Download Biology Notes - Chapter 6 SECTION 1

Biology Notes - Chapter 6
SECTION 1
Your body is made up of many types of specialized cells, but all of them
can be divided into two major categories
1. Somatic cells AKA body cells
a. These make up most of your body tissues and organs
2. Gametes AKA sex cells
a. These are cells that make up the reproductive organs
b. In females – called ova or eggs
c. In male – called spermatozoa or sperm
d. DNA is found in our sex cells and can be passed down to our
children.
As you recall, our DNA is made up of chromosomes. Each chromosome
resembles an X which is made from 2 sister chromatids.
We have 46 chromosomes in our body or 23 pairs of chromosomes.
 One pair or set of 23 chromosomes comes from each parent (one
set from mom and one set from dad)
 Each pair is said to be homologous or having the same structure
Your body is made up of two types of chromosomes
1. Autosomes
a. Made up of pairs 1-22
b. These are homologous chromosomes
2. Sex chromosome
a. Made up from pair 23
b. These determine if you are male (XY) or female (XX)
c. The X chromosome is bigger and can only be given by the
mother since that is all that she has to give. Contains other
genes that are unrelated to sexual characteristics.
d. The Y chromosome is smaller and only contains genes for male
sex traits.
Sexual reproduction vs fertilization
 Sexual reproduction involves the fusion of two gametes that results
in offspring that are a genetic mixture of both parents.
 Fertilization is the actual fusion of an egg and a sperm
Diploid cells have 2 copies of each chromosome, one from the mother
and one from the father
 Di means double
 Represented in charts as a 2n
 Autosomal cells are diploids
Haploid cells have only one copy of each chromosome
 Hap means half
 Represented in charts as an n
 Sex cells are haploids
Polyploids are cells with many copies of each chromosome
 Can be commonly found in plants and some animals such as
goldfish, salmon and salamanders.
 These organisms will usually have even chromosome sets such
as 54, 72 or 90.
Meiosis is a form of nuclear division that divides a diploid cell into haploid
cells.
 Has to occur to produce gametes
 This process is also called reduction division because it reduces
the number of chromosomes by half.
Spermatogenesis vs oogenesis
SECTION 2
Homologous chromosomes - One chromosome comes from the mother
and one comes from the father
There are two cell divisions that occur.
Mitosis is a process of cell division that results in two genetically
identical daughter cells developing from a single parent cell.
Meiosis is the division of a germ cell involving two fissions of the
nucleus and giving rise to four gametes, or sex cells, each possessing
half the number of chromosomes of the original cell.
Mitosis is used by single-celled organisms to reproduce; it is also used
for the organic growth of tissues, fibers, and membranes.
Meiosis is found in sexual reproduction of organisms. The male and
female sex cells (i.e., egg and sperm) are the end result of meiosis;
they combine to create new, genetically different offspring.
Mitosis has one division and meiosis has two divisions. You still have to
remember PMAT, but now you do it twice. You also need to remember
that four cells are created where there was originally one. That's four (4)
cells with half of the amount of DNA needed by a cell. When a cell goes
through meiosis, it's not concerned about creating another working cell.
Meiosis happens when it's time to reproduce an organism. The steps of
meiosis are very simple. When you break it down its just two PMAT's in a
row. Scientists say Meiosis I and Meiosis II, but it's just two PMATs. The
interphase that happens between the two processes is very short and the
DNA is not duplicated.
Step One aka PMAT 1
MEIOSIS I:
 This is basically like the PMAT of a regular mitosis.
 Prophase I the duplicated DNA condenses into compact structures,
the nuclear envelope surrounding the DNA begins to break down.
 Metaphase I the chromosomes align in the center of the cell,
centrioles move to the polar ends of the cell and project thin spindle
fibers to connect to the center (centromeres) of each chromosome.
 Anaphase I the centrioles will begin to pull each chromosome into
two halves called sister chromatids.
 Telophase I the nuclear membrane forms around the chromatids
and they are completely located at opposite ends of the cell. Usually
after Telophase the cell will also divide its cytoplasm and pinch off
into two separate but identical daughter cells.
Step Two aka PMAT 2
MEIOSIS II:
 In Prophase II the DNA that remains in the cell begins to
condense and form short chromosomes.
 In Metaphase II all of the chromosomes line up along the center
of the cell and the centrioles are in position for the duplication.
 Anaphase II shows the chromosomes split and move to
opposite sides of the cell. Each one splits into two pieces. They
don't divide up the DNA between the new cells; they split the
DNA that exists. Each daughter cell will get one-half of the DNA
needed to make a functioning cell.
 Telophase II shows the DNA completely pulled to the sides and
the cell membrane begins to pinch.
 When it's all over, you are left with four haploid cells that are
called gametes.
Mitosis
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Produces genetically identical cells
Results in diploid cells
Takes place throughout an organisms lifetime
Involved in asexual reproduction
Chromosomes are divided here
1 cell division
Meiosis
 Produces genetically unique cells
 Results in haploid cells
 Takes place only at certain times in an organisms life cycle
 Involved in sexual reproduction
 Sister chromatids are divided here
 2 cell divisions
SECTION 3
Heredity is the passing of traits or characters from parents to offspring
Traits are distinguishing characteristics that are inherited and passed
from one generation to the next
 Ex. Eye color, leaf shape, tail length
Genetics is the study of biological inheritance patterns and variations in
organisms
Gregor Mendel is known as the scientist who laid the understanding for
genetics.
 He studied plant variations (peas) because of his background
 He discovered that there were patterns of inheritance based on
appearance.
 He was the first to develop rules that accurately predict
patterns of heredity
Mendel chose to use pea plants in his experiments for several reasons
o
o
o
o
They reproduce quickly
Produce many offspring
He could easily control the mating of them
The sex organs (both female and male) of the plant are in
the flower
o It typically self-pollinates (mates with itself)
o Pea plant contains several traits displayed plainly
Mendel observed seven traits that are easily recognized and apparently
only occur in one of two forms:
1. flower color is purple or white
5. seed color is yellow or green
2. flower position is axil or
6. pod shape is inflated or
terminal
constricted
3. stem length is long or short
7. pod color is yellow or green
4. seed shape is round or wrinkled
Pure breed (sometimes called a true breed)– a type of organism whose
ancestors are genetically uniform
 Organism is only breed or mated with other organisms in the
same breed
 Ex purple flower with a purple flower or Great Dane with a
Great Dane
 All of Mendel’s experiments started with a purebred pea plant.
 All pea plants were allowed to self-pollinate for several
generations to ensure pure bred
Cross or crossbred refers to mating or breeding of two individual
organisms.
Mendel’s first experiments
 He bred a purebred purple flower and a purebred white flower
 He then allowed the purple and white purebreds to cross pollinate
(also known as cross fertilize), these were considered the parental
generation.
 The results were that the flowers had hidden traits because the
flowers were purple with no white showing. These were called the
Filial 1 generation.
 The F1 generation was allowed to self-pollinate.
 The results were that white flowers started to appear. These were
called the Filial 2 generation.
 He continued these experiments with the other traits from the peas
that he was studying.
Parental (P) --- Filial 1 (F1) --- Filial 2 (F2)
Filial means son or daughter
Mendel’s Results from his experiments
 He discovered that in F1 plants that only one form of a trait would
show.
 He discovered that after F1 self-pollination that some of the missing
traits reappeared in the F2 generation.
 He discovered that the results were a 3:1 ratio from this
monohybrid cross.
 These results are now known as Law of Segregation and Law of
Dominance
SECTION 4
Gene
 a segment of DNA (on a specific site on a chromosome) that is
responsible for the physical and inheritable characteristics or
phenotype of an organism.
 A gene is formerly called a factor
Genome is all of an organism’s genetic material
Allele is any of the alternative forms of a gene that may occur at a specific
location.
 Cells have 2 alleles for each gene
 1 on each homologous chromosome
 1 given from each parent
There are 2 types of alleles
1. Homozygous
a. Alleles are identical to each other
i. Both for purple flowers or both or white flowers
2. Heterozygous
a. Alleles are different from each other
i. Ex one for purple flowers and one for white flowers
Mendel’s hypotheses from his experiments make up the Mendelian
Theory of Heredity
1. For each inherited trait, an individual has 2 copies of the gene,
one from each parent
2. There are alternative versions of genes that we call alleles
3. When 2 different alleles occur together one of them may be
completely expressed while the others may have no observable
effect on the organisms appearance
a. We call these dominant and recessive traits
4. When gametes are formed, the alleles for each gene in an
individual separate independently of one another. Thus gametes
carry only one allele for each inherited trait. When gametes unite
during fertilization each gamete contributes one allele.
Genes influence the development of traits
 Dominant trait
o The trait that is shown or expressed when two different alleles
are present
o Represented by a capital letter = T
 Recessive trait
o The trait that is only shown or expressed when two copies are
present
o Represented by a lower case letter = t
 Homozygous
o 2 alleles that are the same in an organism = TT or tt
 Heterozygous
o 2 different alleles in an organism = Tt
Genotype
 A set of alleles that an individual has
 The genetic makeup of a specific set of genes
o Ex BB
Phenotype
 The physical characteristics or traits of an individual that we see
o Ex brown eyes
SECTION 5
Punnett square
o A grid system for prediction all possible genotypes resulting
from a cross
o A diagram that predicts the outcome of a genetic cross by
considering all possible combinations of gametes in the cross.
Monohybrid cross is the Punnett square that only looks at one trait
Dihybrid cross is the punnett square that looks at two traits
Law of independent assortment states that the alleles of different genes
separate independently of one another during gamete formation
Probability is the likelihood that is a specific event will occur
o Expressed as a fraction
o Probablility = # of one kind of possible outcome
Total # of all possible outcomes
o Ex flipping a coin
SECTION 6
Before Mendel people thought offspring were just a mix or blend of
parents
o Ex if one parent was tall and one parent was short then the child
would be medium height
He concluded from experiments that each pea has two separate heritable
factors for each trait, one from each parent
o When gametes (sperm and egg cells) form, each receives only one of
the organisms two factors, or genes, for each trait
Crossing over is an exchange of genes or chromosomes.
o The genes are mixed up, not resulting in a perfect duplicate like
mitosis.
o The cell divides, leaving two new cells with a pair of chromosomes
each.
o This allows for increased genetic diversity
o Occurs in Prophase 1 of meiosis 1
o Very important because it allows genetic combinations to be
practically unlimited
Meiosis is an important process that allows for the rapid generation of
new genetic combinations
3 mechanisms make key contributions to the genetic variations
1. Independent assortment
2. Crossing over
3. Random fertilization
Importance of genetic variations
o Speeds up the process of evolution by genetic recombination’s
o Combining genes from two organisms resulting in a third not
identical to either parent