General Biology I Lecture 6 PDF

# General Biology I ## BAZE UNIVERSITY ABUJA ## DEPARTMENT OF BIOLOGICAL SCIENCES ## BIO 101 ## LECTURE 06 ## By ### MRS ASIYA ABUBAKAR MUHAMMED ## HEREDITY/ HEREDITARY # INTRODUCTION ## GENETICS Branch of science that deals with Heredity (characteristics transmitted from parents to offspring) a...

# General Biology I ## BAZE UNIVERSITY ABUJA ## DEPARTMENT OF BIOLOGICAL SCIENCES ## BIO 101 ## LECTURE 06 ## By ### MRS ASIYA ABUBAKAR MUHAMMED ## HEREDITY/ HEREDITARY # INTRODUCTION ## GENETICS Branch of science that deals with Heredity (characteristics transmitted from parents to offspring) and variation of inherited characteristics. ## VARIATION The differences among the individuals of a species/ population are called variations. ## HEREDITY/HEREDITARY It means the transmission of features/ characters/ traits from one generation to the next generation leading to continuity of the species and variation within it. Hereditary variations are of great importance in the process of evolution of a new species. Asexual reproduction results in a small amount of variation as compared to sexual reproduction. ## CHROMOSOMES In the nucleus are a number of thread-like structures called chromosomes. Each cell has fixed number of chromosomes. Each chromosome is made up of two parallel strands called chromatids each pair of chromatid is connected at one point by a structure called centromere. | | | | | | | | | | | | | |---|---|---|---|---|---|---|---|---|---|---|---| | | | | | | | | | | | | | | | | | | | | | | | | | | | **1** | **2** | **3** | **4** | **5** | **6** | **7** | **8** | **9** | **10** | **11** | **12** | | | | | | | | | | | | | | | | | | | | | | | | | | | | **13** | **14** | **15** | **16** | **17** | **18** | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | **19** | **20** | | **21** | **22** | | **X** | **Y** | | | | | | | | | | | | | | | | | | **autosomes** **sex chromosomes** Along the length of the chromosomes is a series of structures called genes. They are too small to be seen even under a powerful microscope. Genes determine the characteristics of the cell and its progeny. Genes are made up of a protein chemical substance called DNA (Deoxyribonucleic acid) which contain coded information of instructions that dictate the characteristics of the offspring. Most of the time, the chromosomes are too thin to be seen except with an electron microscope. But when a cell is dividing, they get shorter and fatter so they can be seen with a light microscope. Chromosomes are a packaged form of DNA. The DNA normally exists in a non-condensed form in the cell nucleus. It condenses into chromosomes during cell replication. Human cells contain 46 chromosomes, which are in pairs. Sex cells (sperm and ova) contain only 23 chromosomes. The 23 chromosomes comprise one from each pair. ## DNA Each chromosome contains one very long molecule of DNA. The DNA molecule carries a code that instructs the cell about which kind of proteins it should make. Each chromosome carries instructions for making many different proteins. | | | |---|---| | **D** | Thymine | | | Adenine | | | Guanine | | | Cytosine | | **P** | Deoxyribose | | | Phosphate | | **---** | Hydrogen | | | Bond | ## GENES They are the specific parts of chromosomes or deoxyribonucleic acid (DNA) segments which determine hereditary characteristics. Each chromosome is made up of a large number of genes coding for the formation of different proteins which give us our characteristics. The gene responsible for a particular characteristic is always on the same relative position on the chromosome. Every gene has two alternative forms for a character, each of which produces different effects in an organism. These alternative forms are called alleles. Example: In case of pea plants, the stem height is controlled by two alleles-one for tallness and the other for dwarfness. Of the two alleles of a gene, one is dominant, i.e. super ruling and the other is recessive, i.e. subordinate or submissive. A dominant allele is the allele which hides or masks the expression of its corresponding allele, which in turn becomes recessive. The most obvious outcome of the reproductive process still remains the generation of individuals of similar design. The rules of heredity determine the process by which traits and characteristics are reliably inherited. # CELL DIVISION Cell division is the process by which a parent cell divided into two or more daughter cells. Cell division starts with division of nucleus i.e. the chromosome then the cytoplasm. In unicellular organisms the cell divides into two separate daughter cells while in multicellular ones the cell divides into two and continues to divide in the same way. The sequence of events leading to cell division in plants and animals is basically the same. There are two types of cell division namely mitosis and meiosis. ## THE CELL CYCLE Actively dividing eukaryote cells pass through a series of stages known collectively as the cell cycle: two gap phases (G1 and G2); an S (for synthesis) phase, in which the genetic material is duplicated; and an M phase, in which mitosis partitions the genetic material and the cell divides. - **G1 phase.** Metabolic changes prepare the cell for division. At a certain point - the restriction point - the cell is committed to division and moves into the S phase. - **S phase.** DNA synthesis replicates the genetic material. Each chromosome now consists of two sister chromatids. - **G2 phase.** Metabolic changes assemble the cytoplasmic materials necessary for mitosis and cytokinesis. - **M phase.** A nuclear division (mitosis) followed by a cell division (cytokinesis). The period between mitotic divisions - that is, G1, S and G2 - is known as interphase. Mitosis is a nuclear division giving rise to genetically identical cells in which the chromosome number is maintained by the exact duplication of chromosome. Mitosis is the way in which any cell (plant or animal) divides when an organism is: - growing - repairing a damaged part of its body - replacing worn out cells This takes place in all body (somatic) cells of an organism to bring about increase in number of cells, resulting in growth and repair. Growth means getting bigger. An individual cell can grow a certain amount, but not indefinitely. Once a cell gets to a certain size, it becomes difficult for all parts of the cell to obtain oxygen and nutrients by division. In order to grow any more, the cell divides to form two smaller cells, each of which can then grow and divide again. Mitosis is also used in asexual reproduction. For example, sweet potato plant can reproduce by growing adventitious roots or runners which eventually produce new plants. ## PROCESS OF MITOSIS ### Interphase The term interphase is used to describe the state of the nucleus when the cell is just about to divide. During this time the following take place: - Multiplication of genetic material so that daughter cells will have the same number of chromosomes as the parent cell. - Manufacture of cell organelles such as mitochondria, Golgi bodies, centrioles, ribosomes and centrioles. - Energy for cell division is synthesised and stored in form of Adenosine Triphosphate (ATP) to drive the cell through the entire process. During interphase, the following observations can be made: - Chromosomes are seen as long, thin, coiled thread-like structures. - Nuclear membrane and nucleolus are intact. ### Prophase The chromosomes shorten and thicken. Each chromosome is seen to consist of a pair of chromatids joined at a point called centromere. Centrioles (in animal cells) separate and move to opposite poles of the cell. The centre of the nucleus is referred to as the equator. Spindle fibres begin to form, and connect the centriole pairs to the opposite poles. The nucleolus and nuclear membrane disintegrate and disappear. ### Metaphase The nuclear membrane disappears hence chromosomes are free in the cytoplasm. The spindle fibres lengthen. In animal cells they attach to the centrioles at both pole Each chromosome moves to the equatorial plane and is attached to the spindle fibres by the centromeres. ### Anaphase The following events occur during this phase, - Chromatids separate and migrate to the opposite poles due to the shortening of spindle fibres. - The spindle apparatus begins to disappear. - In animal cells, the cell membrane starts to constrict towards the end of the anaphase. ### Telophase This is the final phase of cell division - The chromatids collect together at the two opposite ends of the spindle. - A nuclear membrane forms around each set of chromatids and are now referred to as chromosomes. - The cytoplasm divides into two leading to the formation of two daughter cells. - Chromosomes later become less distinct. ## Significance of Mitosis - It brings about the growth of an organism: - It is a basis of asexual reproduction. - Ensures that the chromosome number is retained. - Ensures that the chromosomal constitution of the offspring is the same as the parents. ## MEIOSIS Meiosis is a reduction division in which the chromosome number is halved from diploid to haploid (i.e. Meiosis involves two divisions of the (diploid) parental cell resulting into four (haploid) daughter cells). Meiosis is the way in which gametes (sex cells) are produced. Gametes have only half the number of chromosome of a normal body cell. They have 1 set of chromosome instead of 2. When they fuse together, the zygote formed has 2 sets. This type of cell division takes place in reproductive organs (gonads) to produce gametes. Human gametes are formed by the division of cells in the ovaries and testes. The gametes produced are haploid, but they are formed from diploid cells, so meiosis involves halving the normal chromosome number -the pairs of chromosomes are separated. During meiosis, the new cells get a mixture of homologous chromosomes from father and mother. A sperm cell could contain a chromosome 1 from father and a chromosome 2 from mother. There are all sorts of combinations --> gametes are genetically different from the parent cells. Meiosis produces genetic variation. When ova are formed in a woman, all the ova will carry an X chromosome. When sperm are formed in a man, half the sperm will carry an X chromosome, half will carry a Y chromosome. ## PROCESS OF MEIOSIS ### Interphase - As in mitosis the cell prepares for division. - This involves replication of chromosomes, organelles and builds up of energy to be used during the meiotic division. ### First Meiotic division #### Prophase I - Homologous chromosomes lie side by side in the process of synapsis forming pairs called bivalents. - Chromosomes shorten and thicken hence become more visible. - Chromosomes may become coiled around each other and the chromatids may remain in contact at points called chiasmata (singular chiasma). - Chromatids cross-over at the chiasmata exchanging chromatid portions. Important genetic changes usually result. #### Metaphase I - Spindle fibres are fully formed and attached to the centromeres. - The bivalents move to the equator of the spindles. #### Anaphase I - Homologous chromosomes separate and migrate to opposite poles. - This is brought about by shortening of spindle fibres hence pulling the chromosomes. - The number of chromosomes at each pole is half the number in the mother cell. #### Telophase I - Cytoplasm divides to separate the two daughter cells. ### Second Meiotic Division - Usually the two daughter cells go into a short resting stage (interphase) - But sometimes the chromosomes remain condensed and the daughter cells go straight into metaphase of second meiotic division. - The second meiotic division takes place just like mitosis. #### Prophase II - Each chromosome is seen as a pair of chromatids. #### Metaphase II - Spindle forms and are attached to the chromatids at the centromeres. - Chromatids move to the equator. #### Anaphase II - Sister Chromatids separate from each other - Then move to opposite poles, pulled by the shortening of the spindle fibres. #### Telophase II - The spindle apparatus disappears. - The nucleolus reappears and nuclear membrane is formed around each set of chromatids. - The chromatids become chromosomes. - Cytoplasm divides and four daughter cells are formed. - Each has a haploid number of chromosomes. ## Significance of Meiosis - Meiosis brings about formation of gametes that contain half the number of chromosomes as the parent cells. - It helps to restore the diploid chromosomal constitution in a species at fertilisation. - It brings about new gene combinations that lead to genetic variation in the offsprings. ## THE DIFFERENCES BETWEEN MITOSIS AND MEIOSIS | | | |---|---| | MITOSIS | MEIOSIS | | - 4 stages in total (plus interphase) | - 8 stages in total (plus interphase) | | - Happens in somatic cells | - Happens in germ cells | | - Purpose is cellular proliferation | - Purpose is sexual reproduction | | - Produces 2 diploid daughter cells | - Produces 4 haploid daughter cells | | - Chromosome number remains the same | - Chromosome number is halved in each daughter cell | | - Genetic variation doesn't change | - Genetic variation increased | ## SIMILARITIES BETWEEN MITOSIS AND MEIOSIS - Both take part in cells - Both involve division of cells (cell multiplication) ## INHERITED TRAITS They are traits that are genetically determined. What exactly do we mean by similarities and differences? We know that a child bears all the basic features of a human being. However, it does not look exactly like its parents, and human populations show a great deal of variation. Inherited traits are passed from parents to offspring according to the rules of Mendelian genetics. Most traits are not strictly determined by genes, but rather are influenced by both genes and environment e.g. language you speak, choice of music etc. ## RULES FOR THE INHERITANCE OF TRAITS The rules for inheritance of such traits in human beings are related to the fact that both the father and the mother contribute practically equal amounts of genetic material to the child. This means that each trait can be influenced by both paternal and maternal DNA. Thus, for each trait there will be two versions in each child. What will, then, the trait seen in the child be? Mendel worked out the main rules of such inheritance, and it is interesting to look at some of his experiments from more than a century ago. ### Mendel's Contributions Gregor Johann Mendel (1822&1884), Started his experiments on plant breeding and hybridization. Mendel was known as Father of Genetics ### Mendel's Laws: - **1. Principle of Segregation:** Two members of a gene pair segregate from each other in the formation of gametes; half the gametes carry one allele, and the other half carry the other allele. What it means: each gene has two copies (alleles) and a parent will give only one copy to a child. The other parent will give another copy, and thus the child will receive two copies (alleles) one from each parent. Each child will literally be half-mom and half-dad! - **2. Principle of Independent Assortment:** Genes for different traits assort independently of one another in gamete production What it means: different genes are inherited separately. For example, the gene which codes for eye color is inherited separately from the gene which codes for nose shape. ### Plant selected by Mendel : *Pisum sativum* (garden pea). Mendel used a number of contrasting characters for garden pea. | Traits | Shape of seeds | Colour of seeds | Colour of pods | Shape of pods | Plant height | Position of flowers | Flower Colour | |---|---|---|---|---|---|---|---| | Dominant trait | Round | Yellow | Green | Full | Tall | At leaf junction | Purple | | Recessive trait | Wrinkled | Green | Yellow | Fat, constricted | Short | At tips of branches | White | **Seven pairs of contrasting traits in pea plant** Mendel used a number of contrasting visible characters of garden peas – round/wrinkled seeds, tall/short plants, white/violet flowers and so on. He took pea plants with different characteristics – a tall plant and a short plant, produced progeny from them, and calculated the percentages of tall or short progeny. In the first place, there were no halfway characteristics in this first generation, or F1 progeny – no 'medium-height' plants. All plants were tall. This meant that only one of the parental traits was seen, not some mixture of the two. So the next question was, were the tall plants in the F1 generation exactly the same as the tall plants of the parent generation? Mendelian experiments test this by getting both the parental plants and these F1 tall plants to reproduce by self-pollination. The progeny of the parental plants are, of course, all tall. However, the second-generation, or F2, progeny of the F1 tall plants are not all tall. Instead, one quarter of them are short. This indicates that both the tallness and shortness traits were inherited in the F1 plants, but only the tallness trait was expressed. Thus, two copies of the trait are inherited in each sexually reproducing organism. These two may be identical, or may be different, depending on the parentage. In this explanation, both TT and Tt are tall plants, while only tt is a short plant. In other words, a single copy of 'T' is enough to make the plant tall, while both copies have to be 't' for the plant to be short. Traits like 'T' are called dominant traits, while those that behave like 't' are called recessive traits. ## SEX DETERMINATION It's the Phenomenon of decision or determination of sex of an offspring. Of the 23 pairs of chromosomes present is each human cell, one pair is the sex chromosomes. These determine the sex of the individual. Male have XY, female have XX. So the presence of a Y chromosome results in male features developing. We have discussed the idea that the two sexes participating in sexual reproduction must be somewhat different from each other for a number of reasons. How is the sex of a newborn individual determined? Different species use very different strategies for this. Some rely entirely on environmental cues. Thus, in some animals, the temperature at which fertilised eggs are kept determines whether the animals developing in the eggs will be male or female e.g. Turtles, Most lizards and the tuatara. In other animals, such as snails, individuals can change sex, indicating that sex is not genetically determined. However, in human beings, the sex of the individual is largely genetically determined. In other words, the genes inherited from our parents decide whether we will be boys or girls. But so far, we have assumed that similar gene sets are inherited from both parents. If that is the case, how can genetic inheritance determine sex? The explanation lies in the fact that all human chromosomes are not paired. Most human chromosomes have a maternal and a paternal copy, and we have 22 such pairs. But one pair, called the sex chromosomes, is odd in not always being a perfect pair. Women have a perfect pair of sex chromosomes, both called X. But men have a mismatched pair in which one is a normal-sized X while the other is a short one called Y. So women are XX, while men are XY. Now, can we work out what the inheritance pattern of X and Y will be? All children will inherit an X chromosome from their mother regardless of whether they are boys or girls. Thus, the sex of the children will be determined by what they inherit from their father. A child who inherits an X chromosome from her father will be a girl, and one who inherits a Y chromosome from him will be a boy.