Human Anatomy & Physiology I Dr. Sullivan Unit IV Cellular Function Chapter 4, Chapter 27 (meiosis only) I. Protein Synthesis: creation of new proteins a. Much of the cellular machinery is devoted to synthesizing a large number of diverse proteins. b. A protein is made up of amino acids held together by peptide bonds. i. The sequence of amino acids is what makes one protein different from another one. c. Genes: a cell s hereditary units, which determine what a cell will look like and what it s structure & function will be. i. Genome: The total genetic information carried in a cell or organism. 1. i.e. blonde hair, blue eyes, 5 10, high cheek bones, big nose, etc. are all part of the genome. d. Proteome: all the proteins in an organism e. Proteins can have many different functions: i. Assemble cellular structures such as the cytoskeleton, plasma membrane, and organelles ii. Serve as non-steroid hormones, antibodies, and contractile elements in muscle tissue iii. Enzymes to regulate the numerous chemical reactions that occur in cells or transporters that carry various materials in the blood 1. i.e. such as transports of oxygen by hemoglobin a. Hemoglobin is a protein f. Gene Expression: the process whereby a gene s DNA (deoxyribonucleic acid) is used to direct synthesis of a specific protein to make sure it synthesizes the correct protein. i. 1 st : the information encoded in a specific region of DNA is transcribed (or copied) to produce a specific molecule of RNA (ribonucleic acid). ii. 2 nd : RNA attaches to a ribosome, where the information is translated into a corresponding sequence of amino acids to form a new protein molecule. II. DNA Structure: a two-stranded double helix of genetic information a. Each strand consists of a backbone of phosphate and a sugar molecule called deoxyribose. b. Attached to the backbone are the nucleotides, Adenine (A), Guanine (G), Thymine (T), and Cytosine (C). c. DNA: deoxyribonucleic acid d. The two strands of phosphate, deoxyribose, and nucleotides are bound together by hydrogen bonds between the nucleotides, forming the double helix formation. i. The arrangement is very specific because each nucleotide will only bind to ONE other specific nucleotide on DNA. ii. Adenine binds to thymine iii. Guanine binds to cytosine -Therefore, there are the following different possible configurations of DNA: 1. P-D-A-T-D-P 2. P-D-T-A-D-P 3. P-D-G-C-D-P 4. P-D-C-G-D-P P stands for Phosphate, D for deoxyribose (sugar), A for adenine, T for thymine, G for guanine, and C for cytosine. e. The sequence of these nucleotides makes up the genetic code for everything about a cell, tissue, and organism. f. Each set of three nucleotides together is code for and represents one amino acid, the building blocks of proteins.
III. IV. i. This set of three is called a base triplet ii. The genetic code is the set of rules that relate the base triplet to the codon and the amino acids they specify. 1. The genetic code describes exactly how the protein is to be built. Transcription: the first step in protein synthesis a. During transcription, the base triplets serve as a template for copying the information as a strand of RNA. b. There are three kinds of RNA made from the base triplet or DNA template. i. Messenger RNA (mrna): directs the synthesis of the protein ii. Ribosomal RNA (rrna): joins with ribosomal proteins to make ribosomes. iii. Transfer RNA (trna): binds to an amino acid and holds it in place on the ribosome until it is incorporated into a protein during translation (the next step) c. Step 1: The enzyme RNA Polymerase catalyzes a reaction breaking the hydrogen bonds between the DNA s nucleotides, separating the backbones of the double helix from one another at the nucleotides. i. This leaves the nucleotides open to bond with free-floating nucleotides in the nucleus, which will now take advantage and bond to the open nucleotides. 1. In this case, there are 4 nucleotides: 2. Adenine (A), Cytosine (C), Guanine (G), and Uracil (U): taking the place of thymine. 3. These nucleotides have a backbone of phosphate and a sugar called ribose. ii. RNA Polymerase must be instructed where on the DNA strands to start the transcription and where to end it. 1. The spot where it starts is called the promoter and where it ends is called the terminator. 2. RNA Polymerase attaches to the DNA at the promoter to start transcription. d. Step 2: the free floating nucleotides form hydrogen bonds with the appropriate DNA nucleotides (A to U, C to G). i. Since we know which nucleotide has to bind with which other nucleotide, the original DNA code (sequence of nucleotides) can be easily deciphered. ii. At this point, each set of three free-floating nucleotides that bond to the DNA nucleotides, is called a codon. Each codon is an indirect copy of a base triplet and represents the same amino acid that the original base triplet represented. e. Step 3: RNA Polymerase catalyzes a reaction bonding the phosphate-ribose backbones of now-bonded, formerly free-floating, nucleotides. i. This forms a strand of nucleotides that preserve the original nucleotide sequence that was on the DNA. f. Step 4: When RNA polymerase reaches the terminator, it removes itself from the DNA, the nucleotides that bonded to the open DNA nucleotides break their bonds from DNA, but maintain their bonds to each other, forming a strand of nucleotides called messenger RNA, or mrna. g. Step 5: With RNA Polymerase now absent, the DNA nucleotides re-form their hydrogen bonds and the double helix is restored. h. Transcription is now finished and the mrna molecule, carrying a code for a sequence of amino acids, can pass through a pore in the nuclear envelope into the cytoplasm for translation. Translation: the process whereby the nucleotide sequence in an mrna molecule specifies the amino acid sequence in the building of a protein. a. Translation is carried out by ribosomes. i. The small subunit of the ribosome has a binding site for mrna. ii. The large subunit of the ribosome has 2 binding sites for trna
1. The P site is where the first trna molecule, holding an amino acid and the A site is where the 2 nd trna molecule attaches holding its specific Amino Acid. b. trna i. trna is a molecule of RNA that has three specific nucleotides attached to it. This set of three on the trna is called an anti-codon. ii. Each anti-codon on trna is the nucleotide complement of a codon on mrna 1. i.e. if the mrna codon is A-C-U, there is a trna molecule with the anti-codon U-G-A, which would bind to its codon. 2. each trna is also bound to an amino acid specific to its anticodon. This amino acid is the same one that the codon is calling for in the mrna molecule. c. Step 1: The mrna, carrying the genetic code binds to the small subunit of a ribosome. i. The first codon, aka the start codon, is in place on the small subunit of the ribosome. d. Step 2: A molecule of trna with the first codon s anti-codon attaches to the P site on the ribosome s large subunit and holds on to its amino acid, the same amino acid represented by the 1 st codon. e. Step 3: A molecule of trna with the second codon s anti-codon, holding the next amino acid, attaches to the A site of the large subunit. f. Step 4: A component of the large ribosomal subunit catalyzes a reaction causing the formation of a peptide bond between the two amino acids, starting the chain that will form a protein. g. Step 5: the trna on the P site detaches from the ribosome, leaving the P site empty. Its amino acid is also released, leaving it bound to the other amino acid. h. Step 6: the trna molecule that is bound to the A site shifts over to the P site, leaving the A site empty. i. Step 7: The mrna shifts so that the 3 rd codon is now bound near the A site. A trna molecule with the corresponding anti-codon binds to the A site carrying the appropriate amino acid. Go to Step 4 and repeat, forming a chain of amino acids bound together by peptide bonds. j. Repeat this process until the final codon, called the stop codon, is reached. k. After the stop codon is reached, the mrna detaches from the small subunit. i. The final molecules of trna detach from the P & A sites and also release their amino acids, so we are left with a peptide chain of amino acids, aka a protein. ii. The protein product, which may be an enzyme, hormone, etc. is then packaged and sorted by the golgi complex for use by the cell. V. The Cell Cycle in Somatic Cells a. Cell Cycle: an orderly sequence of events by which a somatic cell duplicates its contents and divides into two. b. Human body cells (or Somatic Cells) contain 23 pairs of chromosomes. Of the two chromosomes in each pair, one comes from the mother and one form the father. i. Because there are two sets of chromosomes per cell, these cells are called diploid cells. 1. Gametes, which are reproductive cells such as sperm or eggs, only have one set of chromosomes and are called haploid cells. ii. Each pair of chromosomes contains two homologous chromosomes, or homologues. iii. DNA and traits are passed along the next generation of cells. c. The Cell cycle consists of two major phases. When a cell divides, it must replicate all of its chromosomes so that it s i. Interphase: when a cell is not dividing ii. Mitotic Phase: when a cell is dividing
d. Interphase: The cell replicates its DNA into two identical sister chromatids during this time and also produces organelles and other components of the cytosol in anticipation of cell division i. G1 Phase: cell duplicates organelles and cytosolic components but not DNA. ii. S Phase: DNA and centrosome replication occurs iii. G2 Phase: cell growth continues and enzymes and other proteins are synthesized in preparation for cell division. iv. G0 State: Cells that remain in the G1 Phase for a very long time and may never divide again. 1. i.e. most nerve cells e. Mitotic Phase: this phase consists of division of the nucleus, or mitosis, and cytoplasmic division, or cytokinesis. i. Distribution of the two sets of chromosomes into two separate nuclei occurs during mitosis. ii. There are four stages of Mitosis: 1. Prophase: the chromatin fibers condense and shorten into chromosomes. a. Since DNA replicated in the S Phase, each chromosome consists of a pair of chromatids held together by a the centromere. b. Each centromere is surrounded by a protein complex called the kinetochore. c. Tubulins in the pericentriolar material will form a mitotic spindle, which attached to each kinetochore. The mitotic spindle will pulll the chromatids to opposite poles of the nucleus. 2. Metaphase: The microtubules attached to the kinetochore align the chromotid pairs to the exact center of the nucleus, divided by the metaphase plate. 3. Anaphase: the centromeres split, separating the members of each chromatic pair. The separated chromatids are now called chromosomes, and are led by the centromere to opposite poles of the nucleus. During late anaphase/early telophase, a cleavage furrow begins to form down the center of the cell, preparing to divide it in two. 4. Telophase: Once the chromosomes reach the poles, they uncoil and revert to chromatin form, creating a chromatin mass at each pole of the nucleus. A nuclear envelope forms around the chromatin mass while the cleavage furrow continues to form down the center of the nucleus, dividing it in two. a. The mitotic spindle disappears, nucleoli reappear in each side and two daughter cells are formed. 5. During late anaphase and early telophase cytokinesis takes place, whereas the cytoplasm divides. iii. Once Mitosis and cytokinesis is comlete, Interphase begins again in each daughter cell. VI. Meiosis: cell division taking place in the gonads (reproductive organs) producing haploid gametes (a male or female reproductive cell, i.e. sperm or egg) i. Haploid gametes must divide as well. ii. Meiosis is divided into two stages: Meiosis I and Meiosis II. iii. Meiosis I (reduction division): DNA replication during interphase prepares the cell for meiosis I. The purpose of meiosis I is to reduce the number of chromosomes from 46 to 23 (diploid to haploid) II. The DNA is doubled on each chromosome as identical sister chromatids. 1. Prophase I: very similar to prophase of Mitosis with 2 additions:
iv. a. The two sister chromatids of each pair of chromosomes pair off called synapsis, creating a tetrad of chromatids. b. Because of the tetrad formation, parts of the chromatids of two homologous chromosomes may be exchanged. i. This is called crossing-over, permitting an exchange of genes between chromatids of homologous chromosomes. ii. The result is genetic recombination. Each daughter cell will not be exactly like the parent cell. There can be a new combination of genes on each daughter cell now. iii. Therefore, brothers and sister do not look exactly alike all the time. 2. Metaphase I: The tetrads line up at the metaphase plate 3. Anaphase I: Homologous chromosomes separate and are pulled toward the poles a. In this anaphase, the sister chromatids stay together at the centromere, instead of splitting. b. Meaning that one full homologue chromosome of each pair with double DNA goes to each pole 4. Telophase I: The cleavage furrow splits the cell into two haploid daughter cells that are genetically different from one another. 5. Interkinesis: similar to interphase without DNA replication Because one of the chromosomes of each pair goes to opposite poles, the resultant daughter cells will have 23 chromosomes each (haploid). However, each chromosome still has double the DNA from replication prior to meiosis I. Meiosis II (equational division): This consists of Prophase II, Metaphase II, Anaphase II, and Telophase II. 1. The phases of Meiosis II are the same as the phases of mitosis. The sister chromatids of each chromosome are pulled apart at the centromere and migrate to opposite polls. This allows each new cell to have 23 chromosomes and the full complement of DNA resulting in 4 haploid gametes. a. Therefore, in meiosis II, the number of chromosomes starts at 23 and remains 23 in the daughter cells. 2. The total result of Meiosis I & II is 4 genetically different haploid gametes from one diploid parent cell.