Module 3: The Cellular Level of Organization

Lesson 4: The Nucleus and DNA Replication

Nhân Và Quá Trình Sao Chép DNA

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Mỗi bài học (lesson) bao gồm 4 phần chính: Thuật ngữ, Luyện Đọc, Luyện Nghe, và Bàn Luận.
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Dưới đây là danh sách những thuật ngữ Y khoa của module The Cellular Level of Organization.
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Medical Terminology: The Cellular Level of Organization

active transport
form of transport across the cell membrane that requires input of cellular energy
amphipathic
describes a molecule that exhibits a difference in polarity between its two ends, resulting in a difference in water solubility
anaphase
third stage of mitosis (and meiosis), during which sister chromatids separate into two new nuclear regions of a dividing cell
anticodon
consecutive sequence of three nucleotides on a tRNA molecule that is complementary to a specific codon on an mRNA molecule
autolysis
breakdown of cells by their own enzymatic action
autophagy
lysosomal breakdown of a cell’s own components
cell cycle
life cycle of a single cell, from its birth until its division into two new daughter cells
cell membrane
membrane surrounding all animal cells, composed of a lipid bilayer interspersed with various molecules; also known as plasma membrane
centriole
small, self-replicating organelle that provides the origin for microtubule growth and moves DNA during cell division
centromere
region of attachment for two sister chromatids
centrosome
cellular structure that organizes microtubules during cell division
channel protein
membrane-spanning protein that has an inner pore which allows the passage of one or more substances
checkpoint
progress point in the cell cycle during which certain conditions must be met in order for the cell to proceed to a subsequence phase
chromatin
substance consisting of DNA and associated proteins
chromosome
condensed version of chromatin
cilia
small appendage on certain cells formed by microtubules and modified for movement of materials across the cellular surface
cleavage furrow
contractile ring that forms around a cell during cytokinesis that pinches the cell into two halves
codon
consecutive sequence of three nucleotides on an mRNA molecule that corresponds to a specific amino acid
concentration gradient
difference in the concentration of a substance between two regions
cyclin
one of a group of proteins that function in the progression of the cell cycle
cyclin-dependent kinase (CDK)
one of a group of enzymes associated with cyclins that help them perform their functions
cytokinesis
final stage in cell division, where the cytoplasm divides to form two separate daughter cells
cytoplasm
internal material between the cell membrane and nucleus of a cell, mainly consisting of a water-based fluid called cytosol, within which are all the other organelles and cellular solute and suspended materials
cytoskeleton
“skeleton” of a cell; formed by rod-like proteins that support the cell’s shape and provide, among other functions, locomotive abilities
cytosol
clear, semi-fluid medium of the cytoplasm, made up mostly of water
diffusion
movement of a substance from an area of higher concentration to one of lower concentration
diploid
condition marked by the presence of a double complement of genetic material (two sets of chromosomes, one set inherited from each of two parents)
DNA polymerase
enzyme that functions in adding new nucleotides to a growing strand of DNA during DNA replication
DNA replication
process of duplicating a molecule of DNA
electrical gradient
difference in the electrical charge (potential) between two regions
endocytosis
import of material into the cell by formation of a membrane-bound vesicle
endoplasmic reticulum (ER)
cellular organelle that consists of interconnected membrane-bound tubules, which may or may not be associated with ribosomes (rough type or smooth type, respectively)
exocytosis
export of a substance out of a cell by formation of a membrane-bound vesicle
exon
one of the coding regions of an mRNA molecule that remain after splicing
extracellular fluid (ECF)
fluid exterior to cells; includes the interstitial fluid, blood plasma, and fluid found in other reservoirs in the body
facilitated diffusion
diffusion of a substance with the aid of a membrane protein
flagellum
appendage on certain cells formed by microtubules and modified for movement
G0 phase
phase of the cell cycle, usually entered from the G1 phase; characterized by long or permanent periods where the cell does not move forward into the DNA synthesis phase
G1 phase
first phase of the cell cycle, after a new cell is born
G2 phase
third phase of the cell cycle, after the DNA synthesis phase
gene
functional length of DNA that provides the genetic information necessary to build a protein
gene expression
active interpretation of the information coded in a gene to produce a functional gene product
genome
entire complement of an organism’s DNA; found within virtually every cell
glycocalyx
coating of sugar molecules that surrounds the cell membrane
glycoprotein
protein that has one or more carbohydrates attached
Golgi apparatus
cellular organelle formed by a series of flattened, membrane-bound sacs that functions in protein modification, tagging, packaging, and transport
helicase
enzyme that functions to separate the two DNA strands of a double helix during DNA replication
histone
family of proteins that associate with DNA in the nucleus to form chromatin
homologous
describes two copies of the same chromosome (not identical), one inherited from each parent
hydrophilic
describes a substance or structure attracted to water
hydrophobic
describes a substance or structure repelled by water
hypertonic
describes a solution concentration that is higher than a reference concentration
hypotonic
describes a solution concentration that is lower than a reference concentration
integral protein
membrane-associated protein that spans the entire width of the lipid bilayer
intermediate filament
type of cytoskeletal filament made of keratin, characterized by an intermediate thickness, and playing a role in resisting cellular tension
interphase
entire life cycle of a cell, excluding mitosis
interstitial fluid (IF)
fluid in the small spaces between cells not contained within blood vessels
intracellular fluid (ICF)
fluid in the cytosol of cells
intron
non-coding regions of a pre-mRNA transcript that may be removed during splicing
isotonic
describes a solution concentration that is the same as a reference concentration
kinetochore
region of a centromere where microtubules attach to a pair of sister chromatids
ligand
molecule that binds with specificity to a specific receptor molecule
lysosome
membrane-bound cellular organelle originating from the Golgi apparatus and containing digestive enzymes
messenger RNA (mRNA)
nucleotide molecule that serves as an intermediate in the genetic code between DNA and protein
metaphase
second stage of mitosis (and meiosis), characterized by the linear alignment of sister chromatids in the center of the cell
metaphase plate
linear alignment of sister chromatids in the center of the cell, which takes place during metaphase
microfilament
the thinnest of the cytoskeletal filaments; composed of actin subunits that function in muscle contraction and cellular structural support
microtubule
the thickest of the cytoskeletal filaments, composed of tubulin subunits that function in cellular movement and structural support
mitochondrion
one of the cellular organelles bound by a double lipid bilayer that function primarily in the production of cellular energy (ATP)
mitosis
division of genetic material, during which the cell nucleus breaks down and two new, fully functional, nuclei are formed
mitotic phase
phase of the cell cycle in which a cell undergoes mitosis
mitotic spindle
network of microtubules, originating from centrioles, that arranges and pulls apart chromosomes during mitosis
multipotent
describes the condition of being able to differentiate into different types of cells within a given cell lineage or small number of lineages, such as a red blood cell or white blood cell
mutation
change in the nucleotide sequence in a gene within a cell’s DNA
nuclear envelope
membrane that surrounds the nucleus; consisting of a double lipid-bilayer
nuclear pore
one of the small, protein-lined openings found scattered throughout the nuclear envelope
nucleolus
small region of the nucleus that functions in ribosome synthesis
nucleosome
unit of chromatin consisting of a DNA strand wrapped around histone proteins
nucleus
cell’s central organelle; contains the cell’s DNA
oligopotent
describes the condition of being more specialized than multipotency; the condition of being able to differentiate into one of a few possible cell types
organelle
any of several different types of membrane-enclosed specialized structures in the cell that perform specific functions for the cell
osmosis
diffusion of water molecules down their concentration gradient across a selectively permeable membrane
passive transport
form of transport across the cell membrane that does not require input of cellular energy
peripheral protein
membrane-associated protein that does not span the width of the lipid bilayer, but is attached peripherally to integral proteins, membrane lipids, or other components of the membrane
peroxisome
membrane-bound organelle that contains enzymes primarily responsible for detoxifying harmful substances
phagocytosis
endocytosis of large particles
pinocytosis
endocytosis of fluid
pluripotent
describes the condition of being able to differentiate into a large variety of cell types
polypeptide
chain of amino acids linked by peptide bonds
polyribosome
simultaneous translation of a single mRNA transcript by multiple ribosomes
promoter
region of DNA that signals transcription to begin at that site within the gene
prophase
first stage of mitosis (and meiosis), characterized by breakdown of the nuclear envelope and condensing of the chromatin to form chromosomes
proteome
full complement of proteins produced by a cell (determined by the cell’s specific gene expression)
reactive oxygen species (ROS)
a group of extremely reactive peroxides and oxygen-containing radicals that may contribute to cellular damage
receptor
protein molecule that contains a binding site for another specific molecule (called a ligand)
receptor-mediated endocytosis
endocytosis of ligands attached to membrane-bound receptors
ribosomal RNA (rRNA)
RNA that makes up the subunits of a ribosome
ribosome
cellular organelle that functions in protein synthesis
RNA polymerase
enzyme that unwinds DNA and then adds new nucleotides to a growing strand of RNA for the transcription phase of protein synthesis
S phase
stage of the cell cycle during which DNA replication occurs
selective permeability
feature of any barrier that allows certain substances to cross but excludes others
sister chromatid
one of a pair of identical chromosomes, formed during DNA replication
sodium-potassium pump
(also, Na+/K+ ATP-ase) membrane-embedded protein pump that uses ATP to move Na+ out of a cell and K+ into the cell
somatic cell
all cells of the body excluding gamete cells
spliceosome
complex of enzymes that serves to splice out the introns of a pre-mRNA transcript
splicing
the process of modifying a pre-mRNA transcript by removing certain, typically non-coding, regions
stem cell
cell that is oligo-, multi-, or pleuripotent that has the ability to produce additional stem cells rather than becoming further specialized
telophase
final stage of mitosis (and meiosis), preceding cytokinesis, characterized by the formation of two new daughter nuclei
totipotent
embryonic cells that have the ability to differentiate into any type of cell and organ in the body
transcription
process of producing an mRNA molecule that is complementary to a particular gene of DNA
transcription factor
one of the proteins that regulate the transcription of genes
transfer RNA (tRNA)
molecules of RNA that serve to bring amino acids to a growing polypeptide strand and properly place them into the sequence
translation
process of producing a protein from the nucleotide sequence code of an mRNA transcript
triplet
consecutive sequence of three nucleotides on a DNA molecule that, when transcribed into an mRNA codon, corresponds to a particular amino acid
unipotent
describes the condition of being committed to a single specialized cell type
vesicle
membrane-bound structure that contains materials within or outside of the cell
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Dưới đây là các bài văn nằm ở bên trái. Ở bên phải là các bài luyện tập (practice) để đánh giá khả năng đọc hiểu của bạn. Sẽ khó khăn trong thời gian đầu nếu vốn từ vựng của bạn còn hạn chế, đặc biệt là từ vựng Y khoa. Hãy kiên nhẫn và đọc nhiều nhất có kể, lượng kiến thức tích tụ dần sẽ giúp bạn đọc thoải mái hơn.
The nucleus is the largest and most prominent of a cell’s organelles (Figure 1). The nucleus is generally considered the control center of the cell because it stores all of the genetic instructions for manufacturing proteins. Interestingly, some cells in the body, such as muscle cells, contain more than one nucleus (Figure 2), which is known as multinucleated. Other cells, such as mammalian red blood cells (RBCs), do not contain nuclei at all. RBCs eject their nuclei as they mature, making space for the large numbers of hemoglobin molecules that carry oxygen throughout the body (Figure 3). Without nuclei, the life span of RBCs is short, and so the body must produce new ones constantly.

Inside the nucleus lies the blueprint that dictates everything a cell will do and all of the products it will make. This information is stored within DNA. The nucleus sends “commands” to the cell via molecular messengers that translate the information from DNA. Each cell in your body (with the exception of germ cells) contains the complete set of your DNA. When a cell divides, the DNA must be duplicated so that the each new cell receives a full complement of DNA. The following section will explore the structure of the nucleus and its contents, as well as the process of DNA replication.
Like most other cellular organelles, the nucleus is surrounded by a membrane called the nuclear envelope. This membranous covering consists of two adjacent lipid bilayers with a thin fluid space in between them. Spanning these two bilayers are nuclear pores. A nuclear pore is a tiny passageway for the passage of proteins, RNA, and solutes between the nucleus and the cytoplasm. Proteins called pore complexes lining the nuclear pores regulate the passage of materials into and out of the nucleus.

Inside the nuclear envelope is a gel-like nucleoplasm with solutes that include the building blocks of nucleic acids. There also can be a dark-staining mass often visible under a simple light microscope, called a nucleolus (plural = nucleoli). The nucleolus is a region of the nucleus that is responsible for manufacturing the RNA necessary for construction of ribosomes. Once synthesized, newly made ribosomal subunits exit the cell’s nucleus through the nuclear pores.

The genetic instructions that are used to build and maintain an organism are arranged in an orderly manner in strands of DNA. Within the nucleus are threads of chromatin composed of DNA and associated proteins (Figure 4). Along the chromatin threads, the DNA is wrapped around a set of histone proteins. A nucleosome is a single, wrapped DNA-histone complex. Multiple nucleosomes along the entire molecule of DNA appear like a beaded necklace, in which the string is the DNA and the beads are the associated histones. When a cell is in the process of division, the chromatin condenses into chromosomes, so that the DNA can be safely transported to the “daughter cells.” The chromosome is composed of DNA and proteins; it is the condensed form of chromatin. It is estimated that humans have almost 22,000 genes distributed on 46 chromosomes.
In order for an organism to grow, develop, and maintain its health, cells must reproduce themselves by dividing to produce two new daughter cells, each with the full complement of DNA as found in the original cell. Billions of new cells are produced in an adult human every day. Only very few cell types in the body do not divide, including nerve cells, skeletal muscle fibers, and cardiac muscle cells. The division time of different cell types varies. Epithelial cells of the skin and gastrointestinal lining, for instance, divide very frequently to replace those that are constantly being rubbed off of the surface by friction.

A DNA molecule is made of two strands that “complement” each other in the sense that the molecules that compose the strands fit together and bind to each other, creating a double-stranded molecule that looks much like a long, twisted ladder. This double helix can be constructed easily because the two strands are antiparallel, meaning the two strands run in opposite directions. Each side rail of the DNA ladder is composed of alternating sugar and phosphate groups (Figure 5). The two sides of the ladder are not identical, but are complementary. These two backbones are bonded to each other across pairs of protruding bases, each bonded pair forming one “rung,” or cross member. The four DNA bases are adenine (A), thymine (T), cytosine (C), and guanine (G). Because of their shape and charge, the two bases that compose a pair always bond together. Adenine always binds with thymine, and cytosine always binds with guanine. The particular sequence of bases along the DNA molecule determines the genetic code. Therefore, if the two complementary strands of DNA were pulled apart, you could infer the order of the bases in one strand from the bases in the other, complementary strand. For example, if one strand has a region with the sequence AGTGCCT, then the sequence of the complementary strand would be TCACGGA.

DNA replication is the copying of DNA that occurs before cell division can take place. After a great deal of debate and experimentation, the general method of DNA replication was deduced in 1958 by two scientists in California, Matthew Meselson and Franklin Stahl. This method is illustrated in Figure 6 and described below.

Stage 1: Initiation. The two complementary strands are separated, much like unzipping a zipper. Special enzymes, including helicase, untwist and separate the two strands of DNA.

Stage 2: Elongation. Each strand becomes a template along which a new complementary strand is built. DNA polymerase brings in the correct bases to complement the template strand, synthesizing a new strand base by base. A DNA polymerase is an enzyme that adds free nucleotides to the end of a chain of DNA, making a new double strand. This growing strand continues to be built until it has fully complemented the template strand.

Stage 3: Termination. Once the two original strands are bound to their own, finished, complementary strands, DNA replication is stopped and the two new identical DNA molecules are complete.

Each new DNA molecule contains one strand from the original molecule and one newly synthesized strand. The term for this mode of replication is “semiconservative,” because half of the original DNA molecule is conserved in each new DNA molecule. This process continues until the cell’s entire genome, the entire complement of an organism’s DNA, is replicated. As you might imagine, it is very important that DNA replication take place precisely so that new cells in the body contain the exact same genetic material as their parent cells. Mistakes made during DNA replication, such as the accidental addition of an inappropriate nucleotide, have the potential to render a gene dysfunctional or useless. Fortunately, there are mechanisms in place to minimize such mistakes. A DNA proofreading process enlists the help of special enzymes that scan the newly synthesized molecule for mistakes and corrects them. Once the process of DNA replication is complete, the cell is ready to divide. You will explore the process of cell division later in the chapter.

OpenStax. (2022). Anatomy and Physiology 2e. Rice University. Retrieved June 15, 2023. ISBN-13: 978-1-711494-06-7 (Hardcover) ISBN-13: 978-1-711494-05-0 (Paperback) ISBN-13: 978-1-951693-42-8 (Digital). License: Attribution 4.0 International (CC BY 4.0). Access for free at openstax.org.

The nucleus is the control center of the cell. The nucleus of living cells contains the genetic material that determines the entire structure and function of that cell.

Unlike cardiac muscle cells and smooth muscle cells, which have a single nucleus, a skeletal muscle cell contains many nuclei, and is referred to as “multinucleated.” These muscle cells are long and fibrous (often referred to as muscle fibers). During development, many smaller cells fuse to form a mature muscle fiber. The nuclei of the fused cells are conserved in the mature cell, thus imparting a multinucleate characteristic to mature muscle cells. LM × 104.3. (Micrograph provided by the Regents of University of Michigan Medical School © 2012)

Mature red blood cells lack a nucleus. As they mature, erythroblasts extrude their nucleus, making room for more hemoglobin. The two panels here show an erythroblast before and after ejecting its nucleus, respectively. (credit: modification of micrograph provided by the Regents of University of Michigan Medical School © 2012)

Strands of DNA are wrapped around supporting histones. These proteins are increasingly bundled and condensed into chromatin, which is packed tightly into chromosomes when the cell is ready to divide.

The DNA double helix is composed of two complementary strands. The strands are bonded together via their nitrogenous base pairs using hydrogen bonds.

DNA replication faithfully duplicates the entire genome of the cell. During DNA replication, a number of different enzymes work together to pull apart the two strands so each strand can be used as a template to synthesize new complementary strands. The two new daughter DNA molecules each contain one pre-existing strand and one newly synthesized strand. Thus, DNA replication is said to be “semiconservative.”

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Dưới đây là video và các luyện tập (practice) của bài này. Nghe là một kĩ năng khó, đặc biệt là khi chúng ta chưa quen nội dung và chưa có nhạy cảm ngôn ngữ. Nhưng cứ đi thật chậm và đừng bỏ cuộc.
Xem video và cảm nhận nội dung bài. Bạn có thể thả trôi, cảm nhận dòng chảy ngôn ngữ và không nhất thiết phải hiểu toàn bộ bài. Bên dưới là script để bạn khái quát nội dụng và tra từ mới.
Script:
  1. The nucleus is the command center of the cell, containing the genetic instructions for all of the materials a cell will make.
  2. In other words, the nucleus includes all of the functions a cell can perform.
  3. The nucleus is encased within a membrane of two interconnected lipid bilayers, side-by-side.
  4. This nuclear envelope is studded with protein-lined pores that allow materials to be trafficked into and out of the nucleus.
  5. The nucleus contains one or more nucleoli, which serve as sites for ribosome synthesis.
  6. The nucleus houses the genetic material of the cell: DNA.
  7. DNA is normally found as a loosely contained structure called chromatin within the nucleus, where it is wound up and associated with a variety of histone proteins.
  8. When a cell is about to divide, the chromatin coils tightly and condenses to form chromosomes.
  9. There is a pool of cells constantly dividing within your body.
  10. The result is billions of new cells being created each day.
  11. Before any cell is ready to divide, it must replicate its DNA so that each new daughter cell will receive an exact copy of the organism’s genome.
  12. A variety of enzymes are enlisted during DNA replication.
  13. These enzymes unwind the DNA molecule, separate the two strands, and assist with the building of complementary strands along each parent strand.
  14. The original DNA strands serve as templates from which the nucleotide sequence of the new strands are determined and synthesized.
  15. When replication is completed, two identical DNA molecules exist.
  16. Each one contains one original strand and one newly synthesized complementary strand.
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