Chapter 9 - Cellular Reproduction

Prokaryotic Cell Cycle

Prokaryotes (bacteria) do not have a nucleus or multiple chromosomes.
To reproduce, the single circular strand of DNA duplicates itself
The cell elongates and the two circular strands of DNA separate
The cell divides into two cells

This process is called BINARY FISSION

The Eukaryotic Cell Cycle is more complex

Eukaryotes (animals, plants, fungi) have a nucleus that houses multiple chromosomes
A dividing cell must ensure that the two new daughter cells each receive an exact copy of each chromosome.

In order to understand the complexity of this process, you need to understand the structure of chromosomes, and why their numbers are important

Chromosome Structure

Chromosomes also have banding patterns unique to each one. These bands are caused by certain dyes. Chromosome banding can help to determine homologs on a karyotype.

Chromosome Numbers

Somatic Cells - body cells, such as muscle, skin, blood ...etc. These cells contain a complete set of chromosomes (46 in humans) and are called DIPLOID or 2N

Sex Cells - also known as gametes (sperm and egg). These cells contain half the number of chromosomes as body cells and are called HAPLOID or 1N

Chromosomes come in pairs, called Homologous Pairs (or homologs). Imagine homologs as a matching set, but they are not exacly alike, like a pair of shoes.

In humans, Diploid cells have 23 homologous pairs = total of 46

In humans, Haploid cells have 23 chromosomes (that are not paired) = total of 23

22 pairs of chromosomes that do NOT determine sex = autosomes
1 pair of chromosomes that determine sex = sex chromomosomes

Homologous Chromosomes - each chromosome has a match, called a homolog. This is why normal organisms always have an even number of chromosomes. One homolog you received from your mother, the other you received from your father. They are not exactly alike, but they are the same size, shape, and have the same banding pattern.

Homologous Chromosomes Diploid Number = 4

Sexual Reproduction

Not to be confused with reproduction, many organisms reproduce without sex!

Sexual reproduction involves the fusion of haploid gametes:

a haploid human sperm + a haploid human egg
= a diploid zygote

Sex Chromosomes

In humans, there are two chromosomes that determine sex. The X and the Y chromosome.

If you have an XX - you are female
If you have an XY - you are male

X's are much larger than Y chromosomes, as visible on the karyotype below. Y chromosomes actually carrie very few genes, and males must rely on genes on the X chromosomes (mom's) for certain traits.

Karyotype

A picture of a person's chromosomes, arranged by size and grouped into homologous pairs.

Changes in Chromosome Number

Occassionally, errors occur during cell division that result in the offspringing receiving too many or too few chromosomes.

Down's Syndrome occurs when a person has an extra chromosome #21 (This condition is also called Trisomy, because the person has 3, instead of 2 chromosomes of a set)

Other Syndromes - Turner, Klinefelter, Edward, Supermale (XYY)

Chromosome Mutations

Inversion (B)
Deletion (A)
Duplication (D)
Translocation (C)

Terms:

Cell division - process which produces two daughter cells
Cell Cycle - cycle that includes cell growth and then division (and then growth again)
Sexual Reproduction - process where two gametes (sperm and egg) fuse to form a zygote
Asexual Reproduction - reproduction that does not involve the fusion of gametes, offspring are clones
Deoxyribonucleic acid - (DNA) - the code of life, each new cell needs a copy of DNA
Binary Fission - how bacteria cells divide (reproduce)
Chromosome - a structure containing the DNA, cells of different species have different numbers of chromosomes, humans have 46.
Chromatin - DNA exists as this dense, spaghetti-like structure, before division, chromatin coils and becomes chromosomes
Haploid - a cell that contains half the number of chromosomes of a body cell; sex cells (gametes)
Diploid - a cell that contains the full number of chromosomes; body cells (somatic cells)
Homologous Pair - chromosomes that match, humans have 23 homologous pairs of chromosomes
Autosomes - chromsomes that do not determine sex
Sex Chromosomes - chromosome that determine sex
Down Syndrome - a condition that results from having an extra chromosome #21