Biology Final Exam

RNA processing in Eukaryotes

* In bacteria, mRNAs are translated immediately, sometimes even before transcription is complete .
* In eukaryotes, RNA processing occurs before exiting nucleus

Exons

coding sequence found in mature mRNA

Introns

Intervening noncoding sequences that are removed by splicing

* transcribed but not translated

Splicing

removal of introns and connection of exons

Alternative Splicing

allows different mRNAs and proteins to be produced from a single gene

A 5’ cap

* a modified guanine nucleotide
* enables ribosomes to bind and protects from degradation

A 3’ poly (A) tail

* needed for translation and protects degradation

Ribosomes

catalyze translation of the mRNA sequence into protein

transfer RNAs (tRNA)

* bind to amino acids
* transfer them to the growing polypetide in the ribosome

aminoacyl tRNA

a tRNA linked to its amino acid

requires ATP to charge tRNA

Anticodon

* the loop at the opposite end forms this
* can base-pair with the mRNA codon

What is the job of the tRNA in translation

* binds to the mRNA
* transfers the specific amino acid

Ribosomes are comprised of proteins and Ribosomal RNA (rRNA) in two subunits:

1. the small subunit holds the mRNA in place
2. the large subunit is where peptide bonds form

The tRNAs fit into three sites in the ribosome :

1. The A site is the acceptor site for an aminoacyl tRNA
2. The P site is the peptide site where a peptide bone forms
3. The E site is where tRNAs without amino acids exit the ribosome

The ribosome is a molecular machine that synthesizes proteins in a three-step sequence:

1. an aminoacyl tRNA carrying the correct anticodon for the mRNA codon enters the A site
2. A peptide bond forms between the amino acid non the a-site tRNA and the polypeptide on the P-site tRNA
3. The ribosome moves down the mRNA by one codon and all three tRNAs move down one position

The structure and function of ribosomes

mRNA read 5’ → 3’ (same as synthesis)

What are the 3 phases of translation?

1. Initiation
2. Elongation
3. Termination

What phase beings near the AUG start codon?

Initiation phase

* the small ribosomal subunits bind to the mRNA
* mediated by initiation factors

What is the first tRNA called?

initiator tRNA

mRNA, tRNA, and the ribosome form what?

Translation

where does an aminoacyl tRNA bind to the codon?

the A site

* the amino acid on the P-site tRNA is transferred to the amino acid on the A-site tRNA

What do ribozymes do?

catalyze peptide bonds

When does Translocation occur?

When the ribosome slides one codon toward the 3’ end of the mRNA

Elongation factors

help move ribosomes

When does Termination occur?

* when the A site encounters a stop codon
* a protein called a release factor enters the A site
* the newly synthesized polypeptide , tRNAs and ribosomal subunits separate from the mRNA

What do molecular chaperones do?

speed protein folding

What is a blastocyst?

a ball of cells

How many cells in a human?

10 to 50 trillions cells

* all info to assemble entire human is contained in the first single cell

Where is genetic information stored?

Nucleus

What is a pair of chromosomes?

a diploid

How many chromosomes are in human cells?

23 pairs of chromosomes (46 total )

* 1 of each pair from mom and 1 from dad

In diploid species, members of a pair of chromosomes are called ____?

Homologous

* each one is nearly identical in size and genetic composition

What is the origin of your two sets of chromosomes?

one set is maternal and the other is paternal

Observations of embryos confirmed that plants and animals ___

1. start life as single-celled zygotes
2. grow through a series of cell division

What does meiosis produce?

reproductive cells called gametes

* half of the chromosomes passed to daughter cells
* random assortment of alleles from both sets

\
What does Mitosis produce?

all the other cell types (somatic cells )

* ALL chromosomes passed to daughter cells
* Identical genetically

What are mitosis and meiosis usually followed by ?

cytokinesis

What is cytokinesis?

division of the cytoplasm into two daughter cells

How do cells replicate?

1. copy the DNA
2. separate the copies
3. divide the cytoplasm to create two complete cells

What is a chromosome?

* a single long double helix of DNA and protein

Eukaryotic DNA is wrapped around histone proteins

called chromatin

Chromatid

each double-stranded DNA copy is called a chromatid

attached at the centromere

What are chromatids that are attached at the centromere called?

sister chromatids

Growing cells cycle between two phases:

1. a dividing phase called the M (mitotic) phase

* chromosomes are condensed fully


2. A non dividing phase called the interphase

* chromosomes are uncoiled, genes being expressed

Where do cells spend most of their time?

The interphase

Where does DNA replication occurring during the interphase?

The S phase (synthesis)

What happens in the S phase

each chromosomes now consist of two sister chromatids

* exact copies of the same genetic information

Interphase also includes what two gap phases?

G1 phase = cells grow, perform functions

G2 phase= prepare for division

During which phase of the cell cycle does DNA replication occur?

S of interphase

What happens during M phase?

1. Mitosis = the division of the replicated chromosomes
2. Cytokinesis= the division of the cytoplasm

What happens during Mitosis?

1. begins when chromatin condenses fully
2. sister chromatids separate to form independent daughter chromosomes
3. One copy of each chromosome goes to each of the two daughter cells

What are the 5 subphases of Mitosis (PPMAT)

* prophase
* prometaphase
* metaphase
* anaphase
* telophase

What happens during prophase?

* chromosomes finished condensing
* The spindle apparatus forms = moves replicate chromosomes during early mitosis
* pulls sister chromatids apart in late mitosis.

What is the spindle apparatus composed of ?

microtubules

In animal cells, microtubule-organizing centers (MTOCs) are what?

centrosomes, each containing a pair of centrioles

Kinetochore microtubules

attach to chromosome kinetochores at the centromere

Polar microtubules

extend from each spindle pole and overlap with each other

Astral microtubules

hold spindle poles in place

What happens during pro metaphase?

* the nuclear envelope breaks down
* chromosomes are pushed and pulled by microtubules until they reach the middle of the spindle

What happens during Metaphase?

* formation of the mitotic spindle is completed
* chromosomes are lined up on the metaphase plate

What happens during the Anaphase?

sister chromatids separate

* pulled by the spindle fibers toward opposite poles of the cell
* kinetochore microtubules shrink
* motor proteins of the polar microtubules push the two poles of the cell away from each other
* CREATES TWO IDENTICAL SETS oF DAUGHTER CHROMOSOMES

What happens during telophase?

a new nuclear envelope begins to form around each set of chromosomes

* the chromosomes begin to unwind
* mitosis is complete when 2 independent nuclei have formed
* Cytokinesis typically occurs immediately after mitosis
* the cytoplasm divides to form two daughter cells

What happens during cytokinesis in plants?

vesicles from the Golgi apparatus bring membrane and cell wall components to form a cell plate

What happens during cytokinesis in animals and other eukaryotes ?

a ring of actin and myosin filaments contract inside the cell membrane and form a cleavage furrow.

What are centromeres?

specialized regions of chromosomes where sister chromatids are most closely doing to each other

What are kinetochores

The structures on sister chromatid where microtubules attach

What are centrosomes?

the microtubule-organizing center in animals and certain plants and fungi. Each pole in the spindle apparatus is a centrosome.

What are gametes?

reproductive cells

* sperm and eggs
* unite to form a zygote (new individual)

Diploid- dominant species

* multi-cellular organisms is diploid
* haploid gametes are a specialized type of cell

What do sex chromosomes determine?

the sex of an individual

FEMALES = 2 X chromosomes (XX)

MALES = X and Y chromosomes (XY)

What are autosomes ?

non-sex chromosomes

What are chromosomes of the same type called?

homologous chromosomes or homologs

What is a gene?

a section of DNA that influences one or more hereditary traits

What are different versions of a specific gene called?

alleles

* homologs may contain different alleles

Unreplicated chromosome

a chromosome that consists of one double-helical molecules of DNA packaged with proteins

Replicated chromosome

a chromosome after DNA replication. Consists of two identical chromatids each containing one double-helical DNA molecules packaged with proteins.

Sister Chromatids

the two identical chromatids in a replicated chromosome

Homologous chromosomes

have the same genes in the same position and are the same size and shape ( since the alleles of the particular genes are often different between the homologs homologs are not called identical chromosomes)

Non-sister chromatids

chromatids on different members of a homologous chromosome pair ( one of the chromatids on one homolog and the other chromatid is on the other homolog)

Bivalent

paired, replicated homologous chromosomes that exist during prophase 1 and metaphase 1 of meiosis

What happens in Meiosis 1?

* the 2 homologs of each chromosome pair separates into two daughter cells
* each daughter cell has one set of chromosomes
* 2n → 1n (2 haploid cells produced)

What happens in Meiosis 2?

The sister chromatids of each chromosome separate into two daughter cells

* each of the two haploid cells from Meiosis 1 are divided
* produced FOUR haploid cells

What are the phases of Meiosis 1 ?

1. Early prophase 1
2. late prophase 1
3. Metaphase 1
4. anaphase 1
5. telophase 1

What happens during early prophase 1?

* the nuclear envelop begins to breakdown
* replicated chromosomes condense
* Spindle apparatus begins to form

The homolog pairs come together in a pairing process calling ______

synapsis

* forms bivalents

What happens during Late prophase 1 ?

* bivalents are attached to microtubules from opposite poles of the spindle apparatus
* homologs begin to separate

Where are homologs still attached during late prophase 1?

chiasmata

What is “crossing over”?

exchange of parts between homologous non-sister chromatids

* occurs during bivalents and Chiasma

What proteins are homologous pairs held together by ?

synaptonemal complex

In crossing over of meiosis 1, genetic information is exchanged between ________

non sister chromatids

What happens during Metaphase 1?

The paired homologs line up at the metaphase plate

* the alignment is random
* also produces recombination

What happens during Anaphase 1?

the paired homologs separate and migrate to opposite ends of the cell

What happens during telophase 1?

The homologs finish migrating to the poles of the cell

* the cell then divides (cytokinesis)

What does meiosis 1 result in?

* 2 haploid cells (with one recombines set of chromosomes each)

The phases of Meiosis 2:

1. prophase 2
2. Metaphase 2
3. Anaphase 2
4. Telophase 2

What happens during prophase 2?

the spindle apparatus forms, attached to the centromere of each sister chromatid

What happens during metaphase 2?

replicated chromosomes line up at the metaphase plate single file

What happens during anaphase 2?

sister chromatids separate

What happens during telophase 2?

A nuclear envelope forms around each haploid set of chromosomes

What does Meiosis 2 result in?

4 haploid daughter cells with unreplicated chromosomes