Lesson 1 Chapter 1 – Genetic Information

Lesson 1 Chapter 1 – Genetic Information 1 Unit 2 Study Guide Genetic Information Cell Nucleus and Nucleic Acids I. The cell nucleus is surrounded by a double-layered nuclear membrane. At some points, the two layers are fused by nuclear pore complexes that allow for the passage of molecules. II. Nucleic acids include DNA, RNA, and their nucleotide subunits.  The DNA nucleotides contain the sugar deoxyribose, whereas the RNA nucleotides contain the sugar ribose.  There are four different types of DNA nucleotides, each of which contains one of four possible bases: adenine, guanine, cytosine, or thymine; in RNA, the base uracil substitutes for the base thymine.  DNA consists of two long polynucleotide strands twisted into a double helix. The two strands are held together by hydrogen bonds between specific bases; adenine pairs with thymine [A-T] guanine pairs with cytosine. [C-G]  RNA is single-stranded. Four types of RNA are produced within the nucleus: ribosomal RNA, transfer RNA, precursor messenger RNA, and messenger RNA. III. Active euchromatin directs the synthesis of RNA; this process is called transcription.  The enzyme RNA polymerase causes separation of the two strands of DNA along the region of the DNA that constitutes a gene.  One of the two separated strands of DNA serves as a template for the production of RNA; this occurs by complementary base pairing between the DNA bases and ribonucleotide bases. Protein Synthesis and Secretion I. Messenger RNA leaves the nucleus and attaches to the ribosomes. Lesson 1 Chapter 1 – Genetic Information 2 II. Each transfer RNA, with a specific base triplet in its anticodon, bonds to a specific amino acid.  As the mRNA moves through the ribosomes, complementary base pairing between tRNA anticodons and mRNA codons occurs.  As each successive tRNA molecule bonds to its complementary codon, the amino acid it carries is added to the end of a growing polypeptide chain. III. Proteins destined for secretion are produced in ribosomes located on the rough endoplasmic reticulum (RER) and enter the cisternae of this organelle. IV. Secretory proteins move from the rough endoplasmic reticulum to the Golgi complex, which consists of a stack of membranous sac.  The Golgi complex modifies the proteins it contains, separates different proteins, and packages them in vesicles.  Secretory vesicles from the Golgi apparatus fuse with the cell membrane and release their products by exocytosis. DNA Each nucleotide has 3 components [PSN] – – – a phosphate group 5 carbon ring sugar molecule a nitrogenous base RNA RNA is a polymer of nucleotides similar to DNA. As mentioned above, RNA has ribose instead of deoxyribose. It also does not have the base thymine (T) instead it has the base uracil (U): [A-T in DNA but A-U in RNA] Three types of RNA: - Messenger RNA (mRNA) = Transcription Transfer RNA (tRNA) = Making proteins (reading the code) Ribosomal RNA (rRNA) Lesson 1 Chapter 1 – Genetic Information 3 The general steps of protein synthesis are: – – – Transcription Translation and protein synthesis Packaging of synthesized proteins by the Golgi complex Transcription During transcription the DNA sequence of a gene is copied (transcribed) into the sequence of a single stranded mRNA. It is divided into three processes: - Initiation Elongation Termination 1) Initiation starts when the enzyme RNA binds to a specific segment of DNA to be transcribed. RNA polymerase binds the DNA molecule in front of the gene to be transcribed in a region called the promotor. In most genes the promotor sequence contains a string of adenine and thymine bases. The promotor indicates where the RNA polymerase should start transcribing the DNA. (The binding site of RNA polymerase only recognizes the promotor region, therefore it always binds in front of the gene.) The DNA double helix is unwound, exposing the template strand. 2) Elongation (after binding and opening the DNA double helix) RNA polymerase: o Starts building the single-stranded mRNA in a 5’ to 3’ (5prime to 3prime) direction. o The promotor is not transcribed.  Does not require a primer  Copies only one of the DNA strands o The transcribed DNA strand is called the template strand.  The mRNA is complementary to the DNA template except that it contains the base uracil in place of thymine. 3) Termination brings the synthesis of mRNA to continue until RNA polymerase reaches the end of the gene. RNA polymerase recognizes the end of gene when it comes to a stop signal called a termination sequence. Transcription stops and the new mRNA disconnects from the DNA template. Lesson 1 Chapter 1 – Genetic Information 4 RNA polymerase binds to another promotor region and transcribes another gene. Translation The RNA polymerase moves from one nucleotide to another – – – mRNA diffuses out of the nucleus [DNA can’t leave nucleus] tRNA carries message to a ribosome in ribosome – rRNA reads the code • Translation of mRNA into protein is accomplished by the ribosome, an RNA/protein hybrid. Ribosomes are composed of 2 subunits, large and small. • Ribosomes bind to the translation initiation sequence on the mRNA, then move down the RNA in a 5’ to 3’ direction, creating a new polypeptide. The first amino acid on the polypeptide has a free amino group, so it is called the “N-terminal”. The last amino acid in a polypeptide has a free acid group, so it is called the “C-terminal”. • Each group of 3 nucleotides in the mRNA is a “codon”, which codes for 1 amino acids. Transfer RNA is the adapter between the 3 bases of the codon and the corresponding amino acid. Lesson 1 Chapter 1 – Genetic Information 5 Set into elongation and termination stages: 1) Elongation a. The first codon that is recognized by the ribosome is the start codon AUG (methionine) b. The ribosome has two sites for tRNA to attach c. A peptide bond forms between the first and second amino acid d. A ribosome shifts over one codon e. A peptide bond forms between the first and second amino acid f. The ribosome shifts over one codon so now the second tRNA is in the next site. g. This action will release the first tRNA molecule i. Which will allow for another tRNA to bind to the next site. ii. Released tRNAs are recycled in the cytoplasm by adding new amino acids to them. h. The process continues until a stop codon is reached. 2) Termination a. Three stop codons have no corresponding amino acids i. UGA, UAG, and UAA b. A protein known as a release factor recognizes that the ribosome has stalled and releases the polypeptide chain from the ribosome. Lesson 1 Chapter 1 – Genetic Information 6 c. The two subunits fall off the mRNA strand. DNA Synthesis and Cell Division I. Replication of DNA is semiconservative; each DNA strand serves as a template for the production of a new strand.  The strands of the original DNA molecule gradually separate along their entire length and, through complementary base pairing, form a new complementary strand.  In this way, each DNA molecule consists of one old and one new strand. II. During the G1 phase of the cell cycle, the DNA directs the synthesis of RNA, and hence that of proteins. Lesson 1 Chapter 1 – Genetic Information 7 III. During the S phase of the cycle, DNA directs the synthesis of new DNA and replicates itself. IV. After a brief rest (G2), the cell begins mitosis (the M stage of the cycle).  Mitosis consists of the following phases: interphase, prophase, metaphase, anaphase, and telophase.  In mitosis, the homologous chromosomes line up single file and are pulled by spindle fibers to opposite poles.  This results in the production of two daughter cells which each contain fortysix chromosomes, just like the parent cell. V. Meiosis is a special type of cell division that results in the production of gametes in the gonads.  The homologous chromosomes line up side by side, so that only one of each pair is pulled to each pole.  This results in the production of two daughter cells that each contain only twenty-three chromosomes, which are duplicated.  The duplicate chromatids are separated into two new daughter cells during the second meiotic cell division. Prior to the initiation of mitosis or meiosis, the cell must duplicate its chromosomes and manufacture the materials needed for cell division. This is accomplished in a phase that precedes cell division known as interphase. Interphase can be divided into three phases. In the first, called the Gl phase, the cell carries out its basic functions and performs specialized activities. In the second period of interphase, the S phase, each chromosome in the nucleus is replicated so that there are now two identical copies of each chromosome. Identical chromosomes are attached to each other by way of a centromere. When attached by their centromeres, chromosomes are referred to as chromatids. In plants, the mitotic spindle is also formed during the S phase. In animal cells, a pair of centrioles duplicates to form two pairs of centrioles. Like the mitotic spindle in plants, the centrioles will form spindle fibers. Spindle fibers are composed of microtubules and are used to pull the duplicated chromosomes apart during cell division. In the third period of interphase, the G2 phase, proteins and structures that will be used during mitosis or meiosis, are synthesized. After the G2 phase has been completed, the cell can then undergo mitosis or meiosis. Lesson 1 Chapter 1 – Genetic Information MITOSIS IPMAT Although mitosis is a continuous process, it is divided into four phases for ease of understanding. The phases are: prophase, metaphase, anaphase, and telophase. Interphase, plus the four phases of mitosis, are referred to as the cell cycle. The following is a brief synopsis of the events of the four phases of mitosis (the events of interphase are described above): Prophase During prophase, the first phase of mitosis, the chromosomes condense (shorten) and become visible when stained and viewed under a microscope. During the latter part of prophase, the nuclear membrane disappears and the newly formed spindle fibers attach to a region of the centromere. Metaphase During metaphase, the spindle fibers move the replicated chromosomes to the middle of the cell where they line up in single file along the equatorial plane. Anaphase During anaphase, the sister chromatids are "pulled" apart (separated) and move toward opposite poles of the cell. The separated chromatids are now called daughter chromosomes. Telophase During telophase, the last phase of mitosis, the spindle apparatus disappears and nuclei reform around each set of daughter chromosomes. Cytokinesis also occurs during telophase. In animal cells, a cleavage furrow forms and eventually divides the cytoplasm in half among the two daughter cells. In plant cells, cytokinesis is accomplished with the formation of the cell plate. MEIOSIS Meiosis is used for sexual reproductions. It consists of two nuclear divisions {meiosis I and meiosis II) and results in the production of haploid cells called gametes. Because the chromosome number is reduced by one half, meiosis is sometimes referred to as reduction division. Before meiosis can proceed, however, homologous chromosomes must first pair together, a process known as synapsis. This is the time that crossing over can occur. Crossing over results in the exchange of genetic material between homologous chromatids. As in in mitosis, meiosis is a continuous process. 8 Lesson 1 Chapter 1 – Genetic Information Chromosome 9 Lesson 1 Chapter 1 – Genetic Information • • 10 Autosomes – The first 22 of the 23 pairs of chromosomes in males and females – The two members are virtually identical and thus said to be homologous Sex chromosomes – Remaining pair of chromosomes – In females, it is a homologous pair (XX) – In males, it is a nonhomologous pair (XY) Abnormalities Syndrome Down Edward's Turner’s Klinefelter’s XXX XXY Abnormality Trisomy 21 Trisomy 18 Monosomy X XXY XXX XYY Down syndrome (DS) is the most common cause of mental retardation and malformation in a newborn. Edward’s syndrome characterized by mental retardation, abnormal skull shape, low-set and malformed ears, small mandible, cardiac defects, short sternum, diaphragmatic or inguinal hernia, Meckel diverticulum, abnormal flexion of fingers, and dermatoglyphic anomalies. Turner syndrome is a chromosomal disorder affecting females wherein one of the two X-chromosomes is defective or completely absent. Klinefelter syndrome is a chromosomal disorder that affects only males. People with this condition are born with at least one extra X chromosome.