These Notes Will Correspond to Chapter 8 in the Textbook

Biology 101

Section 5 Notes

Cellular Reproduction

These notes will correspond to Chapter 8 in the textbook.

What is cellular division? One cell growing in size and dividing to produce two new cells

This is an important question. Since the first scientist observed a cell dividing, people have wondered why cells do this and how. Early biologists observed that cells reproduce, or make more of them, by dividing in half. Simply one cell splitting into two. However, there is a lot more to it than that. First a cell must increase its size before dividing. If I cut a block of wood in half, I get two blocks of wood, but each only half the size of the original. I get nowhere that way. So a cell grows rapidly to nearly twice normal size and divides. This growth entails an increase in cytoplasm, DNA replication, an increase of membrane and organelles; a basic doubling up of all cellular components.

One of the most important events to occur before a cell can divide is the DNA must be copied, faithfully, error free and totally. This is called DNA Replication. This must happen before a cell can divide. It guarantees that each new cell receives a copy of all the genetic instructions.

Another thing to note: all terms when dealing with cells are in feminine terms. The parent cell is called the mother cell, the news cells are called daughter cells, and the two daughter cells are called sister cells to one another.

Also called "simple cellular division”

Functions for:

1)  Growth and development

Cell division functions for all growth of all parts of the body: skin, hair, nails and any other part that grows or increases. Also any part of the body that develops or changes must develop through cell division.

2)  Repair and reproduction

Anytime the body is damaged or hurt, it is repaired by cells dividing. You cut your hand; the cut seals itself and repairs itself by cell division. All forms of reproduction, from the cellular level to larger organisms, are through cell division. If you are a single-celled organism like protists, then you only reproduce by splitting in half. But even larger, multi-celled, organisms reproduce by dividing cells. Humans reproduce by using reproductive cells called sperm and eggs. A sperm or egg is just one cell. They are produced in the body by cell division in the gonads. They fuse in the process of fertilization to produce a single cell called a zygote. This zygote then divides to grow and form a complete human. All reproduction on Earth involves cellular division.

Reproduction = single or multiple celled organisms producing a new generation

2 Types:

1)  Asexual: production of offspring by a single parent, no sexes or genders

This is the most common form of reproduction. Almost all living things can carry out some form of asexual reproduction. This trait is only lost as we get to higher life forms such as mammals. This is the preferred method for most organisms: it is faster, cheaper (requires fewer resources), less space, and is not reliant on finding a mate.

2)  Sexual: production of offspring by two or more parents resulting in genetic recombination

For the majority of organisms, this method is not used. Even those capable of sexual reproduction will typically reproduce asexually if it is an option. Sexual reproduction requires more resources (water, food, shelter), more time, larger territories to supply the resources, and depends on finding a member of the opposite sex of the same species; which for some species such as Pandas, is actually quite difficult. The only positive this form of reproduction has in its favor is Genetic Recombination!! For some species, such as higher animal forms, this benefit is large enough to out weigh the costs. So animals, like humans, only reproduce sexually. In reality, Genetic Recombination is the main point of sexual reproduction, its purpose for existence. Here a distinction is made between sex (intercourse or copulation) and sexual reproduction. The purpose of sexual reproduction is to shuffle and recombine genetic material (DNA) from two individuals into a new, third person. Not for enjoyment or pleasure. For many animals, this is achieved without any form of copulation or even production of gametes (sex cells).

Purpose: to pass on the genome (An organism’s genome is the total collection of all its genes. Reproduction is an attempt to pass on as many of those genes as possible to the next generation.)

In Prokaryotes: No nucleus!

Remember, prokaryotes have no organelles! This includes the nucleus. However, they still contain DNA, and pass it on to the new cells.

Use process called Binary Fission, the DNA is replicated (copied) and divided between two new cells. This is the most basic form of cell division, simple singular division of one cell into two cloned copies.

In Eukaryotes: Nucleus!!

Eukaryotes have a nucleus! In essence, most of the events and information about cell division in eukaryotes revolve around the events associated with the nucleus, what is done with it, and how the DNA molecule is handled. Because of this, the following notes, especially dealing with mitosis, are explicitly in reference to the nucleus, not the entire cell division process. What happens with the nucleus is called Nuclear Division. The division of the rest of the contents of the cell is called Cytoplasmic Division.

→ Simple Cell Division: has 2 parts

1)  Nuclear Division: division of nuclear contents (I.e. the DNA, chromosomes)

2)  Cytoplasmic Division: division of the cytoplasm (Organelles and everything else)

Nuclear Division (only eukaryotes) can take one of two forms:

MITOSIS

·  This is often referred to as simple cell division

·  Basic, general division of all cells

·  Used for growth of the organism and tissues and repair of damage

·  Used for asexual reproduction in organisms that reproduce asexually

·  One parent cell gives rise to two daughter cells, direct split from one to two

·  Each daughter cell exactly identical genetically to parent, clones

·  Occurs in somatic cells, ordinary cells that make up the body (skin, muscle hair, etc.)

·  Most common form of division in all living things, 99.99999% of all divisions

·  Ex. protistans, fungi, plants and animals (basically everything)

Or

MEIOSIS

·  Also called Reduction Division

·  One purpose only, and that is sexual reproduction

·  Only occurs in those species that are capable of sexual reproduction (so not everything)

·  One parent cell will give rise to four daughter cells

·  Each daughter cell is nonidentical genetically to parent or each other

·  Occurs only in germ cells, cells that make up the gonads and divide special to form the reproductive cells called the sperm and egg, which are collectively called the gametes

·  Reduces number of genes in gametes to 1/2 of parents, so called reduction

Chromosomes and Duplication

Chromosome Numbers and Sets

The DNA molecule in most things is quite large. Too large really. So it is broken up into smaller fragments, each consisting of several thousand genes, called chromosomes (because they are easy to color with stains). The number of chromosomes for each species is specific. Humans have 46 chromosomes. However, they are not all different. Technically, we have duplicates of all chromosomes, one inherited from each parent. We get half our DNA from each parent, but not half our genes in a strict sense. The full complement, all the instructions for “human” will fit onto 23 chromosomes. We just get a complete set of instructions from each parent, and chose to follow Chapter one from one set and Chapters 3, 4, and 5 from the other set. Because of this, scientists say humans have two sets of chromosomes, each set consisting of 23 chromosomes. These are also setup in pairs, so we have 23 pairs of chromosomes, one from the pair from one parent and one from the pair from the other parent. For example, let’s use m = mother and f = father, we get:

Chromosomes

Im If

IIm IIf

IIIm IIIf

IVm IVf

Vm Vf

VIm VIf

VIIm VIIf

. .

. .

XXIIIm XXIIIf

So on and so on for 23 pairs. There are two chromosome number VII, one from each parent. They will contain the same genes on them. So recap: humans have 46 chromosomes, divided into 2 sets, each with 23 chromosomes. These can be paired based on similarity of genes, so we have 23 pair.

Duplication: each daughter cell gets a full copy of genes

When a cell gets ready to divide, it carries out the most important thing it will ever do, and that is copy all its chromosomes. We have many words for this: copy, duplicate, replicate. Technically, it is called DNA Replication. This is to guarantee that each daughter cell gets a full copy of the genetic information. It is done before the cell even begins dividing, sort of like making a full backup of a computers hard-drive.

Duplicated Chromosome Structure: centromeres and sister chromatids

When the chromosomes are replicated, they still remain attached to one another at a small region of unreplicated DNA called the centromere. These two pieces, exact copies of each other, are together called a duplicated chromosome. Both pieces together will continue to be referred to as one chromosome while they are attached at the centromere. Eventually, when the cell is dividing, the last, central region of uncopied DNA will be replicated and the two copies will separate. When this happens, both pieces will now be referred to as chromosomes, sort of like a promotion. While attached and called the duplicated chromosome together, the two pieces of DNA (original and copy) are called sister chromatids. For the original chromosome, if you think about it, appears to be demoted. These duplicated chromosomes are what give the classic X shape to chromosomes so common in photographs shown in textbooks.

The Cell Life Cycle

The cell life cycle covers from one division event to the next. Since the parent cell doesn’t die, but doesn’t continue on, the cycle is shown as a circular diagram. You need to study a figure of the cell life cycle, either in your book or find one from the internet!

Total cycle: Has 2 major parts or phases.

For more information on the cell life cycle and mitosis, refer to the addendum notes in this section on the internet.

1. INTERPHASE

Has 3 Subphases

1) G (gap) one (G-1)

2) S (synthesis) phase (S-phase)

3) G (gap) two (G-2)

2. MITOTIC PHASE (mitosis + cytokinesis)

Has 4 Subphases (4 phases of mitosis)

1) Prophase

2) Metaphase

3) Anaphase

4) Telophase

- Then cytokinesis

Mitosis:

Prophase (early):

·  Chromosomes coil up and become visible

·  Centrioles start to move towards opposite ends of cell

·  Mitotic spindle begins to form

Prophase (late):

·  Centrioles finally at opposite ends of cell

·  Nucleolus degenerates and disappears

·  Nuclear envelope breaks down and disappears

·  Rest of spindle forms

Metaphase:

·  Chromosomes line up in a straight single row

·  Centromeres of each chromosome attach to mitotic spindles from both sides

Anaphase:

·  Centromeres finish duplicating and break apart, separating the sister chromatids of the chromosome

·  Spindles shorten, pulling sister chromatids toward opposite ends of the cell

Telophase:

·  Chromosomes (previously sister chromatids) finish migrating to ends of cell

·  New nuclear envelope forms around each batch of chromosomes

·  Nucleoli reform inside each nucleus

·  Eventually cytokinesis occurs

Differences between animal and plant mitosis:

1.  Cytokinesis: animal cells develop a cleavage furrow or split around the equator of the cell while plant cells stretch and form a new cell wall down the middle of the long cell

2.  Plants lack centrioles, but still have a mitotic spindle which anchors to the cell wall

Homologues

Chromosomes in the body come in pairs, called homologous pairs or homologues

·  One homologue from each pair comes from one parent while the other homologue comes from the other parent (we discussed this earlier)

·  Homologues are similar in that they have the same genes at the same loci (location on a chromosome), same overall shape, size and banding pattern. But they may contain different alleles, or variants, for the genes.

Normal cells are diploid, or 2N, they have 2 sets of chromosomes

Sex cells, gametes (sperm and eggs), are haploid, or 1N, they have 1 set of chromosomes

Meiosis

·  Used for sexual reproduction

·  Only one part of process; also requires gametogenesis and fertilization

Process of Meiosis

·  Reduction division (2N→1N) The number of chromosomes in the gametes or sex cells are reduced to one half. A person cannot pass on all their chromosomes, that would be too many. You can only pass on half. Meiosis prepares these cells for eventual fertilization. Also, the chromosomes are not reduced by a random half, but specifically one full set (chromosomes 1 – 23).

·  Most steps are similar to mitosis. You need to study mitosis and know it well. The majority of functions are identical between the two, basic house keeping chores the cell does before dividing and maintaining itself. If you know mitosis, meiosis will not be much more difficult.