Reproduction:
Reproduction is formally the biological process where individual organisms are produced (Mader). Reproduction is a fundamental feature of all life, and each individual exits as the result of reproduction (Mader). There are two main types of reproduction known as sexual and asexual (Mader). An individual organism can reproduce by itself without another organism of that species help (Mader). This type of reproduction is found in bacteria, and most plants (Mader). The division of bacterial cells into two daughter cells is an example of asexual reproduction (Mader). Asexual reproduction is also not limited to single-celled organisms (Mader). The opposite is sexual reproduction, which does require the involvement of two individuals (Mader). Normal human reproduction is a common example of sexual reproduction (Mader).
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Human life cycle:
Different researchers studying human development have different perspectives of how many stages in life there are. The human life cycle is said to be briefly broken down into four main stages known as infancy, childhood, adolescence, and adulthood. Infancy is the stage of life that is from birth to the age of two (Unknown). This is the age that children learn to use their gross motor skills, such as walking, as well as the time teeth and language develop rapidly (Unknown). This stage is also where you find disproportions of children’s head and body (Unknown).
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The next stage is focused on childhood and lasts from two years old to adolescence (Unknown). This is when bones are teeth are growing rapidly, and the time when permanent teeth develop (Unknown). Intellectual skills starts to broaden and become more defined (Unknown).
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The next stage is focused on adolescence, where the body becomes sexually mature (Unknown). Things that can start to occur during the adolescent stage are changes in growth, physically and mentally, and also voice changes for males. Because girls bodies are undergoing puberty (a time where they can become pregnant), they gain extra weight, in order to compensate for having a child (Unknown). This is the time where there are many raging hormones at work, and because the body is undergoing so many changes, it can be a stressful time for teenagers (Unknown).
The last stage for during the human life cycle is adulthood (Unknown). Adulthood is the time when the body slows down the production of all the necessary hormones, and the body is fully grown by this time (Unknown). Life styles changes may occur in adults, such as a decrease in physical activity because of the aging of the body’s joints (Unknown). However, this decrease in physical activity doesn’t have to occur in the adult stages of life, and if children are active and continue to stay healthy throughout their lives, they will age much better than adults that are healthy (Unknown).
Meiosis:
Meiosis, in terms of biology, is the process by which a diploid eukaryotic cell divides in order to generate four haploid cells called gametes (Mader). These gametes are the “sex cells” of the body (Mader). The term meiosis comes from the Greek word meioun, which translates “to make smaller”. This term is used because meiosis is the result in a reduction in chromosomes in the gamete cell (Mader).
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Meiosis is incredibly important to human beings because is essential for sexual reproduction, and without it, there wouldn’t be sustained life on this earth. Meiosis occurs in all eukaryotes that reproduce sexually, even in less complicated single-celled organisms (Mader). Even though sexual reproduction can occur in all eukaryotes, some have lost the ability to carry out meiosis, and have acquired the ability to reproduce by parthenogenesis (Mader).
During meiosis, the genome of a diploid germ cell, composed of DNA, undergoes DNA replication (Mader). After the DNA is replicated, it is able to be divided, which results in the form of gametes (Mader). Each gamete contains a complete set of chromosomes, or half of the genetic content of the original cell (Mader). This is why two parents genetic content combines to give us offspring that has characteristics of both its genetic donors. The gametes have to fuse together with another gamete of the opposite sex during fertilization, in order to create a new diploid cell, known as a zygote (Mader).
After this fertilization occurs, the division mechanism of meiosis is a reciprocal process to the joining of the two genomes (Mader). The chromosomes of each parent undergo genetic recombination, and each gamete and zygote will have a unique genetic blueprint encoded in its DNA. There are many other features that are unique to meiosis, including pairing and genetic recombination between homologous chromosomes, which means that they share the same origin but have a different function (Mader).
Another thing that occurs during meiosis are several processes including prophase, metaphase, anaphase, and telophase (Mader). In the first stage of prophase, individual chromosomes begin to condense into longs strands within the nucleus (Mader). However, the two sister chromatids are still bond together so tightly that they are indistinguishable from one another (Mader). The next stage during meiosis is metaphase (Mader). During this stage, homologous pairs move together along the phase plate (Mader). The physical basis of the independent assortment of chromosomes is the random orientation of each bivalent along the metaphase plate (Mader). The next stage during meiosis is anaphase (Mader). During this stage the cell elongates in preparation for division down the middle. Also, homologous chromosomes that are closely associated in synapsis exchange segments by crossing over. The last stage during meiosis is telophase (Mader). During this stage, the microtubules that make up the spindle network disappear, and a new nuclear membrane surrounds each haploid set. The chromosomes uncoil back into chromatin. After that, cytokinesis (the pinching of the cell membrane) occurs, which completes the process of creating two daughter cells (Mader).
Fetal development and birth:
In early fetal development, there are three main stages a fetus goes through. These stages include the Germinal Stage, the Embryonic Stage, and the Fetal Stage (Feldman pg 75). The Germinal stage occurs from fertilization to two weeks of age. This is the shortest stage of prenatal development, but important because this is when the placenta is able to be used for nutrients and waste disposal through the umbilical cord (Feldman pg 74). The Embryonic stage occurs two weeks to eight weeks following fertilization (Feldman pg 75). This is the most important stage because the entire development of a child occurs during this stage (Feldman pg 75).
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There are three layers that form a different set of structures as a child’s development proceeds (Feldman pg 75). These layers are called the ectoderm, endoderm, and mesoderm. The ectoderm is where skin, hair, teeth, sense organs, the brain, and spinal cord will develop (Feldman pg 75). The endoderm produces the digestive system, liver, pancreas, and respiratory system (Feldman pg 75). Lastly, the mesoderm is where the muscles, bones, blood, and circulatory system are formed (Feldman pg 75). The last stage in fetal development is the fetal stage. This occurs from eight weeks until birth. Now the child is considered a fetus, and this is where labor comes into play (Feldman pg 75).
During all of these stages genes and chromosomes will be passed down to a child, and different aspects of their life are inherited (Feldman pg 125). Genes are what makes us look, and where different aspects of our behavior come from (Feldman pg 125). Chromosomes also play a very important role, and without even just one of them could cause several problems. For example, Fragile X syndrome occurs when a particular gene is injured on the X chromosome, which results in mild to moderate mental retardation (Feldman pg 56). Down syndrome can also occur when the presence of an extra chromosome is on the 21st pair. Another disorder is Klinefelter’s syndrome. Klinefelter’s syndrome occurs when there is the presence of an extra X chromosome, which accounts for genetic ‘gender’ related abnormalities (Feldman pg 56). These abnormalities occur from receiving the improper number of sex chromosomes (Feldman pg 56). In conclusion, having the correct amount of chromosomes and properly working genes can mean a huge difference in people’s lives.
Citations:
Mader, Sylvia. Human Biology 10th edition. 2008
Feldman, Robert. Development across the life span 4th edition 2006.
Unknown. “Human life cycle” http://***.tqnyc.org/NYC040719/life_cycle.htm
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