Chromosomes, Mitosis, And Meiosis

Chapter 8

Do Now

What is DNA?

Where are your chromosomes located?

How many chromosomes do humans have?

DNA

DNA – deoxyribonucleic acid

Long, thin molecule that stores genetic information

Made up of nucleotide building blocks

(Adenine, Guanine, Cytosine, Thymine)

DNA in a human cell has approximately 6 billion pairs of nucleotides

Chromosomes – rod shaped structures made of DNA and protein

Chromosomes

1. Contained in nucleus

2. When cell is not dividing, genetic info is kept in uncoiled DNA called chromatin

3. When a cell begins to divide the single DNA molecule coils around proteins called histones to form chromosomes

Chromosome Structure4. Non-histone proteins control

activity of specific DNA sequences

5. Each chromosome has two identical parts called – sister chromatids

5. The point at which they meet is called a centromere

6. Division-one chromatid from each chromosome/new cell

Info about Chromosomes

1. Entire human genome mapped – found chromosome location of approximately 23,000 genes

2. Above the centromere is the “p” region (small)

3. Below the centromere is the “q” region (large)

Chromosome Numbers

1. Every Species has a different number of chromosomes

2. Humans have 46 (23 pairs) a. 22 autosomes (do not determine the sex)b. 1 pair sex chromosomes (determine the

sex)3. Females have 2 X chromosomes, males

have an X and a Y chromosome

Haploid/Diploid

1. Homologous chromosomes (paired chromosomes) are the same size and shape and carry genes for the same traits

2. Cells with both chromosomes of a homologous pair are considered diploid (2N) – body cells - somatic cells

3. Cells with one of the two chromosomes is considered haploid (N) – sex cells

Haploid/Diploid

Karyotype

An arranged photomicrograph of chromosomes that helps determine problems in chromosome # or deletion

Karyotypes describe the number of chromosomes, and what they look like under a microscope

Attention is paid to:

chromosome length

position of the centromeres

banding pattern

differences between the sex chromosomes

Do Now

What are chromosomes made of?

What does haploid mean?

What does diploid mean?

Down’s Syndrome

Trisomy 21– 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

Down’s Syndrome

Turner Syndrome

Female missing one X chromosome (X0)

1 in every 2500 girls Infertile – ovaries do not develop Other female characteristics do not

develop Decreased height

Kleinfelter’s Syndrome

Male with an extra X chromosome (XXY)

Occurs in 1 out of every 1700 men Testicles do not form properly –

infertile Delayed motor function and muscle

formation Increased height, increased tiredness

Kleinfelter’s Syndrome

II. CELL DIVISION

*Virchow – all cells are derived from pre-existing cells

*Cell division - the process by which cells produce offspring cells

*It is very different for prokaryotes and eukaryotes

Cell Division in ProkaryotesBinary 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

Mitosis - cell division in eukaryotes

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 Cycle

Repeating set of events that make up the life of the cell

Consists of: mitosis - cell division interphase G0 Phase – cell exits the cell cycle and

does not prepare for another division (nerve cells)

Cell Cycle

Interphase

1. Takes up 80-90% of cells lifespan2. Three distinct parts:

a. G1 (Growth Phase) – cell doubles in size, organelles double in number

b. S1 (Synthesis Phase) – chromatin replicates

c. G2 (Growth Phase) - rapid growth phase – prepare for next division

Do Now

What is cell division in prokaryotes called?

What is cell division in eukaryotes called?

What are the 4 main phases of the cell cycle?

Mitosis

1. 4 phases: Prophase, Metaphase, Anaphase, Telophase

Prophase

1. Chromatin coils and forms chromosomes, nucleolus and nuclear membrane disappear, centrosomes and spindle fibers appear

Metaphase

1. Chromosomes line up at center, or equator of cell

Anaphase

Spindle Fibers pull chromosomes towards poles of cell

Telophase

1. Centrioles and Spindle fibers disappear

2. Chromosomes unwind, and nuclear membrane and nucleolus forms in each cell

Mitosis in Plant Cells (onion)

Cytokinesis

1. Takes place after mitosis

2.Cytoplasm and organelles split and form two daughter cells

3. Each cell receives and identical

copy of the original cell’s

chromosomes and ~ half

of the cytoplasm and organelles

Do Now

What are the 4 stages of mitosis?

What are the structures that pull the chromosomes toward opposite ends of the cell?

How does cytokenesis differ in animals and plants?

Meiosis – reduction division

Process of nuclear division that reduces the # of chromosomes in each new cell to half of the original cell

This is necessary in reproductive cells called gametes so that they can fuse and form a zygote with a normal # of chromosomes

Example – human egg (haploid -1n) and sperm (haploid -1n) form an embryo (diploid -2n)

Meiosis

A. Two nuclear divisions take place

1. Meiosis I – homologous chromosomes are separated

2. Meiosis II – chromosomes are separated into chromatids for each haploid cell

3. 4 haploid cells are produced - gametes

Meiosis I – DNA copies itself just before beginning

Prophase I - similar to mitosis: DNA coils into chromosomes nuclear membrane and nucleolus disassemble spindle fibers appear

THEN Every chromosome lines up with its homologue

and forms a tetrad Synapsis – pairing of homologous

chromosomes – does not occur in mitosis

Meiosis Helps Genetic Diversity

A. During Prophase I, homologous chromosomes line up forming a tetrad

B. Chromatids can exchange pieces, called crossing – over

C. Genetic recombination results

Meiosis I

Metaphase I Tetrads line up randomly along the midline of

the cell Spindle fibers attach to centromeres and poles

of cell

Meiosis I

Anaphase I Each homologous chromosome moves to an

opposite pole of dividing cell 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

Meiosis 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

Meiosis II

DNA does not copy itself before Meiosis II

Occurs in both cells formed in Meiosis I

Exactly like mitosis

Results in four new cells from each cell that began Meiosis

Each new cell (haploid – 1n) has half the number of chromosomes as the original cell (diploid – 2n)

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

Oogenesis

Spermatogenesis

Asexual Reproduction

• Production of offspring from one parent:• no union of gametes• no genetic diversity• offspring genetically identical

• Unicellular organisms -binary

fission or mitosis• Multicellular organisms - budding

Sexual Reproduction

Production of offspring through meiosis and the union of egg and sperm

Offspring are genetically different from parents

Sexual Reproduction

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

Gives an evolutionary advantage in an environment that can change – survival of the fittest

Sexual Reproduction