A&P Lesson 4 - Cellular form and Function; Genes & Cellular Function

Nucleus

largest organelle in the cell and contains cell’s genetic material

How large is the nucleus?

5 um

Anuclear

without a nucleus (ex. erthrocytes)

multinuclear

have multiple nuclei (ex. skeletal muscle cells)

Nuclear envelope

double membrane around the nucleus that regulates molecular traffic

Nuclear lamina

web of protein filaments that supports the nuclear envelope

nucleoplasm

material within the nucleus

What does nucleoplasm include?

chromatin and one or more nucleoli

Chromatin

fine filamentous DNA material complexed with proteins; tightly packed and usually found during mitosis

How big are chromatin?

6 feet long packed in cell nucleus of 5 um diameter

Nucleolus

area in the nucleus that is composed of RNA, enzymes and histones and is abundant in cells that produce proteins

What is the function of the nucleolus?

produces rRNA and ribosomal subunits which are both essential for protein production in the cytoplasm during translation

ribosomes

composed of rRNA and ribosomal proteins

DNA

long, thread-like molecule with uniform diameter but varied length (average 2 inches) that carries genes for the synthesis of proteins

Gene

an information-containing segment of DNA that codes for the production of a molecule of RNA that most often plays a role in synthesizing one or more proteins

• only 2% of total DNA

What determines the amino acid sequence of a protein?

the nucleotide sequence in the DNA

how many DNA molecules (chromosomes) are in the nucleus?

46 (23 maternal and 23 paternal)

What are the 3 components of a nucleotide?

1. a sugar

2. a phosphate group

3. a nitrogenous base

Purines

• double ringed structure

• adenine (A), guanine (G)

Pyrimidines

• single ringed structure

• Cytosine (C), thymine (T), uracil (U)

what type of bond connects the nitrogenous bases in DNA?

hydrogen bonds

Histones

proteins around which DNA winds; cluster in groups of 8 molecules

Nucleosome

DNA wrapped around histones

DNA replication

the copying of DNA to make another molecule of DNA

Sister chromatids

2 parallel filaments of identical DNA

Kinetochores

protein plaques on each side of centromere that play a role in cell division

RNA

single nucleotide chain, smaller than DNA and function mainly in cytoplasm

What are the 3 RNA that are important for protein synthesis?

• messenger RNA (mRNA)

• ribosomal RNA (rRNA)

• transfer RNA (tRNA)

Gene

an information-containing segment of DNA that codes for the production of a molecule of RNA that most often plays a role in synthesizing one or more proteins

What determines the amino acid sequence of a protein?

the nucleotide sequence in the DNA

T/F All body cells except sex cells and some immune cells contain identical genes

true

what genes are activated in cells?

different genes are activated in different cells

how many of its genes does a cell use?

1/3 - 2/3 of its genes

In what order is RNA, DNA, and protein made?

DNA → RNA → protein

Where does transcription occur?

in the nucleus

Transcription

• copying genetic instructions from DNA to mRNA utilizing RNA polymerase enzyme

RNA polymerase

• enzyme that binds to transcription start sequence in DNA and unwinds the DNA double helix

• reads the nitrogenous bases from one strand of DNA to build a complementary strand of mRNA (CGTA → GCAU)

terminator

stop sequence on DNA at the end of a gene

Pre-mRNA

immature RNA produced by transcription that undergoes processing

What processing occurs to pre-mRNA?

enzymes within nucleus remove introns from the pre-mRNA and splice exons together

introns

intervening sequences that are not used to code for a protein

Exons

the RNA sequences that will be expressed

where does translation occur?

in the cytoplasm

Translation

nucleotide language in mRNA is converted into amino acid language

What are the 3 main participants in translation?

mRNA, tRNA, ribosomes

mRNA

• carries the genetic code from the nucleus to the cytoplasm

• has protein cap that is recognition site for ribosome

when is mRNA made?

when a gene is activated

codon

series of 3 nucleotides on mRNA

tRNA

• delivers a single amino acid to the ribosome

• contains an anticodon

anticodon

series of 3 nucleotides that are complementary to the mRNA codon

Ribosomes

• organelles that read the message and build a peptide chain

• contains a large and small subunit

• The E, P and A site on the ribosome are involved in the translation process

Where are ribosomes located?

free in cytosol, on rough ER, and on nuclear envelope

What are the 3 main steps in translation?

initiation, elongation, termination

Initiation

1. ribosomes assemble with mRNA in the cytosol

2. small subunit binds to leader sequence of mRNA and reads the mRNA strand until it reaches the start codon (AUG)

3. the first tRNA brings the first amino acid (methionine) and then the large ribosomal subunit attached

4. protein synthesis begins

Where is the first tRNA?

in the P site

Elongation

5. tRNA with its amino acid arrives and binds to A site

6. ribosome creates peptide bond between first and second amino acids

7. ribosome moves down mRNA by 1 codon so growing peptide is now attached to tRNA in P site

8. process continues and peptide grows

Termination

9. ribosome reaches a stop codon on mRNA

10. the A site binds a protein called a release factor

11. ribosome disassembles and dissociates from the mRNA

what happens to proteins destined for lysosomes or secretion?

they are modified by the ER

polyribosomes

multiple ribosomes in clusters that can translate the same mRNA molecule simultaneously

What is the process of protein processing and secretion?

1. post-translational modification

2. protein pinches off from rough ER in transport vesicle

3. clusters of vesicles fuse and form a new Golgi cistern

4. new cistern migrates through Golgi to opposite face

5. the trans cistern breaks up into vesicles containing cell product

6. some vesicles become lysosomes others become secretory vesicles that fuse with membrane to release their content

Post-translational modification

• protein assembled on ER surface threads itself through a pore in the rough ER membrane into its cistern

• enzymes in the rough ER modify the protein (remove some amino acid segments, fold the protein, stabilize with disulfide bridges, add carbohydrates, etc.)

can cells permanently turn off genes?

yes

ex. liver cells turn off hemoglobin genes

When can cells turn on genes?

only when needed

Gene regulation example: Casein synthesis and secretion

1. Hormone prolactin binds to receptors on membrane of mammary cell

2. Receptors trigger activation of a regulatory protein (transcription activator) in the cytoplasm

3. Regulatory protein moves into the nucleus and binds to the DNA near the casein gene

4. The binding enables RNA polymerase to bind to the gene and transcribe it, producing the mRNA for casein

5. The casein mRNA moves to the cytoplasm, bind to ribosomes, start translation, and ribosomes move to and attach to the rough ER complete translation

6. The Golgi complex packages casein into secretory vesicles

7. The secretory vesicles release the casein by exocytosis, and it becomes part of the milk

What complex outcomes can genes greatly affect?

behavior, aggression, and sex drive

what do cells synthesize?

glycogen, fat, steroids, phospholipids, pigments, other compounds

DNA replication

when a cell duplicates its DNA so that it can give a complete copy of all its genes to each daughter cell

What are the 4 steps of DNA replication?

1. unwinding the helix from the histones

2. unzipping small portion of helix by DNA helicase enzyme separating the strands and forming a replication fork

3. DNA polymerase moves along each strand, reads exposed bases, and synthesizes complementary strands

4. New histones synthesized used to organize new DNA strands into nucleosomes

Semiconservative replication

each new DNA molecule contains old (parental) DNA and newly synthesized DNA

DNA ligase

connects segments on new but discontinuous strand

What are the 2 main parts of the cell cycle?

Interphase and mitotic phase

What is included in interphase?

G1

S

G2

G1

• interval between cell division and DNA replication

• cell carries out normal tasks and synthesizes proteins and materials for next phase

S phase

cell replicates all nuclear DNA and duplicates centrioles

G2

• interval between DNA replication and cell division

• cell repairs DNA replication errors, grows and synthesizes enzymes that control cell division

G0

cells that have left the cycle and cease dividing for a long time or permanently

What cells will never enter G0?

Stem cells- they need to undergo mitosis to replace worn out tissue

Mitotic phase

refers to the events during nuclear division; cell replicates its nucleus

Mitosis

cell division resulting in 2 genetically identical daughter cells

Meiosis

produces egg and sperm cells containing half the genetic information

What are the 4 phases of mitosis?

prophase, metaphase, anaphase, telophase

Prophase

• genetic material condenses into compact chromosomes

• 46 chromosomes (2 chromatids per chromosome

• nuclear envelop disintegrates

• centrioles sprout spindle fibers

Metaphase

• chromosomes are aligned on cell equator

• spindle fibers form lemon shaped array called mitotic spindle

• shorter microtubules from centrioles complete a starlike aster which anchors itself to inside of cell membrane

Anaphase

• enzyme cleaves 2 sister chromatids apart at centromere

• single-stranded daughter chromosomes migrate to each pole of the cell as motor proteins in kinetochores crawl along spindle fibers

Telophase

• chromosomes cluster on each side of the cell

• rough ER makes new nuclear envelope around each cluster

• chromosomes uncoil to chromatin

• mitotic spindle disintegrates

• each nucleus reforms nucleoli

Cytokinesis

not a phase of mitosis!

• occurs during late anaphase of mitosis and causes the cell to pinch in 2 to form new daughter cells

• division of cytoplasm into 2 cells

What are the regulations of cell division?

• when cells have enough cytoplasm for 2 daughter cells

• when they have replicated their DNA

• they have adequate supply of nutrients

• they are stimulated by growth factors

• neighboring cells die opening up space

When do cells stop dividing?

• nutrients or growth factors are withdrawn

• they snugly contact neighboring cells

• they experience contract inhibition

Contact inhibition

the cessation of cell division in response to contact with other cells

Malignant tumors

tumors that cause cancer by spreading into surrounding tissue (invasion)

• grow rapidly

• tend to metastasize

• may stimulate tumor angiogenesis

• distinguished from slow growing encapsulated benign tumors

metastasize

give off cells that spread and seed the growth of tumors elsewhere

Lethal effects of cancer

• replace functional tissue in vital organs

• put pressure on organs blocking their normal function

• weaken immune system

• open door for opportunistic infectinos

• steal nutrients from the rest of the body

cachexia

severe wasting away of depleted tissues