Study Notes - Term 4 Biology

Male Reproduction Spermatogenesis - the meiotic process by which one sperm cell undergoes mitosis to produce an identical daughter cell. This daughter cell then divides into the four spermatozoa, which are used in the fertilisation process. This is done to ensure that there is a never-ending supply of sperm, unlike the women, where there is only a finite number of ovum.In order for the sperm to be able to fertilise the egg via sexual reproduction, it must undergo some changes which are beneficial. From inside the seminiferous tubules, where spermatogenesis is undertaken, it travels through the testis and into the Epididymis. From there, it travels through the Vas Deferens and onto the adjoining Urethra. By this stage however, secretions have come from the Seminal Vesicle, Cowper's Gland and Prostate Gland to benefit the sperm in fertilising the ovum. The combination of these solutions is called Semen Seminal Vesicle: a sugary secretion which provides the sperm a swimming medium and nutrients for the mitochondria (refer to sperm cell diagram above) Cowper's Gland: a secretion which prevents the sperm being contaminated from urine residue in the urethra passage.  Prostate Gland: An alkaline secretion which compensates for the acidic environment of the vagina, as the testis are basic. The solution allows for the spermatozoa to last approximately 72 hours, before the reaction ceases and the sperm is broken down by the acidic environment.  

Female Reproduction Oogenesis - The meiotic cell division which is similar to spermatogenesis where the cell undertake mitosis before meiosis. However, this is different, as there is only one ovum produced from this process. As seen in the diagram below, the others, known as polar bodies are much smaller as the ovum needs a lot of cytoplasm to compensate for the addition of the sperm cell when fertilisation occurs. 

The Menstrual Cycle - The menstrual cycle, is a cycle which undertaken by the female reproductive system, no matter whether fertilisation does or does not occur. There are several phases to the cycle: Menstrual Phase (Days 1 to 5): Occurs when fertilisation of the ovum has not occurred. It sheds the excess endometrium tissue, which contained coiled blood tubules, causing the extended blood flow.   Follicular Phase (Days 6 to 12):  There are secretions of small quantities of estrogen which are used to stimulate follicle development Ovulatory Phase (Days 13 to 15): The follicle ruptures, releasing the ovum. The ovum then enters the fallopian tube where it is most fertile. Luteal Phase (Days 16 to 23): The ruptured follicle then becomes the corpus luteum, which releases estrogen and progesterone. These are used to increase the lining of the uterus (endometrium) for the pregnancy.  Premenstrual Phase (Days 24 to 28):  If fertilisation has occurred, the corpus lutem continues to secret progesterone and estrogen, which is used to continue the growth and development of the fetus. However, if fertilisation does not occur, the corpus luteum will break down. This will decrease the progesterone and estrogen levels. The thickened endometrium is unable to maintain its thickness with the deficiency in hormones, hence menstruation. 

FertilisationThe following diagram illustrates the path that a sperm cell must take from production to reach the ovum of a female. 

Genetics Monohybrid Cross - A monohybrid cross occurs when there are two parents producing an offspring. The main focus with a monohybrid cross is only one gene, which is usually only dominant or recessive which results in only six possibilities for the genotypes of the parents (see example)Di-hybrid Cross - A di-hybrid cross occurs when there are two parents producing an offspring, In this scenario however, there are two genes. There is also the same dominant and recessive scheme. The layout with regards to the dominant and recessives are the same, and the di-hybrid cross can be solved in the same way as the monohybrid crosses, except using two, Co-dominance - The genetic situation where, in the heterzygous state of the allele, both phenotypes are blatantly visible. Inter-dominance - The genetic situation where the heterozygous genes illustrate the phenotypes of both the dominant and recessive alleles. For example, breeding a red flower with a white flower, which results in a pink flower. Gene Mutations Point Mutation - A change in the genes which only affect one point along the strand. Point mutation includes substitution or inversion. Frame Shift Mutation - A change in the genes which affects two or more nucleotides, normal affecting the whole sequence. A frame shift change can include insertion or deletion of data and is shown in the diagram. 

Cell Division Mitosis - The cellular division where one original cell replicates its genetic material as well as structure to result in two identical daughter cells. The steps of Mitosis include:  Interphase: The period between the previous cell division where the parent cell must grow, in order to be able to divide again. The DNA (chromosomes) must also be replicated to a diploid (2N) stage. The DNA has been replicated but is not condensed (they are chromatin instead of chromosomes). Prophase: Chromatin shortens and condenses until they are in the chromosome form. The nuclear membrane is no longer visible. The centrioles beging to become visible placing themselves at the poles of the nucleus. Metaphase: Spindle fibres attach themselves to the chromosome pairs and line themselves in the middle of the nucleus (equatorial plate). Anaphase: Spindle fibres shorten and the chromatids separate, pulled by the centrioles. Telophase: The chromatids reach the poles of their respective spindles. The spindle fibres then distintegrate once a nuclear membrane has formed around the chromatids. Cytokinesis: The process of splitting the daughter cells. Each daughter cell contains the same number and quality of chromosomes (assuming no mutations). Meiosis - The cellular division where one original cell undergoes a repitition of mitosis, followed immediately by another lot of mitosis which does NOT replicate the chromosomes, resulting in four haploid (N) cells. With relation to the human system, this differs slightly. The steps of meiosis are:  Prophase I: DNA coils tightly and individual chromosomes begin to become visible. Homologous chromosome (about the same length and size) match up together and intertwine, resulting in crossing over of DNA. Metaphase I: The nuclear membrane has disappeared and the centrioles begin moving towards the poles. Each pair of chromosomes lines up on the metaphase plate (equatorial).  Anaphase I: The centrioles use the spindle fibres to pull chromosomes towards the specific poles.  Telophase I: The nuclear membrane reforms around the daughter nuclei. The two cells split.  Prophase II: The nuclear membrane immediately breaks down. Metaphase II: Centrioles work at the poles to create spindle fibres to separate the chromosomes.  Anaphase II: Spindle fibres pull one of the pair (lined up on the plate) towards its specific pole. Telophase II: The nuclear membrane forms around the daughter chromosomes, and the cells are then separated. 

Male Reproduction

Female Reproduction

Fertilisation

Genetics

Cell Division