Sexual Reproduction in Humans
Index to this page
Sex Organs of the Human Male
Spermatogenesis
Sperm
LH
FSH
Sex Organs of the Human Female
Oogenesis
Ovulation
Copulation and Fertilization
Pregnancy
The placenta is an allograft
Assisted Reproductive Technology ("ART")
In Vitro Fertilization (IVF)
Intracytoplasmic Sperm Injection (ICSI)
Ooplasmic Transfer
Birth and Lactation
Birth Control
The Problems to be SolvedSexual reproduction is the formation of a new individual following the union of two gametes. In humans and the majority of other eukaryotes — plants as well as animals — the two gametes
differ in structure ("anisogamy") and
are contributed by different parents.Gametes need
motility to be able to meet and unite
food to nourish the developing embryo.In animals (and some plants), these two rather contrasting needs are met by anisogametes:
sperm that are motile (and small)
eggs that contain food.
Sex Organs of the Human MaleThe reproductive system of the male has two major functions:
production of sperm
delivery of these to the reproductive tract of the female. Sperm production — spermatogenesis — takes place in the testes.
Each testis is packed with seminiferous tubules (laid end to end, they would extend more than 20 meters) where spermatogenesis occurs.
Spermatogenesis
The walls of the seminiferous tubules consist of diploid spermatogonia, stem cells that are the precursors of sperm. Spermatogonia
divide by mitosis to produce more spermatogonia or
differentiate into spermatocytes.
Meiosis of each spermatocyte produces 4 haploid spermatids. This process takes over three weeks to complete.
Then the spermatids differentiate into sperm, losing most of their cytoplasm in the process. For simplicity, the figure shows the behavior of just a single pair of homologous chromosomes with a single crossover. With 22 pairs of autosomes and an average of two crossovers between each pair, the variety of gene combinations in sperm is very great.
SpermSperm cells are little more than flagellated nuclei. Each consists of
a head, which has
an acrosome at its tip and
contains a haploid set of chromosomes in a compact, inactive, state.
a midpiece containing mitochondria and a single centriole
a tail
This electron micrograph (courtesy of Dr. Don W. Fawcett and Susumu Ito) shows the sperm cell of a bat. Note the orderly arrangement of the mitochondria. They supply the ATP to power the whiplike motion of the tail.
An adult male manufactures over 100 million sperm cells each day. These gradually move into the epididymis where they undergo further maturation. The acidic environment in the epididymis keeps the mature sperm inactive. In addition to making sperm, the testis is an endocrine gland. Its principal hormone, testosterone, is responsible for the development of the secondary sex characteristics of men such as the beard, deep voice, and masculine body shape. Testosterone is also essential for making sperm.
Link to more on testosterone.Testosterone is made in the interstitial cells that lie between the seminiferous tubules.
LHInterstitial cells are, in turn, the targets for a hormone often called interstitial cell stimulating hormone (ICSH). It is a product of the anterior lobe of the pituitary gland. However, ICSH is identical to the luteinizing hormone (LH) found in females, and I prefer to call it LH.
FSHFollicle-stimulating hormone (also named for its role in females) acts directly on spermatogonia to stimulate sperm production (aided by the LH needed for testosterone synthesis). [Discussion]
Sex Organs of the Human Female
The responsibility of the female mammal for successful reproduction is considerably greater than that of the male. She must
manufacture eggs
be equipped to receive sperm from the male
provide an environment conducive to fertilization and implantation
nourish the developing baby not only before birth but after.
Oogenesis
Egg formation takes place in the ovaries. In contrast to males, the initial steps in egg production occur prior to birth. Diploid stem cells called oogonia divide by mitosis to produce more oogonia and primary oocytes. By the time the fetus is 20 weeks old, the process reaches its peak and all the oocytes that she will ever possess (~4 million of them) have been formed. By the time she is born, 1–2 million of these remain. Each has
begun the first steps of the first meiotic division (meiosis I) and then
stopped.No further development occurs until years later when the girl becomes sexually mature. Then the primary oocytes recommence their development, usually one at a time and once a month.
The primary oocyte grows much larger and completes the meiosis I, forming a large secondary oocyte and a small polar body that receives little more than one set of chromosomes. Which chromosomes end up in the egg and which in the polar body is entirely a matter of chance.
In humans (and most vertebrates), the first polar body does not go on to meiosis II, but the secondary oocyte does proceed as far as metaphase of meiosis II and then stops.
Only if fertilization occurs will meiosis II ever be completed. Entry of the sperm restarts the cell cycle
breaking down MPF (M-phase promoting factor) and
turning on the anaphase promoting complex (APC).Completion of meiosis II converts the secondary oocyte into a fertilized egg or zygote (and also a second polar body).
As in the diagram for spermatogenesis, the behavior o
Saturday, May 30, 2009
Friday, November 7, 2008
LIFE
ಲೈಫ್!!!!!!
When Life Gets You Down Recently I've grown weary. Weary of waiting for an agent to pick me up, weary of writing day after day after day and not seeing any results from something that is my only income. So far that income is looking distinctly unhealthy. I have even become weary of doing this newsletter and have toyed with the idea of giving it up; after all, I've been doing it several years now.Yet, I know that I have a talent for writing; I believe a natural God-given ability to place words in such a manner as to make them interesting and sensible. And every time I think about giving up doing the editorial for this newsletter, I am encouraged by particularly supportive feedback, or God inspires me with a topic when I thought I'd exhausted everything I could write about.Life does get tiresome from time to time. A change can make all the difference - holiday, listening to music, a walk, or changing the routine. What about those other times when we come back and still feel the same? Less than enthused, in fact, depressed about the status of things.This is very much a spiritual issue as those feelings permeate our spirit and tend to colour life as a whole, when really it is only one or two aspects that aren't going so well. A walk may suffice, but for the times when that fails to lift your flagging spirit, try these little pick-me-ups. Go and visit someone, particularly someone who has few friends, is ill, or could do with a helping hand. Take them a gift, card, or packet of biscuits. Out of your own frustration, cheer someone else up and it might catch on.
"A generous man will prosper; he who refreshes others will himself be refreshed." Proverbs 11:25 Take time to remember what you are doing and why. Often anything worthwhile takes time and the rewards can take even longer. Maybe you do need to take stock and make some changes to be more realistic or have a greater chance of achieving what you desire.
"He who works his land will have abundant food, but he who chases fantasies lacks judgement." Proverbs 12:11 Are there attitudes or beliefs you hold which are contributing to a general weary mood? Perhaps the answers to a problem have been provided, but there is unwillingness to follow through or accept that guidance.
"Pride only breeds quarrels, but wisdom is found in those who take advice." Proverbs 13:10 Though it is good to take advice, you also need to weigh it up. Is the advice sound? Does it serve to only boost your pride, or worse, do the opinions of others make you feel worthless and incompetent? The words of others can affect us very deeply, often more so than we care to admit.
"The wise in heart are called discerning, and pleasant words promote instruction." Proverbs 16:21 Congratulate someone on their success. Think about the successes of people in the same area as you're attempting and realise it is possible to succeed. Use this to combat any jealousy that might be colouring your mood.
"A heart at peace gives life to the body, but envy rots the bones." Proverbs 14:30 Examine your motives for wanting to achieve a certain goal. Is it healthy, honest, and worthwhile? Does it honour yourself, your family, your friends or colleagues...God?
"All a man's ways seem innocent to him, but motives are weighed by the Lord." Proverbs 16:2 Take the time to ask God about what is giving you trouble. Pray for guidance and strength, commit the acts/goal to His service and glory. Ask Him to work on your behalf.
"Commit to the Lord whatever you do, and your plans will succeed." Proverbs 16:3There are so many little things we forget as we go about living. How easy it is to get caught up in our own world and the issues within it. Bringing focus onto the broader picture and giving oneself a 'thought-process examination' can help bring perspective back to whatever is bringing us down.May you continue your journey with peace and joy in Christ.Till next time:
When Life Gets You Down Recently I've grown weary. Weary of waiting for an agent to pick me up, weary of writing day after day after day and not seeing any results from something that is my only income. So far that income is looking distinctly unhealthy. I have even become weary of doing this newsletter and have toyed with the idea of giving it up; after all, I've been doing it several years now.Yet, I know that I have a talent for writing; I believe a natural God-given ability to place words in such a manner as to make them interesting and sensible. And every time I think about giving up doing the editorial for this newsletter, I am encouraged by particularly supportive feedback, or God inspires me with a topic when I thought I'd exhausted everything I could write about.Life does get tiresome from time to time. A change can make all the difference - holiday, listening to music, a walk, or changing the routine. What about those other times when we come back and still feel the same? Less than enthused, in fact, depressed about the status of things.This is very much a spiritual issue as those feelings permeate our spirit and tend to colour life as a whole, when really it is only one or two aspects that aren't going so well. A walk may suffice, but for the times when that fails to lift your flagging spirit, try these little pick-me-ups. Go and visit someone, particularly someone who has few friends, is ill, or could do with a helping hand. Take them a gift, card, or packet of biscuits. Out of your own frustration, cheer someone else up and it might catch on.
"A generous man will prosper; he who refreshes others will himself be refreshed." Proverbs 11:25 Take time to remember what you are doing and why. Often anything worthwhile takes time and the rewards can take even longer. Maybe you do need to take stock and make some changes to be more realistic or have a greater chance of achieving what you desire.
"He who works his land will have abundant food, but he who chases fantasies lacks judgement." Proverbs 12:11 Are there attitudes or beliefs you hold which are contributing to a general weary mood? Perhaps the answers to a problem have been provided, but there is unwillingness to follow through or accept that guidance.
"Pride only breeds quarrels, but wisdom is found in those who take advice." Proverbs 13:10 Though it is good to take advice, you also need to weigh it up. Is the advice sound? Does it serve to only boost your pride, or worse, do the opinions of others make you feel worthless and incompetent? The words of others can affect us very deeply, often more so than we care to admit.
"The wise in heart are called discerning, and pleasant words promote instruction." Proverbs 16:21 Congratulate someone on their success. Think about the successes of people in the same area as you're attempting and realise it is possible to succeed. Use this to combat any jealousy that might be colouring your mood.
"A heart at peace gives life to the body, but envy rots the bones." Proverbs 14:30 Examine your motives for wanting to achieve a certain goal. Is it healthy, honest, and worthwhile? Does it honour yourself, your family, your friends or colleagues...God?
"All a man's ways seem innocent to him, but motives are weighed by the Lord." Proverbs 16:2 Take the time to ask God about what is giving you trouble. Pray for guidance and strength, commit the acts/goal to His service and glory. Ask Him to work on your behalf.
"Commit to the Lord whatever you do, and your plans will succeed." Proverbs 16:3There are so many little things we forget as we go about living. How easy it is to get caught up in our own world and the issues within it. Bringing focus onto the broader picture and giving oneself a 'thought-process examination' can help bring perspective back to whatever is bringing us down.May you continue your journey with peace and joy in Christ.Till next time:
genetics
We will begin this course with the question: What is a gene?
This question will take us four lectures to answer because there are actually several different definitions that are appropriate in different contexts.
We will start with a physical definition of the gene. Conceptually this is the simplest and it will give me an excuse to briefly review some of the molecular biology that you probably already know.
Genes are made of DNA
For this course we will mostly think of DNA as an information molecule not as a chemical substance.
In 1953, Watson and Crick deduced that the structure of DNA was a double helix. It was not the helical structure per se, but the discovery of complementary base pairing that revealed how information could be encoded in a molecule and how this information could be exactly duplicated each cell division. R
In order to extract information from the DNA, the cell again uses the complementary base-pairing to make a copy of the information copied onto an RNA molecule. This is known as T. RNA is chemically less stable than DNA and mRNA can be thought of as a temporary copy of DNA’s information.
T r
: enzymes structural proteins membrane channels hormones
G DNA segment needed to make a protein
Genes are typically 103 - 104 base pairs in size although they can be much larger. For example, the human dystrophin gene is 2 x 106 base pairs.
E.coli has about 4,200 genes which isn’t very many considering that at least 1,000 different enzymes are needed carry out just the basic biochemical reactions in a cell. The smallest genome for a free-living organism (i.e. a cell, not a virus) is that of the bacterium Mycoplasma genetalium which encodes only 467 genes. Humans are at the other end of the spectrum of complexity and have about 35,000 genes.
In the demonstration in class you see how a mutation in the Shibire gene in the fly Drosophila gives a heat sensitive protein that is required for synaptic transmission. When the flies that carry this mutation are warmed by the projector lamp they become paralyzed.
Gene — Protein — Cell Process — Organism “disease”
(Shibire) (Dynamin) (Synaptic Signaling) (Paralyzed Fly)
This example illustrates two powerful aspects of genetic analysis. First, we can follow microscopic changes in the DNA such as the Shibire mutation as they are revealed by the macroscopic consequences of the mutation such as a paralyzed fly. Second, we have a very precise way of studying the function of individual proteins by examining the consequences of eliminating just that one protein function in an otherwise normal organism.
A different versions of the same gene
Often alleles are referred to as mutants but actually this usage is often incorrect particularly when we discuss naturally occurring variants in a population.
M an altered version of a gene when we have “witnessed” the alteration but not when it is preexisting in the population.
G all alleles of an individual
W defined standard genotype
The concept of wild-type is used as a defined reference for organisms where we can do breeding experiments. Of course there is no realistic way to define a standard genotype for humans, therefore “wild type” has no meaning when we discuss human genetics.
The physical definition of the gene is a very good one but there are many instances where we wish to study genes whose DNA sequences are not known. For example, say we have isolated a new mutant fly that is also paralyzed and we want to know whether this mutation is also in the Shibire gene. We will see in the next several lectures that we can answer this question without knowledge of the DNA sequence either by a test for gene function known as a complementation test or by a test of the chromosomal position of the mutation by recombinational mapping. In practice, these other ways of defining genes by function or by position are often much more useful than a definition based on the DNA sequence.
This question will take us four lectures to answer because there are actually several different definitions that are appropriate in different contexts.
We will start with a physical definition of the gene. Conceptually this is the simplest and it will give me an excuse to briefly review some of the molecular biology that you probably already know.
Genes are made of DNA
For this course we will mostly think of DNA as an information molecule not as a chemical substance.
In 1953, Watson and Crick deduced that the structure of DNA was a double helix. It was not the helical structure per se, but the discovery of complementary base pairing that revealed how information could be encoded in a molecule and how this information could be exactly duplicated each cell division. R
In order to extract information from the DNA, the cell again uses the complementary base-pairing to make a copy of the information copied onto an RNA molecule. This is known as T. RNA is chemically less stable than DNA and mRNA can be thought of as a temporary copy of DNA’s information.
T r
: enzymes structural proteins membrane channels hormones
G DNA segment needed to make a protein
Genes are typically 103 - 104 base pairs in size although they can be much larger. For example, the human dystrophin gene is 2 x 106 base pairs.
E.coli has about 4,200 genes which isn’t very many considering that at least 1,000 different enzymes are needed carry out just the basic biochemical reactions in a cell. The smallest genome for a free-living organism (i.e. a cell, not a virus) is that of the bacterium Mycoplasma genetalium which encodes only 467 genes. Humans are at the other end of the spectrum of complexity and have about 35,000 genes.
In the demonstration in class you see how a mutation in the Shibire gene in the fly Drosophila gives a heat sensitive protein that is required for synaptic transmission. When the flies that carry this mutation are warmed by the projector lamp they become paralyzed.
Gene — Protein — Cell Process — Organism “disease”
(Shibire) (Dynamin) (Synaptic Signaling) (Paralyzed Fly)
This example illustrates two powerful aspects of genetic analysis. First, we can follow microscopic changes in the DNA such as the Shibire mutation as they are revealed by the macroscopic consequences of the mutation such as a paralyzed fly. Second, we have a very precise way of studying the function of individual proteins by examining the consequences of eliminating just that one protein function in an otherwise normal organism.
A different versions of the same gene
Often alleles are referred to as mutants but actually this usage is often incorrect particularly when we discuss naturally occurring variants in a population.
M an altered version of a gene when we have “witnessed” the alteration but not when it is preexisting in the population.
G all alleles of an individual
W defined standard genotype
The concept of wild-type is used as a defined reference for organisms where we can do breeding experiments. Of course there is no realistic way to define a standard genotype for humans, therefore “wild type” has no meaning when we discuss human genetics.
The physical definition of the gene is a very good one but there are many instances where we wish to study genes whose DNA sequences are not known. For example, say we have isolated a new mutant fly that is also paralyzed and we want to know whether this mutation is also in the Shibire gene. We will see in the next several lectures that we can answer this question without knowledge of the DNA sequence either by a test for gene function known as a complementation test or by a test of the chromosomal position of the mutation by recombinational mapping. In practice, these other ways of defining genes by function or by position are often much more useful than a definition based on the DNA sequence.
Wednesday, November 5, 2008
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