6Gametogenesis Gametes are made by a process called gametogenesis. Female gametes are made in the ovaries and male gametes are made in the testes.Gametogenesis involves a special type of division called meiosis.In meiosis, the number of chromosomes is halved, so that when gametes unite in fertilisation, they have the correct number of chromosomes.Human gametes have only 23 chromosomes (and are called haploid cells), while normal human cells have 46 chromosomes (and are called diploid) cells).In males, meiosis produces 4 gametes, whereas in females it produces only 1.The gametes then undergo a period of maturation. In males this occurs in the epididymis, while in females this occurs in the follicle in the ovary, and the uterine tube shortly after ovulation.
8Meiosis 1 – the first division prophase Ithe chromosomes become visible - each chromosome contains two chromatidshomologous chromosomes move together & a spindle formsmetaphase Ihomologous pairs of chromosomes line up on the equator, attached by their centromeresanaphase Ione of each pair of homologous chromosomes is pulled to either end of the celltelophase Ithe chromosomes condensecytoplasm is split of and cell divides into two daughter cells, each with 23 chromosomeseach chromosome consists of two chromatids
9Meiosis 2 – the second division prophase IIthe chromosomes become visible - each chromosome contains two chromatidsa new spindle forms, at right angles to the old onemetaphase IIchromosomes line up on the equator, attached by their centromeresanaphase IIone of each chromatid in each chromosome is pulled to either end of the celltelophase IIthe chromosomes condensecytoplasm is split of and each daughter cell divides into two (ie total of 4 daughter cells), each with 23 chromosomes - each chromosome consists of one chromatidGametes
10Comparing mitosis and meiosis Where it occursBody cellsGonads (reproductive organs)Why it occursCell repair, growth, asexual divisionSexual reproductionNumber of cells produced24Number of divisions1Number of chromosomes in daughter cellsSame as parent (diploid)Half that of parent (haploid)Amount of variation in daughter cellsNoneLots
11Comparing mitosis and meiosis 2 Advantages for reproductionSimpleRapid divisionAllows variationDisadvantages for reproductionNo variationMore complexSlower reproduction
12DNAMeiosis is necessary in order to make sure that the gametes have the correct amount of DNA.DNA (deoxyribose nucleic acid) in found in the nucleus of all cells, and controls protein synthesis, and cell activity.DNA is also found in mitochondriaNuclear DNA is arranged in structures called chromosomes, which are only visible when the cell is dividing.Humans have 46 chromosomes arranged as 23 pairs.These chromosomes are passed on in meiosis so that each new individual has 1 of each pair from its mother, and 1 of each pair from its father.Mitochondrial DNA is inherited only from the mother as it is the ovum that provides the cell and its organelles (mitochondria) for the new zygote.The sperm provides only nuclear DNA as the mitochondria in the flagella are rapidly destroyed.Mitochondrial DNA can be used to trace ancestry of people.
13The structure of DNADNA consists of two strands of alternating sugars and phosphates, with pairs of nuclear bases forming cross-links between the chains.This is called a double helix.A nucleotide is a nuclear base attached to a sugar and a phosphateThe four bases are cytosine, guanine, adenine and thymine.Each base links up with its base pair - cytosine with guanine, adenine with thymineEach strand is coiled around the histones so they can fit into a small space.When not dividing, the tangled DNA is called chromatin.The base sequences on the DNA acts as a code that controls the action of the cell.Sections of DNA code which control one feature are called genes, so the DNA sequence is called the genetic code.In order to read the code, that section of DNA must untangle and uncoil from its histone.The code sequences are copied as mRNA, which is used to make proteins.Proteins are used as structural components (eg in cell membranes, skin and hair - keratin, messengers (eg hormones) and as enzymes.Protein synthesis occurs in the ribosomes.
15Why are proteins important? Roles of proteins in the body includeStructural proteins eg collagen, keratinEnzymes (organic catalysts) eg digestive enzymesTransport proteins eg haemoglobinRegulatory proteins eg hormonesProtective proteins eg antibodies, clotting factorsTherefore proteins determine what you will look like, and how your body functions
16DNA replication When cells divide the DNA is copied or replicated. DNA replication occurs during meiosis in interphase, just before prophase starts.During DNA replication the strands s_____________New bases are added using the b_______ pair ruleadenine attaches to __________________thymine attaches to __________________cytosine attaches to __________________guanine attaches to __________________The separation and attachment of new nucleotides are controlled by enzymes.
17DNA replication When cells divide the DNA is copied or replicated. DNA replication occurs during meiosis in interphase, just before prophase starts.During DNA replication the strands separateNew bases are added using the base pair ruleadenine attaches to thyminethymine attaches to adeninecytosine attaches to guanineguanine attaches to cytosineThe separation and attachment of new nucleotides are controlled by enzymes.
18IntercourseDuring intercourse semen containing male gametes is introduced into the female’s vagina, so as to allow fertilisation.Semen consists of spermatozoa (produced in the testes, and matured in the epididymis) and fluid secretions from the accessory glands.The seminal vesicles secrete sugary fluid, which provides food for the sperm; the prostate secretes alkaline fluid, which neutralises vaginal secretions; and the bulbo-urethral glands secrete mucus, which acts as lubricant.During sexual excitement the erectile tissue of the penis fills with blood, causing the penis to become erect.Contractions of epididymis, vas deferens, and accessory glands cause semen to travel through the vas deferens and then the urethra and finally to be ejaculated from the penis.When the penis is placed in the vagina during intercourse, the semen is deposited at the top of the vagina, just below the cervix.Sexual excitation in the female causes vaginal widening, cervical relaxation, an increase in mucus and alteration in acid secretions, thus allowing sperm to enter the uterus and survive more easily.The sperm move up through the uterus, using their flagella to propel them, heading towards the uterine tubes, where they may find an ovum to fertilise. .
19FertilisationIf they find an ovum, the sperm must burrow through the outer layer of cells, and the cell membrane.The head of the sperm contains enzymes which do this.Once one sperm has entered the ovum, the cell membrane changes so no other can enter and the sperm’s chromosomes combine with the ovum’s chromosome.Because the gametes have only half the chromosomes, they have a much shorter survival time than normal cells - sperm ~72 hours, ova 12 – 24 hours, so in order for fertilisation to occur sperm should be present just before ovulationAfter fertilisation, the zygote moves down the uterine tube, into the uterus, where it embeds itself in the endometrium, thus initiating pregnancy.Symptoms of pregnancy include: no menstruation, weight gain, enlarged and tender breasts, increased urination, (and sometimes morning sickness)Twins can result in two ways:Identical twins occur when the zygote splits and develops into 2 babies – ie both genetically identicalNon-identical twins occur when both ovaries ovulate and both ova are fertilised, so that babies are not genetically identical
20Multiple choice questions 3 The questions below apply to the following diagram that shows part of an animal cell viewed under a microscope using a 10x ocular lens and a 40x objective lens.The nucleus was measured to be 20 m in diameter. What was the field diameter for the diagram above?10 m50 m100 m200 m
21Multiple choice questions 3 The questions below apply to the following diagram that shows part of an animal cell viewed under a microscope using a 10x ocular lens and a 40x objective lens.The nucleus was measured to be 20 m in diameter. What was the field diameter for the diagram above?10 m50 m100 m200 m
22Multiple choice questions 4 The questions below apply to the following diagram that shows part of an animal cell viewed under a microscope using a 10x ocular lens and a 40x objective lens.Without changing anything else, the observer changed the revolving nosepiece of the microscope to a different magnification. This allowed the image of the entire cell to just fall within the field of view. The cell was then measured to be 200 m long. What was the magnification of the second objective used?5 x10 x20x80 x
23Multiple choice questions 4 The questions below apply to the following diagram that shows part of an animal cell viewed under a microscope using a 10x ocular lens and a 40x objective lens.Without changing anything else, the observer changed the revolving nosepiece of the microscope to a different magnification. This allowed the image of the entire cell to just fall within the field of view. The cell was then measured to be 200 m long. What was the magnification of the second objective used?5 x10 x20x80 x