Cell Growth and
Reproduction
(BIO.B.1)

DNA
DEOXYRIBONUCLEIC ACID

DNA STRUCTURE
• The chemical components of
DNA are:
– sugar (deoxyribose)
– phosphate groups
–Four different nitrogen containing bases

THE FOUR BASES ARE:
1. Adenine
2. Guanine
3. Thymine
4. Cytosine

- made up of a
, a
, and a
group.
• DNA is like a twisted ladder and is called a double helix.

• The uprights of the ladder are made of the sugar and phosphate of the nucleotides.
• The rungs are two nitrogen bases held together by hydrogen bonds .

THE DNA LADDER
Uprights = sugars and phosphates
Each rung = 2 bases connected by H bonds

COMPLIMENTARY BASES
• Adenine binds with Thymine .
• Guanine binds with Cytosine .

Figure 16.5 The double helix

DNA
• Chromosome – a long strand of DNA
– One chromosome (one strand of
DNA) has thousands of genes and millions of base pairs
– Humans have 46 chromosomes and have an estimated 23,000 - 25,000 genes
• Gene – a section of DNA that gives instructions for a protein to be made; the protein controls a trait

• Genome – a sum of all of an organism’s genes
• Genetic code - the order of the bases
– Different order = different genes
– Different order = different people and even different species.
• Allele – the form that a gene takes (ex. some are dominant or recessive)

DNA Replicates
• DNA can make copies of itself
(replicate).
• Why does DNA need to be able to copy itself?
– DNA replicates so when cells divide to make new cells, the new cells have the same DNA as the original cell

REPLICATION OF DNA
• Replication begins with DNA
Polymerase (an enzyme) binding to a part of DNA.

• DNA polymerase breaks
Hydrogen bonds between base pairs (unzips the DNA).
• The enzyme then adds complementary nucleotides to each new strand.

• Each new DNA molecule consists of one original chain and one new chain.
• Proof-reader enzymes - proofread the new chains and check for mistakes (these include
DNA polymerase).

CELL DIVISION

Types of Cell Division
• Mitosis – makes new somatic ( body ) cells in eukaryotes
• Meiosis – makes new gametes ( sex cells) in eukaryotes

WHY DIVIDE?
• Why do multicellular organisms do mitosis ?
– To repair or heal
– To grow and develop
– To replace cells that die
• Why do unicellular organisms do mitosis
( eukaryotes )?
– To reproduce asexually ( cloning )
• Why do multicellular organisms for meiosis ?
– To reproduce sexually

Important part of Cell Division
• Nucleus - contains genetic information ( DNA )
• Chromosomes – strands of DNA
• When a cell divides, the new cells made must have the correct number and kind of chromosomes.
• Most of the time, DNA looks like a lump of spaghetti, and is called chromatin .

Cell Cycle

Cell Cycle
• Interphase
– G
0
– cell at rest; is not preparing for division
– G
1
– cell gets ready for division
– S – chromosomes copy themselves
– G
2
– cell checks for mistakes and prepares for division
• Mitosis – the process of nuclear division
• Cytokinesis – the division of the cells cytoplasm

• Most of the time cells are not dividing.
• Several proteins are involved in starting division.
• Some cells can divide in as little as 20 minutes.

• Cells typically stop dividing when:
– Nutrient levels drop (no food)
–They come into contact with other cells (no room)
–Receive chemical signals to stop.

INTERPHASE
• Cells spend most of their life in interphase.
• Normal cell activities occur.
• To prepare for cell division, the chromosomes make copies of themselves .
• In other words, the DNA replicates.
• Recall, DNA polymerase… etc.

MITOSIS: NUCLEAR DIVISION
FOR EUKARYOTES
Four Stages
1. Prophase
2. Metaphase
3. Anaphase
4. Telophase
Cells Alive Animation John Kyrk Animation McGraw Animation

PROPHASE
• Chromatin condenses and becomes visible as
.
• The nucleus and its various parts disappear.
•
are made and attach to the chromosomes.

• Centrioles separate and move to opposite ends (poles) of the cell.
• Each copy of the chromosome is called a chromatid .

chromatids
Chromosome centromere

METAPHASE
• The chromosomes line up at the middle of the cell
(equator).

ANAPHASE
• The joined chromatids separate and move to opposite ends of the cell.

TELOPHASE
• The chromatids arrive at the opposite poles of the cell.
• Spindle fibers disappear .
• New nuclei form in the two new cells.
• Cytokinesis - pinching in of plasma membrane (formation of cleavage ) in order to form two new cells

END RESULTS OF MITOSIS
• Two new “daughter” cells are made with each containing the same number and kinds of chromosomes as the original (parent) cell.
• They have the same genetic code.

A Little Info on Cancer

CANCER
• Caused by uncontrollable mitosis .
• Cells either fail to stop dividing or get signals to continue dividing.
• An increased number of cells makes a tumor .
• Tumors can be benign (non-cancerous) or malignant (cancerous).
• Cancer cells spread ( metastasis ) throughout body, unlike normal cells!

Some Causes of Cancer
• Genetics
• Radiation
• Carcinogens (mutate DNA)
• Old Age
• Viruses
– Can mutate DNA causing some cancers
– Example: Human papillomavirus (HPV) associated with cervical cancer in women
• Poor diets

CHROMOSOME NUMBER
• The number of chromosomes varies from one kind of organism to another.
• Humans have 46 chromosomes
& fruit flies have 8.
• Some plants have hundreds.

• Karyotype – a picture of chromosomes that shows the number and the size of chromosomes that a cell has
• Amniocentesis and Chorionic
Villi Sampling – tests done on pregnant women in order to look at the chromosomes of the developing baby

• Chromosomes of animal cells are usually paired. WHY?
• Humans have 23 pairs, fruit flies have 4 pairs.
• Haploid - the # of pairs or n
• Diploid - the total number of chromosomes or 2n
• For humans: n=23 and 2n=46

SEXUAL REPRODUCTION
• Fertilization - the combination of chromosomes (genetic information) from two sex cells (combining sperm and egg)
• Each sex cell gives half of the total chromosomes.

• Sex cells are also called
.
–Female sex cell = egg or ovum
–Male sex cell = sperm
• Gametes are haploid (half the chromosomes).

Gamete Production
• Gametes are produced in gonads .
• Eggs are made in ovaries .
– Females begin making their eggs before they are born!
– They stop making eggs during menopause.
• Sperm are made in testes .
– Males make sperm from approximately puberty until death.

• After fertilization, the developing organism is called a zygote and is diploid.
• After fertilization the zygote grows bigger by doing
MITOSIS.
• A zygote becomes an embryo and later a fetus .

MEIOSIS END RESULT
• Meiosis makes 4 haploid, sex cells.
• These 4 cells are sex cells (sperm and egg).
• Each cell has half the number of chromosomes as the parent cell
(haploid).
• Meiosis increases genetic diversity in the offspring.