscle/tissue (repairs w fibrous tissue)

Events of Mitosis

prophase, metaphase, anaphase, telophase

Cytokinesis

division of cytoplasm by cleavage furrow

Early Prophase

• chromosomes become visible with two chromatids joined at centromere

• centrosomes separate and migrate towards opposite poles w help of polar microtubules

• mitotic spindles form

Late Prophase

• nuclear envelope fragments

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

Metaphase

• centromeres of chromosomes aligned at equator

• midway plane between poles = metaphase plate

Anaphase

• centromeres of chromosomes split simultaneously

  • chromatid → chromosome

• chromosome pulled toward poles by motor proteins of kinetochores

Telopase

• begins when chromosome mvmt stops

• chromosomes uncoil to form chromatin

• nuclear membrane forms around each chromatin mass

• nucleoli reappear

• spindle disappears

Cytokinesis

• begins during late anaphase

• actin microfilaments contract to form cleavage furrow

• daughter cells pinched apart → identical

Control of Cell Division

G0 Signals (chemical, availability of space-contact inhibition, volume of cell when area of membrane inadequate for exchange)

Requirements for Mitosis

• cyclins-regulatory proteins

• Cdks (cyclin-dependent kinases) - bind to cyclins → activated

• cyclins destroyed after mitotic cell division

Protein Synthesis

• DNA is master blueprint for protein synthesis

• Gene: segment of DNA with blueprint for one polypeptide

• Triplets (three nitrogen bases) form genetic library

  • codes for order of amino acid in polypeptide

• gene composed of exons and introns

  • exons: code for amino acid

  • introns: noncoding segments

Central Dogma

DNA → Transcription → RNA → Translation → Protein

Roles of RNA

• DNA decoding mechanism and messenger

• 3 Types formed on DNA in nucleus

  • mRNA, rRNA, tRNA

mRNA

carries instruction for building a polypeptide from gene in DNA to ribosomes in cytoplasm

rRNA

structural component of ribosomes that translate message from mRNA

tRNA

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

Transcription

transfer DNA to mRNA

Transcription Factors

gene activators

• loosen histone from DNA to be transcribed

• bind to promoter (sequence specifying start of gene on template strand)

• mediate binding of RNA polymerase to promoter

Stages of Transcription

• Initiation: RNA polymerase binds to promoter, DNA strands separates and initiates mRNA synthesis

• Elongation: unwind double helix and rewinds behind as RNA polymerase moves along strand

• Termination: synthesis ends when termination signal reached

Translation

converts RNA into proteins (involved mRNA, tRNA, rRNA)

Genetic Code

• three base sequence represents codon

• codon = complementary three base sequence on mRNA

• amino acids can be represented by more than one codon

Role of tRNA

• bind specific amino acid at one end (stem)

• anticodon (at head) binds mRNA codon at ribosome by H-bonds

• ribosome coordinates coupling of mRNA and tRNA at 3 sites

  • aminoacyl site, peptidyl site, exit site

Initiation

occurs when 4 components combine

• small ribosomal subunit

• initiator tRNA

• mRNA

• large ribosomal subunit

Elongation

amino acids added to growing peptide chain

• codon recognition: incoming tRNA binds w complementary mRNA codon

• peptide bond formation: polypeptide bound to tRNA at P site is transferred to the amino acid carried by the tRNA in the A site

• Translocation: shifts by one codon along mRNA

Termination

stop codon (UGA, UAA, UAG) releases new polypeptide

Rough ER in Protein Synthesis

• mRNA-ribosome complex directed to rough ER by signal recognition particle (SRP)

• Forming proteins enters ER

• sugar groups can be added to protein altering its shape

• protein enclosed in vesicle for transport to golgi apparatus

Cell Development Aspects

• contain same DNA but not identical

• chemical signals in embryo channel cells into specific developmental pathways by turning genes on/off

• cell differentiation = development of specific and distinctive features in cells

Apoptosis

programmed cell death (cells shrink and are phagocytosed)

Hyperplasia

increase cell number when needed

Atrophy

decreased size results from loss of stimulation or use

Genetic Theory of Aging

Cessation of mitosis and cell aging programmed into genes

Telomeres

may determine # of times a cell can divide (string of nucleotides protecting ends of chromomes)

Telomerase

lengthens telomeres (found in germ cells and absent in adult cells)