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Biology 141. The Cellular Level of Organization Chapter 3: Cell Division and Protein Synthesis. Macromolecule Structure. DNA nucleic acid Right-handed double-helix sugar - phosphate backbone (sides of the ladder).

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Biology 141

Biology 141

The Cellular Level of Organization

Chapter 3: Cell Division and Protein Synthesis

Macromolecule structure
Macromolecule Structure

DNA nucleic acid

  • Right-handed double-helix

  • sugar - phosphate backbone (sides of the ladder).

  • nucleic acid rungs bonded by hydrogen bonds following base-pairing rules of A=T and C=G.

Macromolecule structure dna nucleic acid
Macromolecule StructureDNA nucleic acid

  • DNA is wrapped on histone proteins forming nucleosomes.

  • Nucleosomes wind or fold into thicker structures called chromatin.

  • Prior to cell division

    • the chromatin condenses = chromosome.

    • One strand of DNA per chromosome.

    • humans = 46 chromosomes

Dna replication
DNA Replication

  • DNA makes an exact copy of itself prior to cell division.

  • The copy is attached at the centromere until cell division occurs.

Nucleotide sequence is the biological code
Nucleotide sequence is the biological code.

  • Group of 3 nucleotides is a codon.

  • 1 Codon codes for 1 amino acid.

  • Remember – there are 20 different amino acids.

Macromolecule structure1
Macromolecule Structure

RNA nucleic acid

  • RNA is single-stranded

    • (not double-stranded, can fit through nuclear pores)

  • Ribose sugar

    • (instead of deoxyribose)

  • Uracil

    • (instead of thymine)

  • DNA synthesizes RNA - called transcription.

Dna function protein synthesis
DNA Function = Protein Synthesis

  • carries the blueprint for all proteins and enzymes that determine structure and function of all tissues.

    • Functional unit of DNA = GENE

    • A gene is the DNA template for a specific protein.

Protein synthesis
Protein Synthesis


  • transcription - mRNA synthesis

    • DNA to mRNA transcript.

    • DNA gene is transcribed onto an mRNA molecule.

  • translation - polypeptide synthesis

    • mRNA to amino acid sequence

    • mRNA is translated into protein by ribosomes and tRNA molecules.


DNA codes RNA molecule.Requires 3 steps:

1. Initiation: DNA unwinds.

  • Hydrogen bonds break. - unzips.

    2. Elongation: Nucleotides are joined by RNA polymerase enzyme.

  • mRNA Nucleotides match up according to base pairing rules

    • AT

    • CG

      3. Termination: mRNA falls off upon reaching a stop signal.

      DNA rewinds.

  • New mRNA molecule leaves nucleus.


  • A set of 3 nucleic acid bases on the mRNA is a codon.

  • A tRNA matches to the codon using base-pairing rules.

  • Opposite end of tRNA has one of the 20 amino acids.


  • mRNA leaves the nucleus through the nuclear pore and enters cytoplasm.

  • Once outside the nucleus, the mRNA is translated into protein.

  • Translation Requires 3 Steps: Initiation, elongation and termination.


1. Initiation:

  • Small Ribosome subunit attaches, scans mRNA for start signal.

  • Large ribosome subunit attaches.

    2. Elongation:

  • Once small and large subunits bind, ribosome attracts tRNA molecules.

  • Begins attaching amino acids by binding tRNA molecules according to codons. Amino acids link together with peptide bonds to form a chain.

    • A chain of amino acids = primary structure of a polypeptide (protein).

      3. Termination:

  • Ribosome stops at a stop codon, the tRNA stops attaching amino acids.

  • The ribosome subunits dissociate.

  • mRNA is released, new protein is released.

Dna mutation
DNA Mutation

  • Mutation in a body cell DNA = cancer

  • Mutation in gamete (sex cell) DNA = disease or abnormality.

Cell cycle life
Cell Cycle (life)

  • Cell cycle is divided into 2 phases.

    • Phase 1: interphase

      • G1- protein synthesis, organelles replication begins

      • S- DNA replicates

      • G2 – protein synthesis, organelle replication ends

Cell cycle life1
Cell Cycle (life)

  • Cell cycle is divided into 2 phases.

    • Phase 2: cell division.

      • Prophase

      • Metaphase

      • Anaphase

      • Telophase

      • cytokinesis

Cell division
Cell Division

  • Somatic (body) cell division – mitosis

    • Replaces dead cells

    • New additional cells for growth

    • 46 (23 pair) chromosomes (diploid)

  • Gamete (sex) cell division – meiosis

    • 23 chromosomes (haploid)

    • Egg

    • sperm

Cell division mitosis
Cell Division: Mitosis

  • Nuclear division:

    • Prophase

    • Metaphase

    • Anaphase

    • Telophase

  • Cytoplasm division

    • Cytokinesis


  • Breakdown of nuclear envelope and nucleolus.

  • DNA condenses.

  • Centrioles move to opposite poles, spindle forms.


  • No nuclear membrane.

  • Chromatids separate.

  • Spindle attaches to individual chromatids.

  • Chromatids align on metaphase plate.


  • Chromatids pulled to the opposite poles.

  • Cytokinesis begins.


  • Nuclear envelope reforms.

  • Nucleolus reforms.

  • Spindle breaks down.

  • Cytokinesis continues.

  • DNA unfolds.


  • Cytoplasm completely divides.

    • Animals - furrowing of cytoplasm pinches off two new cells.

Cell division meiosis
Cell Division - Meiosis

  • Nuclear Division of sex cells

  • Meiosis - egg and sperm; Two nuclear divisions results in 4 unique haploid cells.

Meiosis i
Meiosis I

  • Prophase I - chromosomes pair with homologous chromosomes (synapsis) to form a tetrad.

  • Crossing over occurs - source of genetic recombination and variation. Nucleus breaks down.

Metaphase i and anaphase i
Metaphase I and Anaphase I

  • Metaphase I - tetrads align on metaphase plate.

  • Anaphase I - tetrads pulled apart; chromosomes pulled to opposite poles.

Meiosis i1
Meiosis I

  • Telophase I. - Cell divides into two cells. Each cell contains one chromosome from homologous pair.

Meiosis i2
Meiosis I

  • Interkinesis - short rest period between divisions.

  • Chromosomes still consist of sister chromatids (2 chromatids).

Meiosis ii
Meiosis II

  • Prophase II - centrioles move to opposite poles, spindle forms.

Meiosis ii1
Meiosis II

  • Metaphase II - spindle attaches to centromere, chromosomes align on metaphase plate.

  • Anaphase II - sister chromatids separate - pulled to opposite poles.

Meiosis ii2
Meiosis II

  • Telophase II. - Spindle breaks down, nuclear envelope reforms, cytokinesis continues.

Meiosis ii3
Meiosis II

  • Cytokinesis - cell cytoplasm division completes.

  • Results in 4 new cell - each daughter cell with unique genetic haploid number of chromosomes.