Eukaryotic Cells

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1.A) Two main forms of cells exist: eukaryotic cells and prokaryotic cells. Prokaryotic cells are smaller and do not have membrane-bound nucleus or membrane- bound organelles, but do have: plasma membrane, cytosol and cytoplasm, and ribosomes. Prokaryotes contain much less DNA than eukaryotes and have circular chromosomes. Eukaryotic cells have information processing organelles, such as the nucleus which houses most of the cell’s DNA, and ribosomes which use information from DNA to produce proteins. In prokaryotes gene regulation begins during transcription. Transcription begins when RNA polymerase binds to the promoter. RNA polymerase then begins to separate the two DNA strands and initiates copying. Once this has occurred, a terminator sequence signals the end of the gene. The terminator sequence gets transcribed, RNA polymerase detaches, and mRNA is released. Translation in prokaryotes is coupled with translation. The moment a stretch of RNA is synthesized by RNA polymerase, ribosomes attach to it to make a protein. In eukaryotic cells, initiation begins when transcription factors bind to the promoter, followed by RNA polymerase II and TX factors. DNA strands separate and RNA polymerase II begins copying. After this, a polyadenylation signal sequence signals the end of the gene. The polyadenylation signal is transcribed, pre-mRNA is cleaved and released, and RNA II polymerase continues transcribing until it falls off. Eukaryotic cells continue to modify pre-mRNAs after transcription. Modifications to the ends of mRNA are made by the addition of guanine to the 5’ cap and a poly-A-tail to the 3’ cap. Following this RNA splicing occurs. Here noncoding regions (introns) of mRNA are removed and coding regions (exons) are joined. After this has taken place, the mRNA travels to the cytoplasm and mRNA degradation occurs, and translation begins. The genetic material forming the mitochondrial genome is similar in structure to that of the prokaryotic genetic material. The mitochondrial chromosome is a circular DNA molecule, but unlike prokaryotes it is much smaller and several copies are present. In a mitochondrial genome all genes are carried on a single DNA molecule, genetic material is not bounded by a nuclear envelope, DNA is not packed into chromatin, and the genome contains little non-coding DNA. Chloroplast genomes are very similar to mitochondrial genomes in that they are both compact and circular, which shows that they may have originated in prokaryotes. In a nuclear genome the nucleus contains a majority of the cells genes. The nucleus consists of a nuclear envelope, chromosomes, and a nucleolus. In a nuclear genome DNA is organized into chromosomes, which are made up of chromatin. Genomes of chloroplasts like mitochondrial genomes, consists of circular DNA molecules. Chloroplast genomes are made up of two outer membranes, thylakoids:grana, and three internal compartments: intermembrane space, stroma, and thylakoid space.

D) The membrane controls the exchange of materials with the environment. However, the membrane is selectively permeable and only lets some substances pass and prohibits others from passing. Two things decide what passes through the membrane: the phospholipids bilayer and transport proteins. The phospholipid bilayer allows small, hydrophobic molecules through. Hydrophilic molecules have to pass through the membrane via transport proteins, channel and carrier proteins. Channel proteins-function by having a hydrophilic channel that certain molecules or atomic ions use as a tunnel through the membrane and carrier proteins-hold onto their substances and change shape in a way that shuttles them across the membrane. Substances which use transport proteins travel through passive transport or active transport. Passive transport moves substances down its concentration gradient,...
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