2 24 12 Nucleus 3 Main Regions nuclear envelope membrane nucleolus chromatin nucleus the control center of the cell Nucleus controls all activities occurring inside a cell Nuclear envelope similar in composition to the plasma membrane Except the nuclear membrane lacks a glycocalyx In addition the nuclear membrane has pores called nuclear pores openings in the nuclear membrane Nuclear pores allow for the export of substances from the nucleus into the cytoplasm ex mRNA cytoplasm into the nucleus The nuclear pores also allow for the import of substances from the Nucleolus dark spherical structure in the nucleoplasma Site for assembling the ribosomal subunits both small and large Involved in the synthesis of rRNA ribosomal RNA Once the rRNA is synthesized it will form the subunit of the ribosome The small subunit and the large subunit are exported from the nucleus into the cytoplasm through the nuclear pores In the cytoplasm the large and small subunit will fuse to from the functional ribosome Because the nucleolus is involved in the assembly of the ribosomal subunits cells actively producing proteins have an abundance of nucleoli Chromatin appears as thread like beaded structures meandering through the nucleoplasm Chromatin is composed of structural units called nucleosomes A nucleosome consists of 8 histone proteins with a piece of the thread like DNA wrapped around the histone proteins DNA is made of nucleotides pentose sugar deoxyribose or ribose nitrogen containing base A G T U C phosphate group DNA is a type of nucleic acid There are 2 types of nucleic acids DNA and RNA DNA deoxyribonucleic acid Contains the pentose sugar called deoxyribose Nitrogen containing bases A T C G In human ells DNA is a double helical structure Confined to the nucleus RNA ribonucleic acid Contains the pentose sugar called ribose Nitrogen containing bases A U C G Always single stranded Exists in both the nucleus and the cytoplasm Chromatin can be in an active or inactive form Euchromatin active Heterochromatin inactive coiled Cell Life Cycle A series of events involving growth of the cell and the subsequent division of the cell Life Cycle of a cell consists of 2 sequential phases The first phase interphase and the second phase cell division Interphase is divided into 3 subphases 1 G1 2 S 3 G2 2nd phase includes mitosis cytokenisis mitosis DNA replication is referred to semiconservative replication of the old DNA will be in each of the newly synthesized DNA Interphase the 1st phase in the cells life cycle 1 G1 protein synthesis occurs to cause cell growth due to increase in the size of the cell During protein synthesis the chromatin is in the active form Centriole replication begins here 2 S protein synthesis resulting in growth of the cell occurs but in addition the DNA replicates the cell to from 2 identical DNA molecules DNA Replication The double helical DNA uncoils or unwinds and the 2 polynucleatide chains are separated Each polynucleatide chain acts as a template for the synthesis of a new polynucleatide chain
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