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

• Nucleotide - made up of a sugar, a base, and a phosphate 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 LADDERUprights = 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 - proof-read 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

–G0 – cell at rest; is not preparing for division

–G1 – cell gets ready for division

–S – chromosomes copy themselves–G2 – 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. TelophaseCells Alive Animation John Kyrk Animation McGraw Animation

PROPHASE

• Chromatin condenses and becomes visible as chromosomes.

• The nucleus and its various parts disappear.

• Spindle fibers 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.

Chromosome

centromerechromatids

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

MEIOSIS: Making sex cells

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 gametes.–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.