HUMAN PRENATAL DEVELOPMENT
TEACHER PORTAL
INTRODUCTION
Reproduction and Growth
Mitosis vs. Meiosis: Understanding the Differences
Mitosis and meiosis are both processes of cell division, but they serve very different purposes in the body. Understanding their differences is important because it helps explain how organisms grow, repair themselves, and reproduce.
Mitosis: Cell Division for Growth and Repair
Mitosis is the process that occurs when a cell divides to produce two identical daughter cells. It is responsible for the growth of an organism and the repair of tissues. For example, when you get a cut on your skin, mitosis helps to create new skin cells to replace the damaged ones. Each new cell has the same number of chromosomes (DNA) as the original cell. In humans, this means that each new cell has 46 chromosomes arranged in 23 pairs.
Mitosis involves several stages: prophase, metaphase, anaphase, and telophase. The goal of mitosis is to make sure that each daughter cell gets a full set of chromosomes. After mitosis, the two new cells are genetically identical to the original cell and each other.
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Meiosis: Cell Division for Reproduction
Meiosis is a special type of cell division that occurs only in reproductive cells. Its purpose is to produce eggs in females and sperm in males. Unlike mitosis, which produces two identical cells, meiosis results in four genetically different cells. These cells have only half the number of chromosomes—23 in humans—so that when they combine during fertilization, the resulting offspring will have a full set of 46 chromosomes.
Meiosis consists of two rounds of division: meiosis I and meiosis II. During meiosis I, homologous chromosomes (chromosome pairs from each parent) separate, and during meiosis II, the sister chromatids (copies of chromosomes) split apart. This reduction in chromosome number is essential for sexual reproduction, allowing genetic diversity through the mixing of parental genes.
Watch the video below to view the steps in meiosis I and meiosis II:
Key Differences Between Mitosis and Mitosis
- Purpose: Mitosis is for growth and repair, while meiosis is for reproduction.
- Number of Divisions: Mitosis has one division, while meiosis has two.
- Resulting Cells: Mitosis produces two identical cells, while meiosis produces four genetically different cells.
- Chromosome Number: Mitosis maintains the chromosome number, while meiosis halves it.
Embryonic Development: First Trimester
Human pregnancy lasts usually lasts approximately nine months and is typically divided into three 3-month intervals referred to as trimesters.
The first trimester is the very beginning of human life, and it covers the first 12 weeks of pregnancy. It’s an incredibly important time because many things happen quickly to help the baby start growing. Let’s break it down step by step.
Week 1-4: The Start
- It all begins when the sperm from the father fertilizes the egg from the mother. This creates a tiny new cell called a zygote.
- The zygote travels down the fallopian tube to the uterus and starts to divide into more and more cells, forming a blastocyst.
- It then implants itself into the thick lining of the uterus, where it will get all the nutrients it needs to grow.
Week 5-8: Big Changes
- During this time, the baby is called an embryo. It’s still tiny, but it’s starting to grow really important parts.
- The heart begins to beat, and blood vessels start to form.
- The brain, spine, and other organs like the liver and lungs are beginning to develop.
- Arm and leg buds appear, which will grow into arms, hands, legs, and feet.
- By the end of this period, the embryo starts to look a little more like a human, though it’s still only about the size of a blueberry!
Week 9-12: The Fetus Stage
- The embryo is now called a fetus. This means it’s still growing but now has most of its important organs starting to work.
- The fingers and toes start to separate and become more defined.
- Bones are beginning to form, and the muscles are developing, which will allow the baby to start moving soon, even if it’s too tiny to feel.
- The baby’s facial features start to become clearer—you can see little ears, eyes, and a nose forming.
- By the end of the first trimester, the baby is about the size of a plum and is growing fast!
Why Is the First Trimester Important?
- It’s when all the major organs and body parts start to form, so it’s a critical time for the baby’s development.
- It’s important for the mother to stay healthy during this time to help the baby grow properly.
In summary, the first trimester is like the building phase, where all the main parts of the baby are starting to form and develop. It’s a very busy time for the tiny new life! Click on the image below to enlarge the embryo (now a fetus) at the end of the first trimester.
Embryonic Development: Second Trimester
A developing embryo is typically referred to as a fetus after the 10th week of pregnancy. Thus, the entire second and third trimester of pregnancy may be referred to a fetal development.
The second trimester is the period from week 13 to week 26 of pregnancy. This is a time of rapid growth and development for the baby, and it’s often when the mother starts to feel the baby move. Let’s go through what happens step by step!
Weeks 13-16: Growth and Movement
- The baby continues to grow quickly and now looks more like a tiny human. The face becomes more detailed, with eyebrows, eyelashes, and even hair starting to form.
- The baby’s bones are hardening, and the muscles are developing more, allowing the baby to move its arms, legs, and even make little fists.
- By the end of this period, the baby is about the size of an avocado (about 4 to 5 inches long).
Weeks 17-20: Senses and Reactions
- The baby’s nervous system (brain and nerves) continues to develop, and it starts to react to light and sound.
- The baby’s ears are fully formed, so it can even start hearing sounds like the mother’s heartbeat and voice!
- A layer called vernix caseosa (vernix for short) forms on the baby’s skin to protect it. The skin itself is still very thin.
- At this stage, the baby is around the size of a banana, and the mother may feel kicks and movements for the first time.
Weeks 21-24: Organ Development and Chances of Survival
- The baby’s lungs and digestive system are developing, although they still have more growing to do before they’re ready to work on their own.
- The baby is practicing breathing by inhaling small amounts of amniotic fluid, which helps develop the lungs.
- By week 24, the baby is about the size of an ear of corn. This is also when the baby’s chances of survival if born early, start to improve. Babies born at this stage are called premature, and while they would need special medical help, they have a small chance of survival.
Weeks 25-26: Improving Survival Chances
- During these weeks, the baby’s lungs continue to develop, and the air sacs that are important for breathing start to form. The baby is getting better prepared for life outside the womb.
- The baby also begins to put on more fat, which helps regulate body temperature after birth.
- By the end of the second trimester, the baby is around the size of a cabbage and weighs about 1.5 to 2 pounds.
Survival Chances During the Second Trimester
- Before Week 24: If a baby is born earlier than this, the chances of survival are very low because the lungs and other organs are not developed enough.
- Week 24: Survival rates improve but are still low. With special medical care, a baby born at this stage has a 50% chance of survival.
- Week 26: The survival chances are even better, with some babies having about an 80% chance of surviving if born at this point, though they would still need to be in a neonatal intensive care unit (NICU).
In summary, the second trimester is all about the baby growing bigger, moving more, and developing important organs and systems. As time passes, the baby’s chances of surviving outside the womb improve, especially as it gets closer to the third trimester.
Embryonic Development: Third Trimester
The third trimester is the final stage of pregnancy, from week 27 to birth, which usually happens around week 40. During this time, the baby’s body matures, gets bigger, and prepares for life outside the womb. Let’s explore what happens in this important stage!
Weeks 27-30: Growth and Brain Development
- The baby continues to grow rapidly, gaining more weight and muscle. At this point, the baby is about the size of a cauliflower and weighs around 2.5 to 3 pounds.
- The brain is developing quickly, becoming more complex and able to control the baby’s body better. The baby’s movements become stronger and more coordinated.
- The baby’s eyes are open, and it can respond to light and sound. It may even turn its head toward bright light coming from outside the mother’s belly.
- The baby’s lungs are still developing but are getting closer to being ready for breathing on their own.
Weeks 31-34: Getting Ready for Birth
- The baby continues to put on more fat, which helps keep it warm after birth. The skin becomes less transparent, and the baby looks more like a newborn.
- The baby’s lungs are almost fully developed, and it’s practicing breathing by taking in amniotic fluid. This practice is important for lung strength.
- The baby’s bones are fully formed but still soft, making it easier for the baby to fit through the birth canal during delivery.
- By this time, the baby’s size is about that of a butternut squash, and it weighs around 4 to 5 pounds.
Weeks 35-37: Maturity and Positioning for Birth
- The baby’s nervous system and lungs continue to mature, and it’s getting ready for life outside the womb.
- Most babies turn into the head-down position, which is the best position for birth.
- The baby’s movements might feel less intense to the mother because it’s running out of space as it grows larger.
- At this point, the baby is about the size of a pineapple and weighs around 5.5 to 6 pounds.
Weeks 38-40: Full Term and Birth
- At week 37, the baby is considered full term, meaning it is fully developed and ready for birth.
- The baby’s organs are mature, especially the lungs, so it can breathe on its own when born.
- The baby is gaining weight rapidly, putting on half a pound or more each week. By birth, most babies weigh between 6 and 9 pounds and are about the size of a watermelon (18-21 inches long).
Birth
- When the baby is fully developed and ready, the mother’s body starts a process called labor, where the muscles of the uterus contract to help push the baby out.
- The baby is usually delivered head-first, and after the baby is born, the umbilical cord is cut. This cord was the lifeline that provided the baby with nutrients and oxygen while it was in the womb.
- The baby takes its first breath and begins to cry, which helps clear its lungs and start breathing on its own. The lungs are now fully developed and ready to take in air.
- Click on the image below to enlarge the moment of birth. Notice the thick, creamy vernix. This natural, waxy film protects the baby’s skin from the amniotic fluid during development in the uterus. Notice that the umbilical cord is still attached.
Survival and Health
- Babies born after week 37 have the best chance of being healthy and strong because their organs are fully developed.
- Babies born between week 34 and 37 are often okay but might need some extra help, like staying in a special care nursery.
- Babies born before week 34 are considered premature and may need to spend time in a NICU (Neonatal Intensive Care Unit) to help them grow and develop.
In summary, the third trimester is all about the baby getting bigger, stronger, and fully mature to be ready for birth. It’s the home stretch before the baby enters the world!
Human Chromosomes
Think of chromosomes like tiny instruction books that tell our bodies how to grow, develop, and work. We have 46 chromosomes in total, organized into 23 pairs. One pair of these chromosomes is called the sex chromosomes, and they determine whether someone is male or female.
Male and Female Chromosomes
- Females have two X chromosomes, written as XX.
- Males have one X chromosome and one Y chromosome, written as XY.
LAB: You will view human chromosomes under the microscope in the lab. A micrograph is also shown above.
How Do They Work?
When a baby is made, it gets one chromosome from each parent to form the pair. The mother always gives an X chromosome because that’s all she has (XX). The father can give either an X or a Y chromosome.
- If the father gives an X chromosome, the baby will be female (XX).
- If the father gives a Y chromosome, the baby will be male (XY).
So, the father’s chromosome is the one that decides if the baby will be a boy or a girl.
In short:
- Females have XX chromosomes.
- Males have XY chromosomes.
- It’s the father’s chromosomes that determine the baby’s sex.
- Finally, each chromosome contains long strands of the deoxyribonucleic acid (DNA) molecule. The DNA strand is organized into sequences called genes, each of which codes for specific proteins that proform necessary biochemical functions in our cells.
Fertilization
Fertilization is the process where a sperm cell from the father combines with an egg cell from the mother to form a new, genetically unique individual. Only males can produce sperm cells, and only females can produce egg cells (also called ova). Meiosis is required to form both types of gametes (sex cells).
Both sperm and egg cells are haploid, meaning they each have only half the number of chromosomes needed to make a complete set. A complete set of chromosomes in humans is 46, but each haploid cell has only 23 chromosomes.
The Role of Sperm and Egg Cells
Sperm Cell: The sperm cell is produced in the father’s body and carries 23 chromosomes. It is very small and has a tail called a flagellum, which helps it swim toward the egg cell. Click on the illustration of human sperm below to enlarge the image.
Egg Cell: The egg cell (or ovum) is produced in the mother’s body and has 23 chromosomes. It is much larger than the sperm cell and is filled with nutrients to help the developing embryo grow in its early stages.
The Process of Fertilization
When the sperm cell reaches the egg cell, they combine, this is called fertilization. The sperm’s 23 chromosomes and the egg’s 23 chromosomes join together, making a complete set of 46 chromosomes. This new cell is called a zygote.
The zygote has all the genetic information it needs to develop into a new human. It will continue to divide and grow into an embryo and, eventually, a baby.
Why Is Fertilization Important?
Fertilization is important because it combines genetic material from both parents. This means that the baby will have traits from both the father and the mother, like eye color, hair color, and other characteristics. The combination of chromosomes from both parents also makes every person unique from fertilization onward.
In summary, haploid sperm and egg cells are crucial in fertilization because they combine half of the genetic information from each parent to create a new, complete human organism.
The Menstrual Cycle
To understand fertilization, you need to know about the menstrual cycle. The menstrual cycle happens roughly every month in a female’s body to prepare for a possible pregnancy.
The human menstrual cycle typically begins during puberty, which is when a female’s body starts to develop and change. The usual age for this to start is around 12 years old, but it can happen anytime between 9 and 16 years old.
Once the menstrual cycle starts, it generally continues each month until a person reaches menopause, which usually happens between the ages of 45 and 55, when the menstrual cycle stops permanently.
So, the overall age range for the menstrual cycle is roughly 9 to 55 years old, though it can vary slightly from person to person.
The menstrual cycle usually lasts about 28 days but can be a little shorter or longer. Here’s a simple overview:
Days 1-5: The cycle begins with menstruation when the lining of the uterus (the womb) sheds if no fertilization happened in the previous cycle. This lining exits the body as what we call a “period.”
Days 6-14: The body starts to prepare for the next cycle. A new egg cell begins to grow and mature in the ovary. Meanwhile, the lining of the uterus starts to thicken again, getting ready for a fertilized egg to attach.
Around Day 14 (Ovulation): This is the ovulation phase. The egg leaves the ovary and travels down the fallopian tube, where it might meet a sperm. This is the time when fertilization is most likely to happen because the egg is ready to be fertilized.
Days 15-28: If the egg is not fertilized, it breaks down, and the uterus lining gets ready to shed again, starting a new cycle.
Fertilization can happen if sperm enters the female body during or close to the time of ovulation (around Day 14). Sperm cells swim through the uterus to reach the fallopian tube, where they might meet the egg. When one sperm manages to join with the egg, it fertilizes it, and the egg becomes a zygote, the very first stage of a new human life.
After fertilization, the zygote travels to the uterus and attaches to the thickened lining. This is where it will continue to grow and develop into a baby over the next nine months.
In summary:
- Fertilization can only occur if sperm meets the egg when it’s in the fallopian tube, which happens around Day 14 of the menstrual cycle.
- If fertilization doesn’t happen, the egg will eventually break down, and the cycle starts again.
Learn the Lingo
The following list includes Key Terms that are introduced within the Backgrounds of the CELL. These terms should be used, as appropriate, by teachers and students during everyday classroom discourse.
Note: Additional words may be bolded within the Background(s). These words are not Key Terms and are strictly emphasized for exposure at this time.
Investigation 1:
- DNA: nucleic acid that carries genetic information in cells and consists of two complementary chains of nucleotides wound in a double helix
- Protein: large molecule consisting of one or more chains of amino acids (polypeptides)
- Amino Acid: building block of protein molecules
- RNA: group of single-stranded nucleic acids, including mRNA (messenger RNA) that is necessary for transcription and translation
- Codon: three adjacent nucleotides in DNA or mRNA that code for a specific amino acid in a protein
- Mutation: a change in the DNA sequence of a gene
Investigation 2:
- There are no Key Terms introduced in Investigation 2.
Investigation 3:
- Chromosome: structures of DNA and protein in the nucleus of cells, where genes are located
- Mitosis: nuclear division characterized by chromosome replication and formation of two identical daughter nuclei
Get Focused
The Focus Questions in each Investigation are designed to help teachers and students focus on the important concepts. By the end of the CELL, students should be able to answer the following questions:
Investigation 1:
- How does DNA control the functions of an organism? Genes contain DNA sequences that encode for proteins. Proteins are responsible for most of the functions of an organism’s cells.
- Can mutations in DNA cause changes in an organism? Mutations may or may not lead to changes in an organism.
Investigation 2:
- Can mutations in DNA cause changes in an organism? Yes, however not all mutations cause changes in an organism.
Investigation 3:
- Why can mutations in the DNA of a single cell affect the functions of an entire organism? A mutation of the DNA in a gene in a single body cell will get copied when the chromosomes duplicate. All of the new body cells formed through mitosis will then have the mutation. If the mutation causes a change in a protein, all cells will produce the mutant protein. If the cells in an organ are producing mutant proteins, the organ may not function properly. Thus the organism may not function properly.
Note: These are succinct responses to the Focus Questions and are placed here for easy reference. Fully developed responses to the Focus Questions can be found on each PostLab page.
Note: Some questions may be revisited as the CELL progresses. As students acquire additional knowledge, their responses should reflect this.