Genes, Chromosomes and
DNA
(Chapter 2)
The structure of DNA
• Composed of 4 nucleotide
bases, 5 carbon sugar and
phosphate.
• Base pair = rungs of a ladder.
• Edges = sugar-phosphate
backbone.
• Double Helix
• Anti-Parallel
The structure
Figure 2.21of DNA
DNA Replication
•
•
•
•
Adenine (A) always base pairs with thymine (T)
Guanine (G) always base pairs with Cytosine (C)
ALL Down to HYDROGEN Bonding
Requires steps:
– H bonds break as enzymes unwind molecule
– New nucleotides (always in nucleus) fit into place
beside old strand in a process called
Complementary Base Pairing.
DNA
Replication
Figure
2.22a
Remember – the two strands run in
opposite directions
Synthesis of a new (daughter) strand
occurs in the opposite direction of the
old (parental) strand.
Complementary base-pairing occurs
A with T and G with C
G and C have three hydrogen bonds
A and T have two hydrogen bonds
DNA
Replication
Figure
2.22b
DNA
Replication
Figure
2.22c
New nucleotides joined together by enzyme DNA Polymerase
DNA Replication
• Each new double helix is composed of an old
(parental) strand and a new (daughter) strand.
• As each strand acts as a template, process is called
Semi-conservative Replication.
• Replication errors can occur. Cell has repair
enzymes that usually fix problem. An error that
persists is a mutation.
• This is permanent, and alters the phenotype.
Gregor Mendel observed
phenotypes and formed hypotheses
• How do offspring come to resemble their parents physically?
• Genetics begins with the unifying assumption that biological
inheritance is carried by structures called Genes.
• The same basic patterns of inheritance apply to most
organisms.
• The inheritance of some human traits can be explained from
work on plants
• Sex-linked traits in humans is more complicated
• Gregor Mendel
– Was the first
person to analyze
patterns of
inheritance
– Deduced the
fundamental
principles of
genetics
Terms:
• Phenotype
– An organism’s physical traits
• Genotype
– An organism’s genetic makeup
Allele
• Allele: Alternate form of a gene at same
position on pair of chromosomes that affect the
same trait.
• Dominant Allele: Capital Letter--O
• Recessive Allele: lowercase letter--o
• Homozygous Dominant--OO
• Homozygous Recessive--oo
• Heterozygous--Oo
Mendel’s Peas
– These plant are
easily
manipulated
– These plants
can self-fertilize
Stamen
Carpel
Garden pea
Dominant
Flower color
Purple
Recessive
Axial
Terminal
Seed color
Yellow
Green
Round
Recessive
Pod shape
Inflated
Constricted
Pod Color
Green
Yellow
Stem length
Tall
White
Flower position
Seed shape
Dominant
Wrinkled
Dwarf
Monohybrid Crosses
P Generation
(true-breeding
parents)
Purple
flowers
White
flowers
All plants have
purple flowers
F1 Generation
Fertilization
among F1 plants
(F1  F1)
F2 Generation
F2 = 3:1 ratio
3/
of plants
have purple
flowers
4
1/
of plants
have white
flowers
4
• Using a
Punnett square
to explain the
results of a
monohybrid
cross
Gametes
Genotypic ratio
1 PP : 2 Pp : 1 pp
All
P
All p
F1 plants:
(hybrids)
Gametes
F2 plants:
Phenotypic ratio
3 purple : 1 white
PP
PP
P plants
All Pp
1/
2
P
Eggs
1/
P
P
PP
p
2
p
Sperm
p
Pp
Pp
pp
Figure 9.8b
• from the monohybrid crosses, Mendel derived 4
hypotheses….combined, we now refer to these
as…
= Mendel’s Principle of Segregation
– There are alternative forms of genes, now called alleles
– For each characteristic, each organism has two genes
– Gametes carry only one allele for each inherited
characteristic
– Alleles can be dominant or recessive
Mendel’s PrinciPle of indePendent
Assortment
• What happens when you follow the
inheritance of more than a single trait at one time?
• How do two different traits get passed to offspring?
A Dihybrid Cross
RRYY
Dihybrid Cross
rryy
ry
Gametes RY
RrYy
RY
Eggs
RrYy
rY
Ry
ry
RY
RRYy
RrYy
RRYy
RrYy
RrYy
RRyy
rrYy
Ry
RrYY
rrYY
Rryy
rrYy
Rryy
rryy
RrYy
rY
RRYY
RrYY
RrYy
Sperm
ry
9/
3/
Yellow round
16
Green round
16
3/
16
1/
16
Yellow
wrinkled
Green
wrinkled
9:3:3:1
• Mendel’s principle of independent
assortment
– Each pair of alleles segregates independently of
the other pairs during gamete formation
a
P
P
Genotype:
PP
a
aa
B
b
Bb
Using a Testcross to Determine an
Unknown Genotype
• A testcross is a
mating between:
– An individual of
unknown
genotype and
– A homozygous
recessive
individual
Testcross:
Genotypes
P_
pp
Two possibilities for the purple flower:
PP
Gametes
P
P
Offspring
Pp
Pp
All purple
P
p
Pp
p
pp
1 purple : 1 white
Incomplete Dominance in Plants
and People
Red
RR
• In incomplete
dominance F1
hybrids have
an appearance
in between the
phenotypes of
the two
parents
White
rr
Gametes
r
R
Pink
Rr
1/
Gametes
1/
Eggs
1/
2
2
2
R
1/
2
1/
R
r
r
2
Sperm
R
Red
RR
1/
2
r
Pink
rR
Pink
Rr
White
rr
Figure 9.18
Types of cells
•
•
•
•
•
Not all cells of an organisms have the
same number of chromosomes.
Two types of cells:
Somatic Cells
Gametes
Somatic Cells
• Non-sex Cells.
• These cells do not carry
genetic information for
sexual reproduction.
• Contain a full compliment
of chromosomes
• Characteristic to their
species.
• Referred to as the diploid
number of chromosomes.
• Diploid
• Means double number.
• Designated 2n
• All somatic cells in an
organism have the 2n or
diploid number of
chromosomes.
Gametes
• Sex Cells
• Cell which carry genetic
information for sexual
reproduction.
• Haploid
• Means single number.
• Designated n
• Contain one half the
compliment of chromosomes
characteristic to their species. • All gametes formed by an
organism have the n or
haploid number of
• Referred to as the haploid
number of chromosomes.
• chromosomes.
Human Life Cycle
•
•
•
•
Adults produce gametes--egg and sperm.
Gametes fuse to produce zygote.
Zygote grows and develops to produce baby.
Meiosis--process of division that produces
gametes.
• Mitosis--process of replication and division
required for growth.
• Adults, zygote and baby--2n. 2n=diploid
• Gametes--n. n=haploid
Mitosis
• Process of division that produces two daughter
cells with identical chromosomal content of
parent cell.
• Mitosis is one stage of the cell cycle.
• Cell cycle--cycle of stages a cell goes through
in order to grow and divide.
• Stages: I=Interphase, Growth 1=G1, DNA
synthesis=S, Growth 2=G2, Mitosis=M
The Human cell cycle
•
•
•
•
Interphase--G1, S, G2
Mitosis--M
G1--growth
S--DNA Synthesis,
replication
• G2--growth
• M:
– mitosis-- nuclear division
– cytokinesis--cell division
Stages of Division- Mitosis
• Prophase--nuclear envelope breakdown,
chromosome condensation, spindle formation.
• Metaphase--chromosomes are lined up
precisely on the metaphase plate, or middle of
the cell.
• Anaphase--spindle pulls sister chromatids
apart.
• Telophase--chromatids begin to decondense
and become chromatin. Spindle disappears.
• Cytokinesis--divide cell and organelles. Actin
ring, or cleavage furrow splits cell.
Gamete Production -Meiosis
• In order to reproduce we must produce
gametes.
• Gametes are sperm and egg.
• Why is that siblings are not identical?
• Meiosis blends DNA from parental
contributions to produce a mixed up “half”
or haploid, set of DNA.
• Crossing over is critical for producing
haploid DNA with genetic diversity.
The Process of Meiosis
Interphase
– Haploid gametes are
produced in diploid
organisms
– Two consecutive
divisions occur,
meiosis I and
meiosis II, preceded
by interphase
Centrosomes
(with
centriole
pairs)
Nuclear
envelope
Chromatin
Chromosomes duplicate
Prophase -I
Replicated pairs of chromosomes
line up side by side.
These pairs are called Homologous-both have same gene order (gene
for eye color, hair color, etc).
Sister chromatid from one pair
interact with a Sister chromatid from
another pair.
One sister is from father, one sister
from mother, but they have same
gene order.
Prophase -I
• This interaction is called
Synapsis.
• Synapsis results in the
formation of a Tetrad
(4
sisters together).
• Crossing over swaps
sections of homologous
genes.
Meiosis - I
Figure 2.9 (1)
Prophase I
Metaphase I
Anaphase I
Telophase I
Meiosis - II
Figure Prophase
2.9 (2)
II
Metaphase II
Anaphase II
Telophase II
•Meiosis I
Meiosis I: Homologous
chromosomes separate
Prophase I
Sites of crossing over
Metaphase I
Microtubules attached
to Chromosomes
Anaphase I
Sister chromatids
remain attached
Telophase I
and Cytokinesis
Cleavage
furrow
Spindle
Sister
chromatids
Tetrad
Homologous
chromosomes
pair and exchange
segments
Centromere
Tetrads line up
Pairs of homologous
chromosomes
split up
Two haploid cells
form: chromosomes
are still double
•Meiosis II
Meiosis II:
Sister chromatids separate
Prophase II
Metaphase II
Anaphase II
Telophase II
and Cytokinesis
Sister chromatids
separate
Haploid daughter cells
forming
During another round of cell division, the sister chromatids finally separate;
four haploid daughter cells result, containing single chromosomes
Sexual life cycles
• Haploid Gametes join to form a zygote
• Somatic cells divide by Mitosis to produce adult organism
• Meiosis produces gametes in sex cells
Genes on sex chromosomes determine
Sex and sex-linked traits
• Micrograph of the chromosomes of an organism paired and
numbered.
• Used to check for chromosomal abnormalities in individuals.
Sex Determination
• All embryos start on a neutral or
"indifferent" path. The 4 week old
embryo is indifferent
• By 7 weeks, the SRY (sex-related)
gene encoded by the short arm of
the Y chromosome begins to roar!
• Testis determining factor converts
progesterone to testosterone
Sex Determination
• Indifferent embryos have two sets of
ducts:
• Müllerian ducts--will be come the
future oviducts--thus female.
•
Wolfian ducts--will become the
future vas deferens--thus male
– dependent on testosterone for its
continued development
• The testes also produce an antiMüllerian hormone that promotes
regression of the Müllerian ducts
– without SRY, the indifferent embryo
will naturally develop into a female
Sex Determination
Two copies of DAX (double X) inactivate SRY, thus this
individual would be genetically male, but look female.
Sex Determination
SexFigure
Determination
2.16 (3)
Sex Chromosomal Disorders
• Turner Syndrome – XO only one sex
chromosome
• Short, thick neck and stature
• Do not undergo puberty, or menstruate,
• no breast development
• Kleinfelter Syndrome – XXY
•
•
•
•
•
Testis and prostate underdeveloped
No facial hair
Brest development
Long arms and legs: big hands and feet
Can be mentally retarded
An XY Individual with Androgen
Insensitivity Syndrome
Androgen Insensitivity Syndrome is a sex reversal condition where XY
individuals look female. These individuals have the Y chromosome and
functional SRY. These individuals have testis which generate AMH and
testosterone. However, the genetic mutation results in a lack of the
testosterone receptor. Estrogens are made in the adrenal gland which drive
phenotypic development. As adults, these individuals have testes in the
abdomen and lack a uterus and oviducts.
Issues of sex determination
• So, clearly being Male or Female isn’t as cut and dry
as people have been led to believe!
• 17 out of 1,000 people are really neither
– XY, but Female anatomy
– XX, but Male anatomy
– Both Female and Male anatomy
• Other genes, such as testosterone receptor on
chromosomes other than X and Y chromosomes
have a role in sexual development
Issues of sex determination
• So, some people fall between Male and Female
– Due to chromosomal variation
– Variations in SYR gene
– Testosterone receptor gene
• All of the different variations are known as intersex
– These people are altered surgically to make them
conform to one sex or the other
• Should there be five genders instead of two?
Summary
• Genetics is the study of biological traits. These
traits are coded for in genes, which are parts of
chromosomes.
• An Allele is a variant of a gene. These can be
dominant or recessive, and these are the basis of
inherited traits, both structural and behavioral.
• Chromosomes exist as homologous pairs.
Summary
• Somatic Cells - Non-sex Cells. Contain a full
compliment of chromosomes. Characteristic to
their species. Referred to as the diploid number of
chromosomes.
• Gametes - Sex Cells. Cell which carry genetic
information for sexual reproduction. Contain one
half the compliment of chromosomes characteristic
to their species .Referred to as the haploid number
of chromosomes.
The End.
Any Questions?