Exam 1 - Germ Cells Flashcards
What is the difference between Embryology and Developmental bio. logy?
Embryology - more traditional and descriptive (conception to birth)
Developmental Biology - more modern and explanatory (conception to death)
What is the germ line concept?
The understanding of the continuity from one generation to the next. This goes all the way back to Adam and Eve and will continue on down through your children and their children etc.
What is the practical significance of the germ line concept?
- mutations in the germ line are passed on to all generations
- Gene therapy or enhancement will also effect all generations to come
How does CRISPR (gene editing) work?
- a guide RNA finds the mutated or undesired sequence
- Cas9 nucleotide cuts out that sequence
- It is replaced with a new sequence
What is the danger of gene editing?
Off target cuts can lead to harmful deletions that will affect all generations to come
Gonads
Ovaries and Testicles (organs)
Germ cells
oogonia and spermatogonia (reside in gonads)
Gametes
eggs and sperm (produced by germ cells)
Are germ cells (oogonia and spermatogonia) diploid or haploid?
They are diploid
Why are germ cells considered stem cells?
Because they regenerate themselves
How do germ cells regenerate themselves?
By undergoing mitosis
How do primary “cytes” become secondary “cytes”?
By undergoing meiosis I
Mitosis of germ cells results in:
A primary “cyte” (2C) and a germ cell (2C)
What are the 4 stages of Meiosis?
- Prophase
- Metaphase
- Anaphase
- Telophase
(PMAT)
Prophase I
Chromatin folds into chromosomes
Metaphase I
Chromosomes are paired up in synapsis (with possible crossing over) and line up between poles of the cell
Anaphase I
Homologs separate, pulled to opposite poles
Telophase I
Nuclei (containing duplicated but not paired chromosomes) form?
What is the result of meiosis I?
2C primary “cytes” become 2C secondary “cytes”
Prophase II
Chromosomes reappear
Metaphase II
Chromosomes line up
Anaphase II
Chromosome duplicates separate, pulled to opposite poles of the cell
Telophase II
Nuclei, containing one of each chromosome form
What is the result of meiosis II?
secondary “cytes” (2C) become spermatids or eggs (1C)
At what stage is an oocyte fertilized by a spermatid?
During metaphase II
Why is the meiosis that occurs during the formation of gametes so important?
It produces great genetic diversity
What is the independent assortment of chromosomes in EACH gamete (not taking crossing over into consideration)?
8 million
Give two reasons as to why genetic diversity is good?
- God likes it
2. It ensures survival of a species
What risks are presented during the risk of gamete formation and fertilization?
- non-disjunction of chromosomes leading to aneuplodies
- crossing over can result in deletions, mutations, and duplication’s of chr. segments (which can be lethal)
- It is estimated that 50% of fertilized eggs miscarry in humans and that 50% of these are due to chromosomal problems.
How many days does it take for a sperm to undergo meiosis and maturation?
72 days
Where does sperm maturation occur?
It begins in the seminiferous tubules and continues to mature in the epididymus
When does spermatogenesis begin to take place in humans?
At puberty
If sperm maturation takes approx. 72 days, why are males fertile all the time?
Because different regions of the testes normally initiate spermatogenesis are different times (all the sperm in each region develop in sync) resulting in a constant supply of mature spermatids.
Seminiferous tubules
Long, twisted tubes within the testes (spermatogenesis begins here)
In what direction do sperm move within the seminiferous tubules as they develop?
From the periphery of the tubule to the lumen of the tubule (least to most mature)
What are the 5 stages of spermatogenesis?
- spermatogonia
- primary spermatocytes
- secondary spermatocytes
- spermatids
- spermatozoa
Basement membrane of seminiferous tubules
blood-testes barrier, protects against chemicals, bacteria, etc. (protects the germ line)
What are the genetic changes that occur during spermatogenesis?
spermatogonia (2C) -> (mitosis) => primary spermatocytes (2C) -> (meiosis I) => secondary spermatocytes (2C) -> (meiosis II) => spermatids (1C) => spermatozoa (1C)
What are the 7 cytoplasmic changes during spermatogenesis?
- increase in #
- reduction in size
- formation of acrosome
- formation of a flagellum
- aggregation of mitochondria
- condensation of chromosomes
- synthesis of receptors
Why does the sperm reduce in size during spermatogenesis?
Because it sheds most of its cytoplasm and organelles
What is the benefit of the sperm being very small and compact?
It allows it to swim more effectively
What is the acrosome?
It is a membrane-bound vesicle containing enzymes (located in the head of the sperm)
How does the flagellum of a sperm form?
Via a centriole/centrosome in the tail of the sperm, microtubules elongate from the centriole to form the flagellum)
Why is it important for the mitochondria to aggregate at the base of the flagellum (near the mid-piece)?
Because they provide ATP which runs the molecular motor of the flagellum
Where is the nucleus (containing extra, extra folded chr.) located?
Just behind the acrosome in the head of the sperm
How many receptors are synthesized during spermatogenesis?
2, one is located on the head, the other is located in the acrosome
What are the 3 functions of the sertoli cells?
- hold, even “escalate” developing sperm
- create blood testes barrier by linking with each other, thereby protecting the germ line
- provide nutrients to developing sperm which are otherwise cut off
Where are the sertoli cells located?
Among the developing sperm inside of the seminiferous tubules
What are the 2 functions of interstitial cells?
- produce testosterone which stimulates development of males
- with sertoli cells, inhibit development of female reproductive organs.
Where are the interstitial cells located?
Outside of, but around the seminiferous tubules
How many eggs does a woman have before birth?
1-2 million
Does egg production stop after birth for women?
No, it simply gradually declines until menopause
Oocytes are stalled in _____________ until ovulation.
Metaphase I
How is ovulation regulated?
by FSH (from anterior pituitary gland)
Why does ovulation cease at menopause?
Because with eggs and follicles gone, this source of estrogen disappears.
What is non-disjunction?
The formation of bonds between pairs of chromosomes in oocytes while they await ovulation
What is the risk of non-disjunction?
trisomies and monosomies
Why does the risk of non-disjunction increase with age?
Because the longer the oocytes wait for ovulation, the greater chance there is for bonds to have formed.
How many days does it take an oocyte to mature?
120
How is a haploid genome ensured during oogenesis?
Via meiosis
Why is a stockpile of organelles needed inside of the egg?
Because each daughter cell needs a full set of organelles during the initial divisions of a zygote
How is a stockpile of organelles inside of the egg ensured during oogenesis?
Via uneven cytokinesis, giving most of the cytoplasm to one daughter cell and not the other three (polar bodies). This contributes to the large size of most eggs and polar bodies provide a more ethical alternative to PGD
What does PGD stand for?
Pre-implantation genetic diagnosis
Why is it important to have a stockpile of biomolecules inside of an egg?
Because they can be broken down into amino acids providing a source of energy, the subunits themselves can also be used for cell survival up to 10 days
How are RNA’s stockpiled during oogenesis?
4 sets of genes are available for transcription into RNA (before metaphase), 100’s of copies exist on them and they are further duplicated (gene amplification)
mRNAs are stockpiled in the egg only therefore they are referred to as maternal RNA
How are proteins stockpiled during oogenesis?
The mothers liver produces vitellins which are released into the bloodstream and endocytosed by oocytes
Why do lipids need to be stockpiled during oogenesis?
Because they are needed for membrane formation
What is glycogen stored during oogenesis used for?
To make ATP -> energy
What is the purpose of granulosa (follicle) cells?
to help synthesize molecules, they also provide protection to the oocyte and produce estrogen
What is the total accumulation of stockpiled materials during oogenesis called?
The yolk
How are the egg and developing embryo protected?
Via the zona pellucida as well as the granulosa cells
What is the zona pellucida made up of?
It is a thick layer of glycoproteins (aslo called a chorion in other organisms)
How is the incorporation of a single sperm during fertilization accomplished?
Via 2 different receptors (one on the ZP and the other in the PM) and also via cortical granules (vesicles)
How do cortical granules (vesicles) aid in ensuring that only one sperm fertilizes an egg?
They contain enzymes which destroy sperm receptors immediately upon fertilization
What 3 things aid in providing developmental signals for embryonic development?
- transcription factors - by switching certain genes on and off
- certain molecules - in specific regions signaling for development into specific cells
- location of the oocytes nucleus - may provide a pole from which the embryo develops
