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DNA & Cell Reproduction. MODERN BIOLOGY Chapters 10-1 & 8. DNA. What is DNA? Deoxyribonucleic acid or DNA is a long thin molecule (nucleic acid) that stores genetic information. Main Function:

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dna cell reproduction

DNA & Cell Reproduction


Chapters 10-1 & 8


What is DNA?

Deoxyribonucleic acid or DNA is a long thin molecule (nucleic acid) that stores genetic information.

Main Function:

Store and transmit the genetic information that tells cells which proteins to make and when to make them.


structure of dna
Structure of DNA

DNA -The Double Helix

Scientists James D. Watson and Francis Crick determined the structure of DNA in 1953, using the X-ray crystallography work of Rosalind Franklin and Maurice Wilkins that indicated DNA had a helical structure (shaped

like a twisted ladder).

structure of dna1
Structure of DNA

Made of smaller, repeating subunits called NUCLEOTIDES.

Each nucleotide is made of 3 parts:

1. Sugar (deoxyribose)

2. Phosphate

3. 4 different Nitrogenous Bases (A, G, C, T)

structure of dna2
Structure of DNA

*Sugar and phosphate group - same in all nucleotides .

*4 different Nitrogen bases:

Adenine, thymine, guanine and cytosine

structure of dna3
Structure of DNA

The bases, adenine, thymine, guanine, and cytosine, are often labeled A, T, G and C.

Each base always pairs with its ‘partner’ base; referred to as complementary base pairing’.

structure of dna4
Structure of DNA

Complementary Base Pairing

Adenine always bonds with thymine to form a step

Cytosine always bonds with guanine to form a step

Deoxyribose sugar and phosphate are the backbone or sides of the ladder


A DNA molecule can have billions of ‘steps’. Approx. six billion in the human cell !!

These steps are made up of different combinations of the base pairs, A, G, T, C.

The number and pattern of pairs form a genetic "code" that makes up genes-the genetic code that controls growth, development and characteristics of an organism.

Using just these four bases, nature can create an almost unlimited variety of genetic messages.


dna replication
DNA Replication

Cells that divide must pass exact

copies of their DNA to their

offspring cells.

REPLICATION-the process of

duplicating a DNA molecule.

During replication , the double helix

unwinds & separates.

Each chain serves as a template

for a new chain, or strand of DNA.

dna replication1
DNA Replication

The Steps of DNA Replication:

1. To begin the process, the double helix unwinds. As with all metabolic activities, an enzyme (DNA helicase) is needed for this process.

2. DNA helicase unzips the 2 strands of nucleotides in between the bases – point called the replication fork.

Now the bases are exposed.


dna replication2
DNA Replication
  • The Steps:
  • 3. Complementary nucleotides floating in the nucleus pair with the exposed bases.
  • DNA polymerase (another enzyme!) assists in the addition and bonding of the complimentary nucleotide.
  • Result: 2 new strands of DNA - each consisting of ½ new DNA and ½ old DNA. (identical to the original)
  • http://highered.mcgraw-hill.com/sites/9834092339/student_view0/chapter14/how_nucleotides_are_added_in_dna_replication.html




accuracy repair of dna
Accuracy & Repair of DNA
  • DNA replication is accurate - usually only 1 error in every 10,000 paired nucleotides
  • But any error, called a mutation, can have serious effects
  • Radiation, chemicals , heat can damage DNA (mutation)
  • Cell has proofreading techniques – like spell check
  • Over 20 or more repair enzymes fix errors


cell reproduction
Cell Reproduction

All cells come from the division of pre-existing cells. Cell division or Cell Reproduction allows for growth and repair, carries on life and passes genetic information to future generations.


Cell Reproduction

1. Before a cell can divide, the loosely packed genetic material (DNA) in the nucleus of the eukaryotic cell must make exact copies of itself. This loosely packed genetic material (DNA) is referred to as chromatin.

2. Chromatin then coils into very compact structures called chromosomes.

Each chromosomes consists of two identical strands of DNA tightly wrapped around proteins called histones.



3. These two identical halves are referred to as chromatids (or ‘sister chromatids’)

Centromere - Holds the two chromatids together.

4. During division, sister chromatids from each chromosome separate and each new cell will get one.


Chromosome Numbers

Every Species has a different number of chromosomes

Humans have 46 chromosomes (23 pairs)

Human and animal chromosomes are either:

  • Autosomes
  • sex chromosomes

44 autosomes(do not determine the sex)

2 sex chromosomes (determine the sex)

Females have 2 X chromosomes

Males have an X and a Y chromosome


Chromosome Numbers

Every cell of an organism produced by sexual reproduction has two copies of each autosome

Autosomal Homologous chromosomes (sometimes called homologues) are the same size and shape and carry genes for the same traits.


Chromosome Numbers


*An arranged photomicrograph of chromosomes.

*Shows the number of chromosomes, and what they look like under a microscope.

*Helps determine problems in chromosomes. **Attention is paid to:

chromosome number & length

position of the centromeres

banding pattern

sex chromosomes


Chromosome Numbers

Trisomy 21 – Down Syndrome

  • 3 copies of chromosome 21 instead of 2
    • 1 in 900 births
    • More common with older mothers
    • Some impairment of cognitive ability and physical growth
    • A particular set of facial characteristics

Chromosome Numbers

Cells having two sets of chromosomes

are said to be diploid (2n).

Diploid - having both chromosome for each homologous pair.

All human cells (somatic), except reproductive cells (sperm & egg) are diploid cells.

46 - 22 pairs homologous chromosomes & 2 sex chromosomes.


Chromosome Numbers

Cells with only one of the two chromosomes is considered haploid (1n)

Sperm and egg cells are haploid.

Haploid cells have half the number of chromosomes that are present in diploid cells.

When two haploid cells combine, each new cell will be diploid.


Cell Division

Every Cell has a life of its own!

Every Cell comes from pre-existing cells!

  • single-celled organisms
  • billion of cells that make up the bodies of plants and animals.

Each cell has:

  • A beginning (growth)
  • A middle (genetic material replicates and cell prepares for cell division)
  • And an end (Mitosis & Cytokinesis )



Cell Division: Eukaryote vs. Prokaryote

Cell division - the process by which cells produce offspring cells

It is very different for prokaryotes and eukaryotes


Cell Division: Eukaryote vs. Prokaryote

  • Cell Division in Prokaryotes:
  • Binary fission – division of a prokaryotic cell into two offspring
  • Single chromosome copies itself
  • Cell grows to 2X its size
  • Cell wall forms between the 2 chromosomes
  • Each new cell is identical

Cell Division: Eukaryote vs. Prokaryote

  • Cell division in eukaryotes
  • Mitosis –
  • Division of a cell’s nucleus that produces a genetically identical cell
  • It does not change the amount of DNA in a cell
  • Mitosis occurs in the reproduction of unicellular organisms and in the addition of cells to a tissue or organ of a multicellular organism.

Cell Division: The Cell Cycle

The Cell Cycle is the process or stages cells go through to grow, copy their DNA, and divide to make new cells.

The cycle starts when a cell is formed and ends when a cell divides to make new cells.



Cell Division: The Cell Cycle

The Cycle consists of:

Interphase – time between cell division

Mitosis – nucleus of cell divides

Cytokinesis – division of cytoplasm = 2 new cells!


Cell Division: The Cell Cycle

  • Interphase
  • Takes up 80-90% of a cells lifespan
  • Three distinct parts:
  • G1 (Growth Phase) – cell doubles in size, organelles double in number
  • S (Synthesis Phase) – chromatin (DNA) replicates
  • G2 (Growth Phase) - rapid growth phase & cell check *prepare for next division
  • G0 (resting phase) – muscles cells, retina cells, nerve cells – those you don’t need a lot of go into a resting phase

Cell Division: The Cell Cycle


  • Chromosomes separate
  • Nucleus divides

There are four important stages of Mitosis:

  • Prophase
  • Metaphase
  • Anaphase
  • Telophase
the four stages of mitosis mnemonic
The Four Stages of Mitosis Mnemonic

“(P)LEASE” - Prophase

“(M)AKE” - Metaphase

“(A)NOTHER” - Anaphase

“(T)WIN” - Telophase


Cell Division: Mitosis

  • Prophase
  • Chromatin coils and forms chromosomes
  • nucleolus and nuclear membrane disappear
  • centrosomes and spindle fibers appear.
  • Chromosomes can be seen under a microscope.

Cell Division: Mitosis

  • Metaphase

spindle fibers assist in aligning all chromosomes at the center of the cell.


Cell Division: Mitosis

  • Anaphase

Spindle fibers pull chromosomes apart.

One set of chromosomes is pull toward one pole of the new cell and the other set toward the other.


Cell Division: Mitosis

  • Telophase
  • Centrioles and spindle fibers disappear.
  • Middle of an animal cell pinches together.
  • A nuclear membrane forms around each new set of chromosomes.
  • In plant cells, a new cell wall and a new cell membrane form down the middle of the cell.

Cell Division: Mitosis

  • Cytokinesis
  • Cytoplasm and organelles split and form two daughter cells – exactly like the parent cell
  • Each cell receives an identical copy of the original cell’s chromosomes and half of the cytoplasm and organelles
now take the mitosis quiz record your results special thanks to
Now Take the Mitosis Quiz Record your resultsSpecial thanks to

Cell Division: Mitosis

Please watch this video


Cell Division: Meiosis

Body cells (somatic cells) are reproduced through mitosis. The number of chromosomes remain the same in each daughter cell produced.

However, there is another type of cell division called Meiosis. The number of chromosomes is cut in half in all daughter cells produced.


Cell Division: Meiosis

Meiosis–Called the “Reduction Division”

  • Type of cell division that reduces the # of chromosomes (diploid - 2n) in each new cell to half of the original # (haploid - 1n)
  • Meiosis produces sex cells (sperm and egg cells), each with that reduced number of chromosomes.

Cell Division: Meiosis

When mitosis is completed, each new cell's chromosome number remains the same as in the original parent cell (diploid – 2n).

When meiosis is completed, each sex cell's chromosome number is half (haploid- 1n). The sperm and egg cells have half the number of chromosomes that are found in a normal body cell (somatic cell).


Cell Division: Meiosis

Sex Cells, also called gametes fuse during sexual reproduction and form a zygote with a normal # of chromosomes




Cell Division: Meiosis

Meiosis actually involves two separate cell divisions, and they occur one right after the other.

Meiosis I

Meiosis II


Cell Division: Meiosis

Meiosis I: Similar to Mitosis I

  • Prophase I: DNA has already duplicated. Chromosomes become visible. The nuclear membrane disappears. Spindle fibers appear. Centrosomes (containing centrioles) begin to move.


Synapsis – pairing of

Homologous chromosomes –

does not occur in mitosis

(Every chromosome lines up with

its homolog and forms a tetrad.)


Meiosis – Helping Genetic Diversity

  • Prophase I:

When the homologous chromosomes form a tetrad, chromatids can exchange pieces, called crossing – over

Genetic recombination results


Cell Division: Meiosis

Meiosis I: Similar to Mitosis I

  • Metaphase I:
    • Tetrads line up randomly along the midline of the cell
    • Spindle fibers attach to centromeres and poles of cell

Cell Division: Meiosis

Meiosis I: Similar to Mitosis I

  • Anaphase I:
    • Each homologous chromosome moves to an opposite pole of the dividing cell

Cell Division: Meiosis

Meiosis I: Similar to Mitosis I

  • Independent assortment – the random separation of homologous chromosomes
  • Genetic recombination occurs again due to random separation of maternal (mom’s) and paternal (dad’s) chromosomes

Cell Division: Meiosis

Meiosis I: Similar to Mitosis I

  • Telophase I:
  • Chromosomes reach the opposite ends of the cell - Cytokinesis begins
  • New cells are now haploid (1n)
      • they contain only one homologue from each homologous pair
      • But they have two copies of each chromosome because the DNA copied itself right before beginning Meiosis I

Cell Division: Meiosis

Meiosis II

  • Prophase II: chromosomes DO NOT duplicate. If a nuclear membrane formed, it will break apart again.
  • Metaphase II: chromosomes line up in the center of the cell
  • Anaphase II: sister chromatids split and separate.
  • Telophase II: cells pinch together. Nuclear membranes form. Four daughter cells are formed. Each cell has only half the number of chromosomes as the parent cell.

Cell Division: Meiosis

Gamete Formation

  • Meiosis produces haploid reproductive cells called gametes
  • Meiosis only occurs in cells of the reproductive organs
  • Humans:
    • Oogenesis - female ovaries produce 1 egg and 3 polar bodies (disintegrate)
    • Spermatogenesis - male testes produce 4 sperm


Take a look at a family with more than one child. You can see the importance of meiosis in genetic variation. The children are different from each other even though they have the same parents. With the exception of identical twins, offspring from the same family never look exactly alike.



Because of genetic recombination during meiosis, sexual reproduction results in offspring that are different from each other and from their parents.


Great Meiosis Video!

Animated video of Meiosis!

Now, Take the Meiosis Quiz!








Prophase II

Prophase I

Metaphase II

Metaphase I

Anaphase II

Anaphase I

Telophase I

Telophase II

mitosis vs meiosis
Mitosis vs. Meiosis

Mitosis vs. Meiosis Interactive


Asexual Reproduction

Production of offspring from one parent:

    • no union of gametes
    • no genetic diversity
    • offspring genetically identical (clone)
  • Unicellular organisms –binary fission (prokaryotes; i.e. bacteria) or mitosis (eukaryotes; i.e. algae, some yeasts, and protozoans, such as paramecium) budding, too
  • Multicellular organisms – budding (i.e. hydra)

Sexual Reproduction

  • Except in identical twins, offspring are unique combinations of their parents’ genes

Sexual Reproduction

Sexual reproduction is the formation of a new individual following the union of two gametes.

Sexual reproduction gives an evolutionary advantage in an environment that can change – “survival of the fittest”