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Development
Chapter Three
Why are we interested in
Development?
1. Genes regulate every step of
development
2. Understanding what is normal will help
frame what is not
3. It affects every one of us here
How are genes inherited?
• Genes are carried in the DNA
• DNA is condensed into chromosomes
• Each individual has two copies of every
chromosome
• Sex cells (sperm or eggs) each have one
copy of every chromosome
• Mating leads to one copy of every
chromosome coming from one parent and
other copy coming from the other parent
– Variances are mixed in offspring
Reproductive System
• Gametes are formed in the gonads
– Diploid cells are divided into separated
haploid gametes
• Males – form sperm in seminiferious
tubules
• Females – form oocytes in ovaries
• Sexual reproduction allows mixing of two
haploid cells into one diploid zygote
Sexual vs. asexual
Reproduction
Sexual:
Asexual:
• Two sexes
• Must come together
to mate
• Only half population
has offspring
• One sex (gender)
• Doesn’t need another
to reproduce
• All of population has
offspring
• Introduce variety into
population
• Every individual is
genetically identical
Germline vs. Somatic Cells
Germline:
Somatic:
• “Sex cells”
• Non sex cells
– Sperm and oocytes
– Hair, eyes, gut, etc
• Gametes
• Haploid (1N)
• Formed by Meiosis
• Not gametes
• Diploid (2N)
• Formed by Mitosis
Meiosis
Two Stages:
1. Meiosis I – dividing and reducing
•
•
•
2N becomes 1N
2 chromosomes become 1 chromosome
4 chromatid become 2 chromatid
2. Meiosis II – dividing
•
•
Exactly the same as Mitosis
2 chromatid become 1 chromatid
Meiosis
Phases
Prophase – chromosomes condense and
become visible
Metaphase – chromosomes align
Anaphase – paired chromosomes or sister
chromatids separate by centromeres
Telophase – cytokinesis divides into two
cells
Crossing Over
• Not the TV show
• Homologous
Chromosomes actually
carry different alleles of
same genes
• Genotype – represents the
two alleles carried
• In Metaphase I, crossing
over can occur
Sex Introduces Variety
1. Crossing Over – changes which alleles
are on which chromatid
2. Meiosis – which alleles will be inherited
together
3. Sexual Reproduction – mixing of two
parents’ alleles
Evolution can act upon different alleles
•
keeping “beneficial” and removing
“detrimental” alleles
Spermatogenesis
Insert figure 3.7
Oogenesis
Insert figure 3.11
Important Differences:
Male Meiosis:
Female Meiosis:
1. Begins at puberty
2. Completes meiosis
immediately
1. Begins before birth
2. Meiosis is arrested at
Prophase I from
birth until ovulation
Arrested again at
Metaphase II unless
fertilized
3. Produces one ovum,
and three polar
bodies
3. Produces 4 equal
sperm cells
Fertilization:
Insert figure 3.13
After Fertilization
• Ovum’s nuclear membrane dissolves
• Chromosomes from sperm cell and
chromosomes from ovum come together
• Forming “Pronucleus”
• Zygote – now has 23 pairs of
chromosomes again (2N)
• Zygote begins mitosis or cleavage
Implantation
Insert figure 3.14
The Embryo Develops:
Zygote
Morula
Blastocyst
Blastocyst
Gastrula
Early Embryo Timeline
Ovulation
Day 1
Day 2 Day 3
Fertilization
Morula
Day 4 Day 5
Implantation
Day 6 Day 7
Blastocyst
Day 8 Day 9
Day 14
Gastrulation
Gastrulation:
• Blastocyst begins to fold forming
three layers:
Ectoderm: Outer layer
– Skin, nervous tissue
Mesoderm: Middle layer
– Muscle, bones
Endoderm: Inner layer
– Internal organs
Embryo Timeline
Fertilization
Week 1 Week 2
Implantation
Gastrulation
Fetus
Week 3 Week4 Week5 Week 6 Week 7 Week 8
Week 9
Organogenesis:
Embryo transforms from three simple layers to form
all the structures that will be present at birth:
• Spinal cord and brain (3 - 4 weeks)
• Internal organs and limbs (3 - 7 weeks)
• Muscles and bones (7 - 8 weeks)
Placenta
• ~ 3 weeks after conception chorionic villi
connect the developing embryo to the
mother’s blood supply
• Chorionic villi becomes placenta
• ~ 10 weeks, the placenta is fully formed
• “3rd Brain”
– Secretes hormones
– Send nutrients and O2 to fetus
Twins
• Monozygotic – come from one zygote
– MZ twins
– Absolutely identical DNA (100 %)
– May have different environments
• Dizygotic – arise from two eggs, forming
two separate zygotes
– DZ twins
– Siblings that happen to share a womb
– Share 50 % of DNA
Fetal Period
• Weeks 8 to 40 (until birth)
• Sex differences physically appear 6 to 12
weeks
– Determined by SRY gene on Y chromosome
• Fetus develops:
– Hair, nails, vocal cords, and digestive system
– Lungs develop last (~ 37 weeks)
• Mostly this is a time for specialization and
growth
Birth Defects
1. Genetic Abnormalities
•
•
•
Inherit mutated version of a gene
De novo mutation
Chromosomal abnormalities
2. Environmental exposures
•
•
Toxins, drugs
Poor diet or lifestyle of mother
3. Viruses
•
Rubella, HIV, etc
Next Class:
• Enjoy Labor Day Holiday!
• Read Chapter Four
(Now we are going to start actually learning
Genetics!)
• Homework – Chapter Three Problems;
– Review: 1, 4, 5
– Applied: 3, 5, 6, 9, 11