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
receives 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 exactly 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 chromosomes
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
Xs are much larger than Y chromosomes, as visible on the karyotype
below. Y chromosomes actually carry 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
Occasionally, errors occur during cell division that result in the
offspring 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