Genes, Chromosomes and DNA

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Genes, Chromosomes and DNA

• (Chapter 2)

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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

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Figure 2.21
The structure of DNA
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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.

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Figure 2.22a
DNA Replication
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
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Figure 2.22b
DNA Replication
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Figure 2.22c
DNA Replication
New nucleotides joined together by enzyme DNA
Polymerase
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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.

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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

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• Gregor Mendel

• Was the first person to analyze patterns of
  inheritance
• Deduced the fundamental principles of genetics

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Terms

• Phenotype

• An organisms physical traits
• Genotype
• An organisms genetic makeup

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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

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Mendels Peas

• These plant are easily manipulated
• These plants can self-fertilize

Stamen
Carpel
Garden pea
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Dominant
Recessive
Dominant
Recessive
Pod shape
Constricted
Inflated
Flower color
White
Purple
Pod Color
Green
Yellow
Flower position
Axial
Terminal
Seed color
Yellow
Green
Stem length
Dwarf
Tall
Seed shape
Round
Wrinkled
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Monohybrid Crosses
P Generation (true-breeding parents)
Purple flowers
Whiteflowers
All plants have purple flowers
F1 Generation
Fertilization among F1 plants (F1 ? F1)
F2 Generation
1/4 of plants have white flowers
F2 31 ratio
3/4 of plants have purpleflowers
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PP
PP
P plants

• Using a Punnett square to explain the results of
  a monohybrid cross

All
P
Gametes
p
All
F1 plants (hybrids)
All Pp
P
Gametes
1/2
1/2
p
P
P
Sperm
Eggs
F2 plants
PP
p
p
Phenotypic ratio 3 purple 1 white
Pp
Pp
pp
Genotypic ratio 1 PP 2 Pp 1 pp
Figure 9.8b
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• from the monohybrid crosses, Mendel derived 4
  hypotheses.combined, we now refer to these as

Mendels 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

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Mendels 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
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Dihybrid Cross
RRYY
rryy
ry
Gametes
RY
RrYy
RY
RY
Sperm
Eggs
RrYy
RrYy
rY
rY
RRYY
Ry
Ry
RrYY
RrYY
ry
ry
RRYy
rrYY
RRYy
RrYy
RrYy
RrYy
RrYy
Yellow round
9/16
RRyy
Green round
rrYy
rrYy
3/16
Yellow wrinkled
Rryy
Rryy
3/16
rryy
1/16
Green wrinkled
9331
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• Mendels principle of independent assortment

• Each pair of alleles segregates independently of
  the other pairs during gamete formation

a
P
B
a
b
P
aa
Bb
PP
Genotype
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Using a Testcross to Determine an Unknown Genotype

• A testcross is a mating between

Testcross

• An individual of unknown genotype and
• A homozygous recessive individual

Genotypes
P_
pp
Two possibilities for the purple flower
PP
Pp
Gametes
P
p
P
P
p
Pp
pp
Pp
Offspring
All purple
1 purple 1 white
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Incomplete Dominance in Plants and People
Red RR
White rr

• In incomplete dominance F1 hybrids have an
  appearance in between the phenotypes of the two
  parents

r
Gametes
R
Pink Rr
1/2
1/2
r
R
Gametes
R
1/2
R
Sperm
1/2
Eggs
Red RR
1/2
r
r
1/2
Pink rR
Pink Rr
White rr
Figure 9.18
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Types of cells

• Not all cells of an organisms have the
• same number of chromosomes.
• Two types of cells
• Somatic Cells
• Gametes

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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.

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Gametes

• Haploid
• Means single number.
• Designated n
• All gametes formed by an organism have the n or
  haploid number of
• chromosomes.

• 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.

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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. 2ndiploid
• Gametes--n. nhaploid

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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 IInterphase, Growth 1G1, DNA
  synthesisS, Growth 2G2, MitosisM

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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

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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.

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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.

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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
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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.
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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.

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Figure 2.9 (1)
Meiosis - I
Prophase I
Metaphase I
Anaphase I
Telophase I
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Figure 2.9 (2)
Meiosis - II
Prophase II
Metaphase II
Anaphase II
Telophase II
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• Meiosis I

Meiosis I Homologous chromosomes separate
Telophase I and Cytokinesis
Anaphase I
Prophase I
Metaphase I
Microtubules attached to Chromosomes
Sister chromatids remain attached
Cleavage furrow
Sites of crossing over
Spindle
Sister chromatids
Tetrad
Centromere
Tetrads line up
Homologous chromosomes pair and exchange segments
Two haploid cells form chromosomes are still
double
Pairs of homologous chromosomes split up
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• Meiosis II

Meiosis II Sister chromatids separate
Telophase II and Cytokinesis
Prophase II
Anaphase II
Metaphase II
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
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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

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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.

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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

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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 anti-Müllerian hormone
  that promotes regression of the Müllerian ducts
• without SRY, the indifferent embryo will
  naturally develop into a female

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Sex Determination
Two copies of DAX (double X) inactivate SRY, thus
this individual would be genetically male, but
look female.
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Sex Determination
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Figure 2.16 (3)
Sex Determination
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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

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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.
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Issues of sex determination

• So, clearly being Male or Female isnt 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

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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?

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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.

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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.

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The End.

• Any Questions?