Cell Cycle, DNA, and Protein Synthesis Chapters 8 and 10

The Cell Cycle We have learned that the basic unit of life is the cell. Like all living things, the cell goes through a cycle of growth and reproduction. The sequence of growth and division of a cell is called the Cell Cycle. Most of the cell’s life is spent in the growth phase known as interphase –made up of three phases: G1, S, and G2 The shortest phase in the cycle is the cell division phase known as mitosis and cytokinesis.

The Cell Cycle (see diagram on page 228) G1 – GAP 1 – Chromosomes are not visible (chromatin) Cell is rapidly growing and synthesizing proteins for daily functions S Stage - Synthesis Chromosomes are replicated to form a pair of sister chromatids connected by a centromere G2 – Gap 2 - Cell is growing and producing proteins needed for mitosis Mitosis - Cell divides the nucleus followed by cytoplasm division (cytokinesis) resulting in two identical daughter cells

Mitosis During mitosis, one parent cell divides into two identical daughter cells. All somatic cells (cells other than the sex cells that make eggs and sperm) undergo mitosis. There are four phases of mitosis: –Prophase –Metaphase –Anaphase –Telophase

Prophase This is the first and longest phase in mitosis. The nuclear envelope disappears Chromatin coils to become visible chromosomes The two halves of the doubled structure are called sister chromatids. Sister chromatids are exact copies of each other and are held together by a centromere. In animal cells, the centrioles move to opposite ends of the cell and start to form spindle fibers

Metaphase The second and shortest phase in mitosis The spindle fibers attach to the centromere The sister chromatids are then pulled to the middle of the cell and line up on the midline or equator One sister chromatid from each pair points to one pole while the other points to the opposite pole

Anaphase The centromeres split and the sister chromatids are pulled to opposite poles of the cell

Telophase Chromosomes uncoil Spindle is broken down Nuclear envelope reappears Cytokinesis begins

Cytokinesis Cytoplasm is split forming two daughter cells each with its own nucleus and cytoplasmic organelles –In animals: a cleavage furrow is formed that pinches the two cells apart –In plants: a cell plate forms between the two new cells to start the formation of the cell wall (this does not occur in animal cells!) Cell Plate

Name the Phase Prophase Metaphase Anaphase Metaphase Telophase Anaphase

Controlling the Cell Cycle The cell cycle is driven by a chemical control system telling the cell when to turn on and off cell division –Internal signals – cell senses the presence of enzymes produced within the cell –External signals – cell senses the presence of chemicals (such as growth factors) produced by other specialized cells Cells also respond to physical signals –When cells are packed in too closely, division is turned off –When cells are not in contact with other cells, division is turned on

Controlling the Cell Cycle The cycle control system is regulated at certain checkpoints At each checkpoint, the cell decides if it should go on with division –G1 – makes sure conditions are favorable and cell is big enough for division –G2 – cell checks for any mistakes in the copies of DNA –Mitosis – cell makes sure chromosomes and spindle are arranged properly Specific stimuli are required to initiate cell division. Cell division in most animal’s cells is in the “off” position when no stimulus is present

Mitosis Out of Control Cancer cells are an example of cells that do not listen to the cell’s control system Cancer cells keep dividing even though they may be closely packed together or no growth factor is present. Cancer begins as a single cell This cell is normally found and destroyed by the body’s immune system. If not, this cell could divide into a mass of identical daughter cancer cells that: –Impair the function of one or more organs – malignant tumor Cells can break off, enter the blood and lymph systems and invade other parts of the body and become new tumors. –Remain at their original site – benign tumor

DNA Deoxyribonucleic Acid (DNA) contains the information for life – all the instructions needed to make proteins (including enzymes) A segment of DNA that controls the production of a protein is called a gene. Hundreds of genes together make up a chromosome. DNA  genes  chromosomes DNA is a polymer made up of a chain of nucleotides Each nucleotide has three parts: –simple sugar (deoxyribose) –phosphate group –Nitrogen base (adenine, guanine, thymine, or cytosine)

DNA Nucleotide Structure

DNA Structure Nucleotide Single ring nitrogen bases always bind with a double ring nitrogen base: Adenine to Thymine Cytosine to Guanine

Nucleotide Sequence The DNA of all living things has the same four nitrogen bases. They are different due to the different sequences of those bases. –For example, the code ATTGAC would code for a different protein than TCCAAA Because the order of these bases is so important, DNA must carefully replicate itself when the cell divides to ensure an exact copy is passed on to each daughter cell

DNA Replication DNA is un zipped and unwound by the enzyme helicase The enzyme Polymerase attaches and reads the DNA DNA nucleotides find their compliments on each side of the DNA strand New bases keep attaching until two identical molecules of DNA are completed. This is called semi-conservative replication. Replication Forks increase the speed of replication. Mitosis would then follow where each daughter cell would be given matching copies of the original DNA

Protein Synthesis Flow of genetic information-- DNA→RNA→Protein DNA holds the code for protein synthesis but it cannot leave the nucleus. Protein synthesis is performed at the ribosomes in the cytoplasm The cell uses RNA to copy the code from DNA and bring it to the ribosomes RNA (ribonucleic acid) has three parts: –Simple sugar (ribose) –Phosphate group –Nitrogen base (adenine, cytosine, guanine, and uracil) There is no thymine in RNA – it is replaced with uracil

RNA Structure

Transcription Copying the portion of DNA that carries the code for a protein is called transcription. A portion of DNA that codes for a specific protein is unwound RNA nucleotides find their compliment DNA - ATTGCTCCG RNA - UAACGAGGC The RNA strand (mRNA) releases from the DNA strand mRNA strand is edited and released from the nucleus

Chapter 10 Transcription

Translation Translation is the process of interpreting mRNA to build a chain of amino acids that make up a protein mRNA leaves the nucleus and heads to the ribosomes where translation will occur Each sequence of three nucleotides is called a codon. Each codon codes for a specific amino acid. UAA CGA GGC codon

Translation Steps Amino acids are brought to the ribosome by tRNA There are 20 different tRNA molecules, one for each type of amino acid tRNA anticodons find their compliment codon on the mRNA mRNA codons – UAA CGA GGC tRNA anticodons – AUU GCU CCG Peptide bonds forms between the amino acids forming a polypeptide Translation stops when a stop codon is reached tRNA

Translation Steps

Chapter 10 Genetic Code The nearly universal genetic code identifies the specific amino acids coded for by each three-nucleotide mRNA codon. The Human Genome: The entire gene sequence of the human genome, the complete genetic content, is now known. Approximately 30,000 genes.

Processes and Code Transfer Replication – copies DNA to make another identical double strand of DNA Transcription – makes a copy of a section of DNA and creates a single strand of mRNA Translation – reads the sequence of mRNA nucleotides to build a protein

Protein Secretion The polypeptide chain that is made during translation is sent to the endoplasmic reticulum (ER) for any further structural components Golgi bodies package the protein and send it to the cell membrane The protein is then secreted from the cell and sent where the body needs it

Mutations If the mRNA does not copy the code correctly, the amino acid chain will be altered – this is called a mutation