Exam 3 Biology

Synthesis or S phase goal

To create 2 identical copies of genome

Covalent phosphodiester bonds

Bonds that hold the backbone together, hard to break

Hydrogen bonds

Bonds that hold the 2 strands together, easier to break

Initiation

Proteins assemble at the ORI to unwind/stabilize DNA

ORI

Original of replication, proteins in the replication complex bind to a DNA sequence

Prokaryotes

Tend to have circular chromosomes

Eukaryotes

Tend to have linear chromosomes, initiation here involves many of sites on each chromosome

DNA helicase

Separates the strands of the double helix

SSBinding proteins

Stabilizes single strands of DNA to prevent unwanted hydrogen bonds between base pairs

Topoisomerase

Relieves strain caused by unwinding a twisty molecule

Primase

Lays down an RNA primer

Why can’t the DNA polymerase just start adding molecules?

It needs a double strand to bind to and provide the 3’-OH group

This is provided by an enzyme called Primase

Primase

Connects a few complimentary RNA bases to the template strand, called an RNA primer

Elongation

DNA polymerase binds and adds complementary DNA bases, but only to the 3’ end of new strand

What direction does the DNA polymerase build new DNA?

From 5’ to 3’

Template strand

Existing strand of DNA acts as a template for the new strand, template strand is read in the 3’-5’ direction

Lagging strand synthesis

Lagging strand elongation needs 3 steps more than the leading strand

1. RNA primers between Okazaki fragments are removed

2. DNA polymerase I comes and fills in the nucleotides that were once RNA

3. DNA ligase creates covalent bonds in the backbone between Okazaki fragments

Mitotic phase

DNA condenses and is divided equally into two cells

Includes prophase, metaphase, anaphase, telophase, and cytokinesis

Histones

Organize DNA in the nucleus

Cytoskeleton

Mediates the movement of chromosomes

Cytokinesis

Division of cytoplasm

Bacterial cells divide but _____

Do not undergo mitosis

Summary of DNA structure

2 antiparallel strands of DNA arranged in double helix

Phosphodiester backbone=covalent bonds between monomers, gives DNA structure and protects nitrogenous bases in the middle

Orientation of deoxyribose sugar determines strand polarity

Nitrogenous bases bond together through hydrogen bonds, annealing 2 strands of DNA into double helix

Order of nitrogenous bases holds the information

Why is it important for a cell to get checked before dividing?

If a cell with a mutation continues to divide, all new cells made from that cell will inherit that mutation, also makes sure the organism can keep up with resource demands

What is checked at the G1 checkpoint?

1. Are new cells needed?

2. Can new cells be supported by the environment?

3. Does this cell have any DNA damage?

4. Are the telomeres long enough?

Signaling pathways converge on _____ synthesis to pass checkpoints

Cyclin

CDK

Check point specific CDK always present during the cell cycle, but are not active until cyclin proteins are made, these proteins activate other proteins to pass checkpoint

P53 proteins

Will inactive CDK-cyclin proteins activate other complex to stop cell division if DNA damage is present

Mutagens

DNA damage, can be from chemicals, x-rays, UV radiation that cause mutations

Nucleotide excision repair

One of many DNA repair mechanisms

UvrA

Protein that recognizes DNA damage and signals to start repair mechanisms

RecA

Protein that facilitates DNA repair

Telomeres

Act as buffer for chromosomes following DNA replication

An enzyme that contains its own RNA primer and restores telomere length, maintains replication capacity of cells

What is being checked at G2 checkpoint?

1. Did all chromosomes replicate successfully?

2. Is there any DNA damage?

3. Is active MPF present?

What does DNA polymerase need for a condensation reactions to polymerize DNA?

Needs a free 3’ OH group

Examples include the lagging strand which does not have a 3’ OH group

Hayflik limit

Average number of times a normal human cell population will divide before becoming senescent and dying

Predicting life span

Telomere shortening rate

Once telomeres are gone, the cell will no longer pass cell cycle checkpoint 1

DNA polymerase can_____

Proofread and fix mismatches

MPF

M phase promoting factor, the name of the active cyclin-CDK complex that intimidates mitosis or M-phase

Metaphase checkpoint

1. Is the spindle attached correctly?

2. Is MPF absent?

Cancer

Disease of the cell cycle

Varies in type and lethality but see a common set of behaviors

BRCA1 mutation

Often a predictor of cancer,

The enzyme made from the BRCA1 gene fixes DNA damage

Gametes

Reproductive cells, includes sperm or eggs, usually unite to form a new individual

Fertilization

When two gametes come together to form a individual

Meiosis

Nuclear division that leads to halving of chromosome number

Gametes contain half the chromosome number

Autosomes

Most of the chromosomes, besides sex chromosomes

Sex chromosomes

Associated with an individual’s sex

Female sex chromosomes

Two X chromosomes

Male sex chromosomes

X and Y chromosomes

Human karyotype

23 pairs of homologous chromosomes

2n=46

Number 1-22 for autsomes approximately by size and pair of sex chromosomes

Homologous chromosomes

Chromosomes of the same type

Homologous pair

Pair of homologs, contain the same genes in the same position along the chromosome, not typically identical

Gene

A section of DNA that influences a trait

Alleles

Difference versions of specific gene

Diploid

Organisms that have two homologs of each chromosome

They have two alleles of each gene

Haploid

Organisms that have only one of each type of chromosome

They have just one allele of each gene

Haploid number

N, the number of distinct types of chromosomes present

Example: humans n=23

Ploidy

Indicates the number of complete chromosome sets it contains

What happens before meiosis?

Each chromosome in the diploid parent cell is replicated, S phase, same in mitosis

What happens after replication?

Each chromosome is comprised of two identifcla sister chromatids which remain attached until later in Meiosis II

Functions of meiosis

Reduce chromosome number from diploid to haploid

Ensure each haploid cell has a complete set of chromosomes

Generate diversity among the daughter cells

Key features of meiosis

Two nuclear divisions but DNA is replicated only once

1. Begins in a diploid cell with all chromosomes in pairs

2. Ends with haploid products

In Meiosis I

Homologous chromosomes pair and exchange genetic information, then segregate from each other

Two homologs of each chromosome pair separate into two daughter cells

In Meiosis II

Sister chromatids separate from each other

The sister chromatids of each chromosomes separate into daughter cells

What is chromosome movement coordinated by in Meiosis

Microtubules of the spindle apparatus

Microtubules

Attach to kinetic horse at the centromere of each chromosome

Gametogenesis

When some or all of the daughter cells become eggs or sperm

Zygote

A diploid cell resulted from fertilization

Unique events in Meiosis I

Paring occurs during Prophase I called synapsis

The four chromatids of each homologous pair form a bivalent

Can lead to crossing over between non-sister chromatids

Unique events after metaphase 1

Cells are the end are haploid but each chromosome still contains 2 chromatids

Maternal and paternal centromeres of each pair segregate to opposite poles

Phases of Meiosis I

1. Early prophase I

2. Later prophase I

3. Metaphase I

4. Anaphase I

5. Telophase I

Early prophase I

Nuclear envelope begins to break down

Chromosomes condense

Spindle apparatus begins to form

Homolog pairs come together and pair by synapsis

Later prophase I

The two homologs become attached to Microtubules from opposite poles of the spindle apparatus

The homologs being to separate

Chromosome remain joined at chiasmata

Chiasmata

Where genetic exchange or crossing over occur between homologous non-sister chromatids

Metaphase I

Paired homologs line up at the metaphase plate

Alignment of the homologs is random

Anaphase I

Paired homologs separate and migrate to opposite ends of the cell

Telophase I

The homologs finish migrating to the poles of the cell

Phases of Meiosis II

1. Prophase II

2. Metaphase II

3. Anaphase II

4. Telophase II

Prophase II

Spindle apparatus forms

Microtubules from each pole attach to kinetochores on opposite sides of each chromosome

Metaphase II

Replicated chromosomes line up at the metaphase plate

Anaphase II

Sister chromatids separate

Resulting daughter chromosomes begin moving to opposite sides of the cell

Telophase II

Chromosomes arrive at opposite sides of the cell

Nuclear envelope forms around each haploid set of chromosomes

Each cell then undergoes cytokinesis

Mitosis versus meiosis

Homologs pair in meiosis but not in mitosis

Homologous pairs separate during Anaphase I, resulting in a reduction division

Mitosis produced two diploid daughter cells that are identical to the parent cells

Meiosis produces four haploid daughter cells that are genetically distinct from each other and the parent cells Meiosis produces

Human male timing in prophase

Prophase I lasts about 1 week, and 1 month for entire meiosis cycle

Human female timing of events in prophase I

Prophase I begins in utero, pauses, then resumes at puberty

Sperm formation

All 4 haploid cells from meiosis can become functional sperm

Egg formation

Only 1 of the 4 haploid cells from meiosis becomes the functional egg nucleus

Genetic recombination

Independent shuffling of egg-derived and sperm-derived chromosomes

Crossing over during meiosis

Sexual reproduction

Produces offspring with unique chromosome complements

Asexual reproduction

Produces clones that are genetically identical to one another and to the parent

Independent assortment

Random alignment and separation of homologous chromosomes during Meiosis I that can result in a variety of combinations of egg-derived and sperm-derived chromosomes

Crossing over produces

New combinations of alleles within a chromosome

Combinations that did not exist in either parent

Adds to genetic variability resulting from independent assortment

Role of fertilization

Each gamete is genetically unique

A given sperm and egg come together at random

Random joining of gametes increases genetic variation in offspring

What happens when things go wrong in meiosis?

Errors are common

As many as half of conceptions are spontaneously terminated because of problems in meiosis

Down syndrome caused by extra copy of chromosome 21

Nondisjunction

When both homologs or both sister chromatids move to the same daughter cell

Trisomy zygote

Fertilization of an n+1 gamete

Monopsony zygote

Fertilization of an n-1 gamete

Aneuploid

Cells with too many or too few chromosomes

Oogenesis

Involves human egg development

Sexual reproduction in prokaryotes and eukaryotes

Sexual reproduction using meiosis is absent in prokaryotes but is widespread in eukaryotes