Organisms have tens of thousands of genes that determine individual traits. The more closely related two organisms are, the more genes they’ll have in common
Genes are lined up on chromosomes that can hold thousands of genes.
In body cells of animals and most plants, chromosomes occur in pairs .
One chromosome in the pair came from the male parent and one came from the female parent.
These pairs are called homologous chromosomes – each pair has genes for the same traits
A cell with two of each kind of chromosome is called diploid ( 2n ).
Organisms produce gametes (sex cells) that contain one of each kind of chromosome.
A cell with only one of each kind of chromosome is called haploid ( n ).
Sex cells have one of each kind of chromosome so that when they combine (as egg and sperm do during fertilization), the resulting cell is diploid .
Each species has a specific number of chromosomes.
Humans have 23 pairs ( 46 total)
Fruit Flies have 4 pairs ( 8 total)
Dogs have 39 pairs ( 78 total)
Humans have 23 pairs of chromosomes (46 total)
22 pairs of autosomes
1 pair of sex chromosomes
Half of each pair came from one parent and half came from the other parent
Mitosis divides one diploid cell to form two diploid cells
For example: A human cell with 46 chromosomes divides to form two cells with 46 chromosomes.
If each parent were to pass on a diploid cell to the offspring, that offspring would then have 4 copies of each chromosome
46 chromosomes from each parent would yield a 92 chromosome offspring
Meiosis allows for two divisions to divide a one diploid cell into four haploid cells.
Meiosis: Where and Who?
Meiosis takes place in the gonads (sexual organs)
For humans , these are the ovaries and testes
The process of meiosis produces egg and sperm cells
Two gametes come together by fertilization
The haploid sperm and egg join to form a diploid zygote
Before Meiosis (just like before Mitosis) the cell must prepare for division:
Cells increase in size
DNA is replicated
Necessary proteins and RNA are synthesized
During this phase, chromosomes are not yet visible .
Meiosis: Prophase I
Chromosomes become visible
Nuclear envelope disappears
Centrioles head to opposite poles and spindle forms
Homologous chromosomes (one pair of sister chromatids from the mother and one from the father ) pair up to form a tetrad
The tetrad pairs up so tightly that crossing over occurs
Meiosis: Metaphase I
Spindle fibers attach to the centromeres
Tetrads line up along the equator (or middle of the cell)
Note that homologous chromosomes line up together along the equator in Meiosis where in Mitosis, they lined up independently to one another.
Meiosis: Anaphase I
Homologous chromosomes separate and head to opposite ends of the cell
Centromeres DO NOT split – Sister chromatids will stay together until the next division
Meiosis: Telophase I and Cytokinesis
Spindle is broken down
Cytoplasm divides into two cells
Meiosis: Prophase II
Chromosomes become visible
If nuclear membrane reformed after Telophase I, it will break down now
Meiosis: Metaphase II
Spindle pulls the sister chromatids to the middle of the cell where they line up along the equator in random order (just as they did during Mitosis)
Meiosis: Anaphase II
Centromere of each sister chromosome splits and each sister chromatid heads for an opposite pole
Meiosis: Telophase II and Cytokinesis
Nuclei reform (nuclear envelope reappears)
Spindle breaks down
Cytoplasm divides into a total of four haploid cells that will become gametes
Each cell contains one chromosome from each homologous pair
Let’s See it!
Meiosis has a large role in maintaining variability in a species.
Through sexual reproduction , offspring are not simply replicas of one organism but a genetic combination of two organisms
Crossing over during Prophase I insures that a parent organism can pass on different gametes each time it reproduces, creating a variety of offspring.
Chromosomal mutations can happen when chromosomes break and do not repair correctly.
Errors can also occur during Meiosis .
Sometimes the homologous chromosomes do not separate properly – this is called nondisjunction
This results in gametes with either an extra copy of a chromosome or no copy at all.
Normal Example Nondisjunction Examples
Types of Nondisjunction
Remember: In normal fertilization, a zygote would get one copy of a chromosome from each parent resulting in one pair of each type of chromosome (humans: 23 pairs)
Monosomy – when the zygote gets a copy of a chromosome from only one parent so it is missing one chromosome
Most zygotes with monosomy do not survive
One exception is the case of Turner’s Syndrome
Females have only one X chromosome instead of two
These people will still have female sexual characteristics but they will generally be underdeveloped
Types of Nondisjunction
Trisomy – In this case, the zygote gets one copy of a chromosome from one parent and two copies from the other parent resulting in three copies rather than the normal two copies.
Down Syndrome (Trisomy 21) – This person has three copies of the 21 st chromosome. This can lead to mental retardation, susceptibility to certain illness or diseases, and a shorter life span
Klinefelter's syndrome ( XXY ) – This person has two copies of the X chromosome as well as a copy of the Y chromosome. This person will be male but may suffer from underdeveloped testicles and infertility.
Identifying Chromosomal Disorders
To determine whether or not an organism has the proper number of each chromosome, one can look at a karyotype
To make a karyotype a photograph is taken of the paired chromosomes during metaphase
These pairs are cut out and arranged in a chart according to length and location of centromere
Once arranged, it is easy to see if there are any extra or missing chromosomes
This individual has an extra Y chromosome
In the development of most multicellular organisms, a single cell ( fertilized egg ) gives rise to many different types of cells, each with a different structure and corresponding function .
The fertilized egg gives rise to a large number of cells through cell division , but the process of cell division alone could only lead to increasing numbers of identical cells.
As cell division proceeds, the cells not only increase in number but also undergo differentiation becoming specialized in structure and function.
The various types of cells (such as blood, muscle, or epithelial cells) arrange into tissues which are organized into organs , and, ultimately, into organ systems .
Nearly all of the cells of a multicellular organism have exactly the same chromosomes and DNA .
During the process of differentiation, only specific parts of the DNA are activated; the parts of the DNA that are activated determine the function and specialized structure of a cell.
Because all cells contain the same DNA, all cells initially have the potential to become any type of cell .
Once a cell differentiates, the process can not be reversed .
Stem cells are unspecialized cells that continually reproduce themselves and have, under appropriate conditions, the ability to differentiate into one or more types of specialized cells.
Embryonic cells, which have not yet differentiated into various cell types, are called embryonic stem cells .
Stem cells found in adult organisms, for instance in bone marrow, are called adult stem cells .
Scientists have recently demonstrated that stem cells, both embryonic and adult , with the right laboratory culture conditions, differentiate into specialized cells .