Genetics Part 1: Cell reproduction

LS1I: Cell Reproduction and how the process of genetic recombination and sexual reproduction produces variation in offspring.

A Quick Review Do you remember where the nucleus is found? The nucleus is found inside all eukaryotic cells. What is the function or purpose of the nucleus? The nucleus is the control center of the cell. What is stored inside the nucleus? DNA is stored inside the nucleus. Why is DNA important? DNA contains the instructions on how to maintain

the function and activity of our cells. It is the molecule that essentially controls the expression and process of “life”.

Organelles What is an organelle? Definition: An organelle is a structure found within

a cell that has a specific job it performs to keep cells functioning properly.

What is DNA? DNA Stands for “Deoxyribonucleic Acid” Definition: The molecule that holds the

instructions for building an organism and how to keep it biologically functioning.

A. Found in the nucleus of all cells B. Stored as structures called chromosomes C. In our cells, these chromosomes always are

found in pairs. 1. There is one exception we will talk about

later.

The shape of DNA is described

as a “double helix”

Chromosomes Definition: The structure in the nucleus

that is used as the storage form of DNA A. Each species has its own unique set

Humans have 23 pairs for a total of 46 chromosomes (23 from mom, 23 from dad).

B. Only visible during replication C. The pairs are called “homologous pairs”. D. Different chromosomes contain

different genes. E. Two types of chromosomes

1. Autosomes: 22 pairs in our cells 2. Sex Chromosomes: 1 pair, this

determines your gender.

How Many Chromosomes? Humans have how many singular chromosomes?

Pairs? Can you guess how many chromosomes these

organisms have?

Goldfish = 94 Fern = 64 Mosquito = 6 Strawberry = 56

Cat = 38 Dog= 78

Karyotype

A Karyotype is a “genetic inventory” of the chromosomes present. A female will have 2 “X” chromosomes, while a male will have one “X” and one “Y” chromosome.

Genes Definition: A segment of DNA that is

responsible for the control or expression of a very specific trait.

A. Examples of traits controlled by genes:

1. Eye color 2. Hair texture 3. Ability to break down

lactose (milk sugar). 4. Ability for your blood

to clot if you get a cut.

Vocabulary terms

Diploid: Refers to cells that have 23 pairs of chromosomes (all 46 in humans)

A. Abbreviated as “2n”. B. Chromosomes are in pairs, like shoes. Haploid: Refers to cells that only have HALF the

complete set of chromosomes A. Abbreviated as “n”. B. For humans, that number is 23 chromosomes. C. There are no pairs, only singles.

Did you Realize…….. The human body is composed of roughly 100

TRILLION cells. Very few cells, like the neurons that compose the

nervous system, live “forever”. The rest of our cells, have definite lifespans: Bone Cells: 25 – 30 yrs Pancreas cells: 1 year or so Red Blood Cells: 4 months Skin Cells: 2 – 4 weeks Cells lining our stomach: 2 days

So how do our cells get replaced?

Reproduction of Cells Reproduction of cells occurs in one of 2

ways 1. Mitosis- The reproduction of regular

body (somatic) cells. (growth & repair) 2. Meiosis- The production of

reproductive cells (egg or sperm) a. Only occurs in the testes in males, ovaries

in females.

The Cell Cycle There are 5 phases in the Cell Cycle

1. G-1: The growth phase of the cell. Cell grows rapidly and carries out it’s normal functions. This phase occupies the major portion of time for a cell. 2. S Phase: DNA replication occurs, all chromosomes have been copied, so there are double the number. 3. G-2 Phase: Preparations made for cell division (organelles are duplicated). 4. M Phase: Mitosis occurs; nucleus divides in two. 5. C-Phase: “Cytokinesis” (the cell splits into two cells).

Overview of Mitosis

2n (46)

4n (92)

2n (46)

2n (46)

1. Parent cell contains 23 pairs of chromosomes

2. At the end of mitosis, both daughter cells are identical to each other AND the original (parent) cell.

3. This type of reproduction occurs in all our regular body cells.

Replication of DNA

Stages of Mitosis Mitosis is made up of four stages: 1. Prophase: Chromosomes condense, become visible.

Nuclear membrane dissolves. 2. Metaphase: Chromosomes move to center of cell. 3. Anaphase: Chromosomes separate. 4. Telophase: Spindle fibers pull chromosomes to opposite

poles of the cell. 5. Following telophase is cytokinesis. 6. Then the cell enters “interphase”: the G-1 phase

What does it look like?

Splitting Of The Chromosome The chromosome as we

recognize it is made up of sister chromatids held together by a centromere.

A. The reason it looks like an “x” is because it has replicated.

B. Typically, the chromosome will appear as a single strand.

Stages of Mitosis

If you observe cells under a microscope, you can see the nuclei In different stages (phases) of mitosis.

As you look at the cells, they are not all in the same phases or stages of mitosis

What Does it Look Like?

Watching Mitosis Here is what it looks like in action:

Meiosis Definition: the process by which haploid

cells are created from diploid parent cells

1. Has two phases: Meiosis 1 Meiosis 2

2. Occurs only in the ovaries or testes 3. Results in 4 haploid cells from each

diploid cell 4. These cells are called “gametes” > Eggs and Sperm

Definition: cells produced by organisms that reproduce

sexually. The gametes combine during fertilization to create offspring.

A. Both cells are haploid (only have half the full set of chromosomes)

B. Dad’s 23 in the sperm (hap) + mom’s 23 in the egg (hap)= 46 total (diploid)

Gametes

Overview of Meiosis 2n

(46)

2·2n (92)

Meiosis I

Meiosis II

2n (46)

2n (46)

n (23)

n (23)

n (23)

n (23)

2n = 23 pairs of Chromosomes for a Total of 46

n= 23 single Chromosomes, NO pairs

Replication of DNA

Crossing over

Segregation/Independent Assortment of genes

Because of crossing over & segregation/I.A., these gametes are not identical

Watching Meiosis

Parent A: Full set

2 copies of all 23 chromosomes

Parent B: Full set

2 copies of all 23 chromosomes

½ set ½ set

1copy 1 copy

½ set ½ set

1copy 1 copy

Offspring: Full set

2 copies of all 23 chromosomes

Meiosis

Sexual Reproduction

Even more Variation: Crossover

Def: when two copies of a chromosome exchange sections to produce a new combination of the existing genes from the parents.

Even More Variation: Independent Assortment Independent Assortment is another way in which

offspring are unique from their parents and siblings.

When the homologous pairs of chromosomes separate, there is no predictable pattern of which will end up in a given egg or a given sperm cell.

mistakes in Meiosis

Trisomy Def: when diploid cells have three copies of a

chromosome A. Result of uneven distribution during

anaphase B. One gamete ends up with two copies

instead of the normal one copy

What is Down’s Syndrome?

1. It is called a “chromosomal disorder” because there is an extra copy or fragment of chromsome #21

2. Lower than average cognitive ability. 3. Physical differences A. Small chin B. Overly round face C. Almond shaped eyes D. Protruding tongue

Other Trisomy Examples 1. Most fetuses with trisomies die within the womb early

in development. 2. Trisomy 16: Edward’s Syndrome A. Kidney and heart defects B. Small head, low set ears, widely spaced eyes and heavily

drooping eyelids. 3. Trisomy 13: Patau Syndrome A. Mental/motor challenged B. Extra fingers or toes C. Low set ears, eye defects and small head

Downs Syndrome

Patau Syndrome

Edwards Syndrome

Monosomy Def: when diploid cells have

one copy of a chromosome 1. Result of uneven

distribution during meiosis II

2. One gamete ends up

with no copies of the given chromosome

Turner’s Syndrome

1. All or part of an X chromosome is missing 2. Short stature, swelling, broad chest, low hairline low set ears and webbed neck. 3. Non-working ovaries and an absence of a menstrual cycle.

Summary 1. The nucleus, the control center of the cell, stores

DNA in the form of structures called “chromosomes”.

2. In our body cells, chromosomes occur in pairs, in our gametes, they occur as singles.

3. Cells need to reproduce to allow for growth, repair and continuity of life. Mitosis is the process that allows this to happen.

4. Our eggs and sperm (gametes) are formed by a process called meiosis which takes place in specialized cells in the testes or ovaries.